Active Food Packaging

ACTIVE FOOD PACKAGING

Edited by

M.L. ROONEYPrincipal Research Scientist

CSIRODivision of Food Science & Technology

North RydeNew South Wales

Australia

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Preface

Food packaging materials have traditionally been chosen to avoid unwantedinteractions with the food. During the past two decades a wide variety ofpackaging materials have been devised or developed to interact with thefood. These packaging materials, designed to perform some desired roleother than to provide an inert barrier to outside influences, are termed activepackaging. The benefits of active packaging are based on both chemical andphysical effects.

Active packaging concepts have often been presented to the food industrywith few supporting results of background research. This manner ofintroduction has led to substantial uncertainty by potential users becauseclaims have sometimes been based on extrapolation from what little proveninformation is available. The forms of active packaging have been chosen torespond to various food properties which are often unrelated to one another.For instance, many packaging requirements for postharvest horticulturalproduce are quite different from those for most processed foods.

The objective of this book is to introduce and consolidate informationupon which active packaging concepts are based. Scientists, technologists,students and regulators will find here the basis of those active packagingmaterials which are either commercial or proposed. Some types of activepackaging are inevitably omitted but the book should assist the inquirer tounderstand how other concepts might be applied or where they should berejected.

Chapter 1 is the editor's overview of the field. Here I have sought todefine active packaging and to establish its limits and the background to itsdevelopment. The history of oxygen scavenging is used as a case study.

Chapter 2 is contributed by Dr Devon Zagory of Devon Zagory &Associates Inc., California. Dr Zagory has been a major contributor topresent knowledge of modified atmosphere packaging of horticulturalproduce. In this chapter he discusses the background to the use of activepackaging to remove ethylene from the headspace of respiring produce. Hediscusses the options currently available and their limitations.

Chapter 3, by Dr Kit Yam of Rutgers University, New Jersey and DrDong Sun Lee of Kyungnam University in South Korea, address theinterface between active packaging and equilibrium modified atmospherepackaging now in use. They introduce a simple method of modelling gasatmospheres to show where additional active packaging concepts arerequired.

Chapter 4 is the editor's discussion of the field of active packaging basedon polymers. This includes use of thermoplastics for films and more rigidcontainers but also provides background useful when consideration is givento other polymer-based coatings. The aim has been to unify the manyalternative unrelated concepts being offered to packers of both fresh andprocessed foods.

Chapter 5 is contributed by Dr Bernard Cuq and Dr St6phane Guilbertof CIRAD-SAR, and Dr Nathalie Gontard of ENSIA-SIARC, all ofMontpellier, France. Their research into the edible coating of foods needs nointroduction. They describe in this chapter how edible coatings are oftenalready active packaging and introduce the current and potential use asdelivery systems for food additives.

Chapter 6 is contributed by Dr J.P. Smith of McGiIl University, Montreal,Canada, Yoshiaki Abe, President of Mitsubishi Gas Chemical EuropeGmbH and Dr Jun Hoshino, Chief Microbiologist of Mitsubishi GasChemical Company, Inc., Tokyo. Jim Smith has published the results ofmany of the key investigations of the impact of modified atmosphere andactive packaging on food microbiology. Mr Abe and Dr Hoshino have beenresponsible for the introduction of 'Ageless' oxygen scavenging sachets,particularly outside Japan. In this chapter they consolidate knowledge ofsachet-based technologies.

Chapter 7 is co-authored by Dr John Budny, President of PharmaCal Ltd.,California and Dr Aaron Brody, who is Managing Director of RubbrightBrody Inc. Dr Brody was formerly with Schotland Business Research ofPrinceton and is a widely respected packaging consultant and author. Theseauthors provide the background to and opportunities for the use of enzymesin active packaging. This is still a frontier field and requires the carefullyexplained background presented in this chapter.

Chapter 8 is contributed by Dr Fred Teumac of ZapatA Technologies Inc.of Hazleton, Pennsylvania, USA, who was jointly responsible (withAdvanced Oxygen Technologies Inc.) for developing the first commerciallysuccessful oxygen scavenging closures for bottled beer. In this chapter hepresents this and other systems as case studies in active package develop-ment.

Chapter 9, by Stanley Sacharow, President of The Packaging Group Inc.,New Jersey, is an examination of the role played by active packaging incurrent commercial use. Stan Sacharow is a leading international consultantin packaging and has written several books and many articles on packagingtechnology. This chapter provides the necessary understanding of the statusof active packaging in the USA today. Active packaging for microwaveablefoods is a key topic.

Chapter 10 is by Dr Jeremy Selman, Head of the Food TechnologyDivision of the Campden Food and Drink Research Association in the UK.Dr Selman is widely respected for his work on food quality monitoring via

time-temperature indicators (TTIs). In this chapter he provides the back-ground to TTIs and summarizes their role with much tabulated information.He provides substantial unity to the TTI field by discussing their recentintroduction to thermal process validation.

Chapter 11 completes this work by drawing together the implications ofactive packaging for food safety. Prof. Joseph Hotchkiss of CornellUniversity, New York, has contributed widely to research and discussion ofthe role of packaging in food safety. In this chapter he draws together thesafety issues which arise from the use of active packaging and illustrateshow active packaging can itself contribute to food safety in the use ofantimicrobial films.

Acknowledgements

I extend my thanks to all the contributors, many of whom I have known andall of whom I have respected for several years. I appreciate theircommitment in working within the tight schedule required. I also thank LynKeen for her tolerant preparation and reorganization of much of themanuscript, and Andrew Sennett for preparing so many versions of some ofthe graphics. My thanks also go to Drs Bob Holland, Brian Patterson, BobJohnson, Mark Horsham, Alister Sharp, Candiera Albert and MichaelMcNaIIy for refereeing my contributions within CSIRO. I appreciate theadvice and forbearance of the staff at Blackie A&P. Finally, I thank my wifeSally, and my children Helen, James and Kathy for their quiet patience andactive support during the preparation of this book.

M.L.R.

Contributors

Y, Abe Mitsubishi Gas Chemical Europe GmbH, Immermann-strasse 45, Deutsch-Japannische Center, 40210Dusseldorf, Germany

AX. Brody Rubbright-Brody Inc., 733 Clovelly Lane, Devon, PA19333-1808, USA

JA. Budny PharmaCal Ltd., 31308 Via Colinas, Suite 107, WestlakeVillage, CA 91362, USA

B. Cuq CIRD-SAR, 73 rue J.F. Breton, BP 5035, 34032Montpellier, France

N. Gontard ENSIA-SIARC, 1101 Avenue Agropolis, BP 5098, 34033Montpellier, France

S. Guilbert CIRD-SAR, 73 rue J.F. Breton, BP 5035, 34032Montpellier, France

J. Hoshino Mitsubishi Gas Chemical Company, Inc, 1-1, Nijuku6-Chome, Katsushika-ku, Tokyo 125, Japan

J.H. Hotchkiss Institute of Food Science, Cornell University, 119Stocking Hall, Ithaca, NY 14853-7201, USA

D.S. Lee Department of Food Engineering, Kyungnam University,449 Wolyoung-dong, Masan City 630-701, Korea

M.L. Rooney Principal Research Scientist, CSIRO Food Research Lab-oratory, PO Box 52, North Ryde, New South Wales 2113,Australia

S. Sacharow The Packaging Group Inc, PO Box 345, Milltown, NJ08850, USA

J.D. Selman Director of Food Technology Division, CFDRA, ChippingCampden, Gloucestershire, GL55 6LD, UK

JLP. Smith McGiIl University, Department of Food Science andAgricultural Chemistry, Macdonald Campus 21, 111Lakeshore, Ste-Anne-de-Bellevue, Quebec, H9X 3V9,Canada

F.N. Teumac Vice-President Research and Development, ZapatA

Technologies Inc., PO Box 2278, Forest Road, HumboldtIndustrial Park, Hazleton, PA 18201, USA

K.L. Yam Assistant Professor, Department of Food Science, RutgersUniversity, PO Box 231, New Brunswick, NJ08903-0231, USA

D. Zagory Devon Zagory & Associates, Postharvest TechnologyConsultants, 759 North Campus Way, Davis, CA 95616,USA

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Contents

Preface ............................................................................... iii

Acknowledgements ............................................................ v

Contributors ........................................................................ vi

1. Overview of Active Food Packaging ........................ 1 1.1 Active, Intelligent and Modified Atmosphere

Packaging ................................................................. 1 1.2 Origins of Active Packaging ...................................... 3

1.2.1 Why Active Packaging ................................ 3 1.2.2 Historical Development .............................. 4

1.3 Literature Review ...................................................... 10 1.4 Scope for Application of Active Packaging ................ 12

1.4.1 Do-it-yourself Active Packaging .................. 17 1.5 Physical and Chemical Principles Applied ................ 20 1.6 Implications for Other Packaging .............................. 27

1.6.1 Whole Packages Designed to Be Active .... 29 1.7 Limitations of Current Approaches ............................ 31 1.8 Future Potential ......................................................... 32 1.9 Regulatory Considerations ........................................ 33 References .......................................................................... 33

Contents ix

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2. Ethylene-removing Packaging .................................. 38 2.1 The Chemistry of Ethylene ........................................ 38

2.1.1 Synthesis .................................................... 38 2.1.2 Degradation ................................................ 39 2.1.3 Adsorption and Absorption ......................... 40

2.2 Deleterious Effects of Ethylene ................................. 41 2.2.1 Respiration ................................................. 42 2.2.2 Fruit Ripening and Softening ...................... 42 2.2.3 Flower and Leaf Abscission ....................... 42 2.2.4 Chlorophyll Breakdown .............................. 43 2.2.5 Petal Inrolling in Carnations ........................ 43 2.2.6 Postharvest Disorders ................................ 43 2.2.7 Susceptibility to Plant Pathogens ............... 43

2.3 Interactions of Ethylene and Other Gases ................ 44 2.3.1 Oxygen ....................................................... 44 2.3.2 Carbon Dioxide ........................................... 44 2.3.3 Ozone ......................................................... 45

2.4 Ethylene Sources in the Environment ....................... 45 2.4.1 Combustion ................................................ 45 2.4.2 Plant Sources ............................................. 45 2.4.3 Ripening Rooms ......................................... 46 2.4.4 Fluorescent Ballasts and Rubber

Materials ..................................................... 46 2.4.5 Microorganisms .......................................... 46

2.5 Commercial Applications in Packaging ..................... 46 2.5.1 Potassium Permanganate-based

Scavengers ................................................ 46 2.5.2 Activated Carbon-based Scavengers ......... 47 2.5.3 Activated Earth-type Scavengers ............... 48

x Contents


2.5.4 New and Novel Approaches to Ethylene-removing Packaging ................................... 50

Acknowledgements ............................................................. 51 References .......................................................................... 51

3. Design of Modified Atmosphere Packaging for Fresh Produce ............................................................ 55 3.1 Introduction ............................................................... 55 3.2 Literature Review ...................................................... 57 3.3 Feasibility Study ........................................................ 59

3.3.1 Optimum Conditions ................................... 60 3.4 Respiration Rates ...................................................... 61

3.4.1 Temperature Effect ..................................... 61 3.5 Measurement of Respiration Rates ........................... 62

3.5.1 Flow-through System ................................. 62 3.5.2 Closed System Method .............................. 63

3.6 Model Equations and Package Requirements .......... 64 3.6.1 Unsteady-state Equations .......................... 65 3.6.2 Steady-state Equations .............................. 66

3.7 Polymeric Films for MAP Applications ...................... 67 3.7.1 Perforation and Microporous Films ............. 68 3.7.2 Temperature Compensating Films ............. 70 3.7.3 Ceramic-filled Films .................................... 70

3.8 Concluding Remarks ................................................. 70 Nomenclature ...................................................................... 71 References .......................................................................... 72

4. Active Packaging in Polymer Films ......................... 74 4.1 Introduction ............................................................... 74

Contents xi


4.2 Oxygen Scavenging .................................................. 74 4.2.1 Forms of Oxygen-scavenging Packaging ... 76 4.2.2 Plastics Packaging as Media for Oxygen

Scavenging ................................................. 77 4.2.3 Brief History of Oxygen-scavenging

Films ........................................................... 80 4.2.4 Chemistry of Oxygen Scavenging .............. 83 4.2.5 Chemical Barrier to Oxygen Permeation .... 92

4.3 Moisture Control Films .............................................. 94 4.3.1 Liquid Water Control ................................... 95 4.3.2 Humidity Buffering ...................................... 96

4.4 Removal of Taints and Food Constituents ................ 99 4.5 Ingredient Release .................................................... 102

4.5.1 Antioxidant Release from Plastics .............. 103 4.6 Permeability Modification .......................................... 105 4.7 Current Use Commercially ........................................ 106 4.8 Regulatory and Environmental Impacts .................... 106 References .......................................................................... 107

5. Edible Films and Coatings as Active Layers ........... 111 5.1 Introduction ............................................................... 111 5.2 Use of Edible Active Layers to Control Water Vapor

Transfer ..................................................................... 114 5.3 Use of Edible Active Layers to Control Gas

Exchange .................................................................. 121 5.4 Modification of Surface Conditions with Edible

Active Layers ............................................................. 126 5.5 Conclusion ................................................................ 134 Acknowledgements ............................................................. 135 References .......................................................................... 135

xii Contents


6. Interactive Packaging Involving Sachet Technology ................................................................. 143 6.1 Introduction ............................................................... 143 6.2 Oxygen Absorbents ................................................... 144

6.2.1 Classification of Oxygen Absorbents .......... 145 6.2.2 Main Types of Oxygen Absorbents ............ 149 6.2.3 Factors Influencing the Choice of Oxygen

Absorbents ................................................. 152 6.2.4 Application of Oxygen Absorbents for

Shelf-life Extension of Food ....................... 153 6.2.5 Advantages and Disadvantages of

Oxygen Absorbents .................................... 161 6.2.6 Effect of Oxygen Absorbents on

Aflatoxigenic Mold Species ........................ 164 6.3 Ethanol Vapor ........................................................... 164

6.3.1 Ethanol Vapor Generators .......................... 166 6.3.2 Uses of Ethicap for Shelf-life Extension of

Food ........................................................... 168 6.3.3 Effect of Ethanol Vapor on Food

Spoilage/food Poisoning Bacteria ............... 171 6.3.4 Advantages and Disadvantages of

Ethanol Vapor Generators .......................... 171 6.4 Conclusion ................................................................ 172 References .......................................................................... 172

7. Enzymes as Active Packaging Agents .................... 174 7.1 Enzymes ................................................................... 174 7.2 Potential Roles of Enzymes in Active Packaging ...... 176 7.3 History ....................................................................... 178 7.4 Oxygen Removal ....................................................... 179

Contents xiii


7.5 Antimicrobial Effects .................................................. 186 7.6 Time-temperature Integrator-indicators ..................... 188 7.7 Lactose Removal ...................................................... 189 7.8 Cholesterol Removal ................................................. 190 References .......................................................................... 191

8. The History of Oxygen Scavenger Bottle Closures ..................................................................... 193 8.1 Background ............................................................... 193 8.2 Oxygen Measurements ............................................. 193

8.2.1 Techniques for Measuring the Oxygen Content of Bottles ....................................... 193

8.2.2 Results of Measurements ........................... 194 8.2.3 Oxygen Ingress .......................................... 195 8.2.4 Combining the Effect of Initial and Ingress

Oxygen ....................................................... 196 8.3 Oxygen Scavenger Liners ......................................... 197

8.3.1 Theoretical .................................................. 197 8.3.2 Commercial Activity .................................... 197 8.3.3 Health and Environmental Concerns .......... 199

8.4 The Effect of Scavenging Closures on Beer Flavor .. 199 8.5 The Advantages of Oxygen Control Bottles .............. 200 8.6 The Future of Oxygen Scavenging Closures ............ 200 References .......................................................................... 201

9. Commercial Applications in North America ............ 203 9.1 Packaging Overview ................................................. 203 9.2 Marketplace Susceptors ............................................ 203

9.2.1 Susceptor Types ........................................ 204 9.2.2 Field Intensification Devices ....................... 206

xiv Contents


9.2.3 Susceptor Applications ............................... 208 9.3 Application of Temperature Indicator to

Microwaveable Packaging ........................................ 209 9.4 Active Packaging – Produce ..................................... 209

9.4.1 Oya Produce Bags ..................................... 209 9.4.2 Oya Test Results ........................................ 210 9.4.3 Modified Atmosphere Produce ................... 211

9.5 Oxygen Absorber Food Applications ......................... 211 9.5.1 Bottle Closures – Oxygen Scavengers ....... 213

9.6 Other Applications ..................................................... 213 References .......................................................................... 214

10. Time-temperature Indicators .................................... 215 10.1 Introduction ............................................................... 215 10.2 Indicator Systems ...................................................... 217 10.3 Indicator Application Issues and Consumer

Interests .................................................................... 227 10.4 Chemical Indicators for Thermal Process

Validation .................................................................. 230 10.5 Conclusions ............................................................... 234 References .......................................................................... 234

11. Safety Considerations in Active Packaging ............ 238 11.1 Introduction ............................................................... 238 11.2 Packaging and Food Safety ...................................... 238 11.3 Passive Safety Interactions ....................................... 240

11.3.1 Barriers to Contamination ........................... 240 11.3.2 Prevention of Migration .............................. 241

11.4 Active Safety Interactions .......................................... 242 11.4.1 Emitters and Sorbers .................................. 243

Contents xv


11.4.2 Active Packaging and Migration ................. 243 11.4.3 Barrier to Contamination ............................ 244 11.4.4 Indirect Effects on Safety ........................... 244 11.4.5 Indicators of Safety/spoilage ...................... 245 11.4.6 Direct Inhibition of Microbial Growth ........... 246 11.4.7 Modified Atmosphere Packaging ................ 246 11.4.8 Antimicrobial Films ..................................... 248 11.4.9 Rational Functional Barriers ....................... 250 11.4.10 Combined Systems .................................... 252

11.5 Conclusions ............................................................... 252 References .......................................................................... 253

Index .................................................................................. 256

1 Overview of active food packaging

M.L. ROONEY

1.1 Active, intelligent and modified atmosphere packaging

Packaging may be termed active when it performs some role other thanproviding an inert barrier to external conditions. Hotchkiss (1994) includesthe term 'desired' when describing the role, and this is important in that itdifferentiates clearly between unwanted interactions and desired effects.This definition reflects the element of choice in how active packagingperforms and the fact that it may play some single intended role andotherwise be similar to other packaging in the remainder of its properties.These latter two aspects also reflect that active packaging is something thatis designed to correct deficiencies which exist in passive packaging. Asimple example of this situation is when a plastics package has adequatemoisture barrier but an inadequate oxygen barrier. Active packagingsolutions could be the inclusion of an oxygen scavenger, or an antimicrobialagent if microbial growth is the quality-limiting variable.

Active packaging has developed as a series of responses to unrelatedproblems in maintenance of the quality and safety of foods. Accordingly arange of types of active packaging has been developed. Each of these has arange of descriptive terms. Horticultural produce has for some years nowbeen packaged in 'smart films', and oxygen has been removed from packageheadspaces by oxygen scavengers, free-oxygen absorbers (FOAs) anddeoxidisers. Carbon dioxide can be released by emitters or can be absorbedby getters, and microwaves can be controlled in packages by susceptors orshields (Sacharow and Schiffman, 1992). Regional differences in terminol-ogy are also seen. The terms 'freshness preservative' and 'functional' and'avant garde' are also used to describe active packaging materials (Katsura,1989; Louis and de Leiris, 1991). There has been a range of trade names forthose packages where a generic form has not been coined, with the resultthat we have SmartCap (ZapatA - Advanced Oxygen Technologies) forclosures for beer bottles and Oxyguard for Toyo Seikan Kaisha Ltd. for capsfor similar use.

Smart packages have been defined by Wagner (1989) as 'doing more thanjust offer protection. They interact with the product, and in some cases,actually respond to changes'. In this sense, most packaging media are activeto some degree. However, there are forms of packaging which are clearlydistinct subclasses. The term equilibrium modified atmosphere (EMA)

packaging is used to distinguish the situation where the choice of thepermeability ratio of oxygen/carbon dioxide determines whether respiringhorticultural produce generates a viable gas atmosphere or not (Gill, 1990).Thus where modified atmosphere packaging (MAP) is used with processedfoods and involves merely flushing with an initial gas mixture the packagingis not active. EMA packaging is one of the borders between active andpassive packaging. Some aspects of EMA packaging are discussed in thisbook because the limitations of conventional packaging films are beingincreasingly addressed using active materials.

If the physical interactions of a package with the food are removed we areleft only with the chemical (and increasingly, biochemical) effects. Such arestriction is probably unduly strict and in time we should expect to seefurther subdivision of active packaging to take account of whether 'activity'is a property of the packaging material itself or of inserts within thepackage.

We are beginning to see reference to the benefits of active packaging inthe popular press with reference to 'packaging that is niftier and cooks yourfood' and 'Hi-Tech' packaging (Sprout, 1994). The active packaging sodescribed includes susceptors and reflectors in microwaveable packs as wellas horticultural smart films that absorb ethylene. These are describedtogether with temperature sensitive labels that help determine when food iscooked, i.e. 'doneness indicators'.

There are other areas of packaging developing concurrently and there areareas of overlap with active packaging as noted with MAP above. Probablythe closest area is Intelligent Packaging, a grouping of technologies definedin the CEST publication by Summers (1992) as 'an integral component orinherent property of a pack, product or pack/product configuration whichconfers intelligence appropriate to the function and use of the product itself.This grouping covers the area of product identity, authenticity, traceability,tamper evidence, theft protection, and quality as indicated by time-temperature indicators. The latter was originally included by Labuza (1987)in his seminal review of active packaging. Time-temperature indicators alsofit the definition of active packaging given above; they play a role indefining the steps that need to be taken to ensure the quality and safety of thepackaged food. A somewhat related field of packaging which so far hasfallen between the two definitions is that of gas composition indicators. Todate they have been used in the form of tablets to indicate when oxygen-scavenging sachets have achieved their purpose (Anon, undated). There havebeen steady efforts made for several years to produce oxygen-indicatingprinting inks but thus far, like the pellets, these indicators largely changecolour at oxygen levels below 0.1%.

The description of this field as interactive packaging is also seen. There issome benefit in such a description as it links desired and undesiredinteractions of foods and their packaging, such as flavour scalping (Hirose et

al, 1989). However, this lack of distinction is probably a disadvantageoverall, hence the naming of this book.

1.2 Origins of active packaging

1.2.1 Why active packaging

Inherent in the definition is the point that active packaging now plays a rolein the protection of the food which is additional to the classic purposes ofany packaging, viz. containment, protection, convenience and communica-tion (Robertson, 1993). This role can be addressing a single aspect of thepackaging requirements of a food, such as making up for an inadequacy ofa packaging material - which is already a compromise. Thus, for semi-aseptic or retort trays of steamed rice, oxygen-scavenging sachets arebonded to the lid to consume oxygen passing through the trays, especiallywhen retorted. Ethanol-releasing sachets are used with bakery products ofhigh aw to suppress mould growth because low oxygen barrier packaging isused. These examples show that there are opportunities to reduce the cost ofpackaging materials or packaging processes by use of active packaging withcheaper passive packaging. This has been demonstrated by Alarcon andHotchkiss (1993), who showed that crusty bread rolls packaged in a mediumbarrier plastics laminate had a shelf-life similar to that provided by a moreexpensive foil laminate (see Chapter 6). The key to using chemical forms ofactive packaging for economic benefit is that the benefit must be achievablebefore the chemical is exhausted. This is particularly important in the area offresh and extended shelf-life foods as originally described by Labuza andBreene (1989) but not only in that area. For example, the suppression ofenhanced oxygen permeation of retortable trays (caused by retorting) byapplication of a desiccant layer is important while the water is beingabsorbed rapidly during retorting, in order to impart multi-year shelf-life atambient temperatures.

The key focus of active packaging therefore is the match of the packageproperties to those of the food, a target that normally has been considered torequire compromise. The result of this matching is therefore optimisation ofshelf-life and permitting processes, formulations and presentation whichwere previously impossible. It is possible to add hurdles to enhance thesafety of food packaging processes. It is in this area that antimicrobialpackaging may make a substantial contribution as it develops from itscurrent early stage.

It should be emphasised that active packaging is not a generally appliedconcept like, for instance, water vapour barrier packaging plastics. It israther the application of specific packaging properties to specific situations.In this way, we see a large number of niche markets, some niches being very

large indeed as with oxygen scavenging crown closures for bottled beer. Insome instances we see the need for application of two forms of activepackaging to achieve a goal. This has been demonstrated by Naito et al(1991) who showed that use of an oxygen absorber alone suppressed growthof bacteria and of yeast (Hansenula anomala) in packages of sponge cakebut that the inclusion of an ethanol or carbon dioxide emitter was even moreeffective. The use of combined-effect sachets, for instance, is nowcommonplace - see Chapter 6.

1.2.2 Historical development

Because it applies to a collection of niche markets, active packaging has avery uncoordinated history. This aspect can best be examined by seeking tosubdivide it, albeit somewhat artificially, into processed food packaging andfresh food packaging.

Probably the earliest form of active packaging was the wine skin which,while causing a variety of unwanted interactions, was intended to collapse asthe wine was consumed without necessarily increasing the quantity ofoxygen within. Just how good a barrier the skin is to oxygen permeation isnot known, so this is a far from perfect example. The most obviouscommencement of active packaging was the use of tinplate for constructionof cans. The tin is sacrificially corroded, protecting the iron base can, whileconcurrently saving the food from contamination with large amounts of iron.The latter, with its two readily accessible oxidation states, can function as anautoxidation catalyst when residual oxygen is present. The tin also acts as areducing agent for food constituents such as pigments. It also contributes todevelopment of the flavour associated with die 'traditional' canned orangejuice. This became economically significant with the introduction ofaluminium cans when it was observed that the 'traditional' flavour was notgenerated in the absence of tin.

A subsequent development was the introduction of sulfur-stainingresistant lacquers (or enamels). The sulfur staining is due to the decomposi-tion of sulfur-containing amino acids in foods to produce compounds whichreact with the tinplate, so staining the metal surface. The introduction of zincoxide results in a reaction which forms products not observable in theotherwise white lacquer.

1.2.2.1 Iron-based oxygen scavengers. Since tinplate cans were the basicpackaging of processed foods for most of the past century, it is notsurprising that the next development was also targeted at canned food.Tallgren (1938), in a patent, described the use of iron, zinc or manganesepowders to remove oxygen from the headspace of cans. This inventionpaved the way for the subsequent development of the iron-based oxygenscavengers of commerce today.

Table 1.1 History of iron-based oxygen scavengers

Additional reagents Form Reference Country

Water absorbents powder Tallgren, 1938 FinlandIron compounds powder Isherwood, 1943 UKSodium carbonate powder Buchner, 1968 GermanyCarbon, water powder Nawata el al., 1977 JapanAlkali metal halides powder Mitsubishi, 1977 JapanAmmonium chloride powder Kureha, 1982 JapanSodium chloride powder Fujishima, 1985 JapanUn-named film Koyama/Oda, 1992 Japan

The development of iron-based oxygen scavengers is summarised inTable 1.1 which shows movement from Europe to Japan as development ofcommercially useable systems occurred.

Concurrent with the development of iron-based scavengers was thedevelopment of other inorganic and organic systems, both based on the useof sachets. The reaction of sodium dithionite with oxygen can be triggeredby the presence of water from a food headspace. The steps involved in thescavenging of oxygen by such agents are shown in the following equations(Nakamura and Hoshino, 1983).

Na2S2O4 + Ca(OH)2 + O 2 - * Na2SO4 + CaSO3 + H2O (1.1)

Na2S2O4+ O2 -> Na2SO4+ SO2 (1.2)

Ca(OH)2 + SO2 -> CaSO3+ H2O (1.3)

The reactions involved in the scavenging of oxygen by iron-basedcompositions are discussed in Chapter 6.

Additional processes involving organic chemicals have also been devel-oped, commencing with catechols and related substances and resulting in thewidespread claims of use of ascorbic acid in patents in recent years. Therecent growth in the number of patent applications, found via the DerwentIndex, for systems which do not involve iron can be seen in Figure 1.1. TheDerwent Index catalogues patent applications by the earliest nationalapplication. Any applications for coverage of the same matter in othercountries are indexed therewith. These results were generated by taking thenumbers of earliest applications without consideration of the number ofadditional countries covered. Not all applications are necessarily granted butpatenting activity is an indication of interest in developing new ideas.

The results in Figure 1.1 show the distribution of such early applicationsfor composition claims over the 20-year period from 1975 to the first half of1994. The incremental unit is 2 years and the applications were dis-tinguished by their involvement with or frequent inclusion of zinc, copper ornickel since non-food applications also constitute a very large potentialmarket. The interest in systems not involving elemental metals was initiallythe strongest although claims for metallic systems peaked around 1980 after

rearsFigure 1.1 Substrates described in patent applications for oxygen scavengers.

Mitsubishi Gas and Chemical Co. released Ageless sachets in 1978. The fallin the number of patent applications in the mid-1980s is reflected in both thenumber of metallic compositions and that of other compositions. Thesubsequent, increasingly strong activity in both areas of chemistry reflectsthe need for processes which overcome deficiencies in existing products.

If the preparation of an active packaging material or process is viewed asa product development topic in its own right then there are two facets whichshould be evident. The first is the composition and the second is the designof the remainder of the product, including its packaging. Some activepackaging technologies have been based on the introduction of a sachet intothe package of food and thus have the characteristics of the protective andfunctional packaging of any other sensitive product.

The results in Figure 1.2 show that, initially, innovation was directedtowards establishing claims for novel compositions and that the level ofactivity between 1977 and 1982 was almost the same as that between 1989and 1994. What has changed are the chemical reactions involved in thesecompositions. Initially, greatest interest was shown in the reactions under-gone by various sulfites in the presence of alkali powders which absorb thesulfur dioxide formed. Subsequent developments were based on oxidation ofiron powder or ferrous compounds and this has continued with the growth inthe number of compositions based on organic reactions. Prominent amongthese is oxidation of ascorbic acid or of ethylenically unsaturated com-pounds such as fatty acids, squalene and rubbers.

App

licat

ions

Metal Other

Years

Figure 1.2 Patent applications for oxygen scavengers based on composition or design.

Evidence of the maturity of this field of active packaging is shown by thetrend in patent applications for novel designs aimed at overcomingdeficiencies in either handling or effectiveness found in the early versions.The ease of triggering an active state in oxygen-scavenging compositions ismore important than with most other forms of active packaging. Designpatents were scarcely considered for the first 8 years of this period until theproducts of companies such as Mitsubishi Gas and Chemical Co. andToppan Printing Co. made an impact on food packaging in Japan. Since thattime the number of applications for new designs has increased sharply, withthe numbers of design applications over the last 10 years exceeding those fornew compositions. This maturity of the market is seen in the progressiveexpansion of use of such products in the USA, Europe and Australasia. Thecurrent production of oxygen-scavenging sachets alone is understood toexceed 7 billion per annum in Japan, several hundred million in the USAand some tens of millions in Europe.

Another of the aspects of oxygen-scavenger development is the respectivepositions of sachet and film-based technologies. These aspects are con-sidered in detail in Chapter 4.

A recent trend is the incorporation of scavengers into the packagingmaterial itself, rather than being used in a sachet which hitherto had been themain commercial application. Chapter 4 deals with development ofscavenging plastics for packages in general, and the specific commercialapplication to bottle closures, especially as liners for crown seals for beerbottles, is discussed in Chapter 8.

Besides the main lines of research and development of oxygen scavengersreferred to above there have been other, less popular, lines of research and

Appl

icat

ions

Composition Design

commercial development. The catalytic conversion of hydrogen to water,initially in tinplate cans then in laminate pouches, was first described byKing (1955) who saw the need for removal of oxygen from spray-driedcanned milk powder.

There has also been continuous research into ways of stabilising enzymiccatalysts for oxygen removal. The oxidation of glucose, catalysed by glucoseoxidase, has been studied intermittently following the work with OxyBan byScott and Hammer (1961). This subject is discussed in detail in Chapter7.

1.2.2.2 Composite systems. An early observation was that in-packremoval of oxygen from package headspaces creates a correspondingdecrease in pressure which results in either package collapse or developmentof a partial vacuum. The latter may be tolerable in rigid packaging where theseal integrity is good, but in flexible packs pressure decreases of as little asa few kPa result in package collapse where the headspace is small. Thesimultaneous release of carbon dioxide by sachets which consume oxygenwas devised by several companies (see Chapter 6). This concurrent releaseof carbon dioxide can be additionally useful in the suppression of microbialgrowth (Naito et al9 1991). The concentration of this gas needs to reach20%, preferably more, and is potentially useful where packages already havesome carbon dioxide in the atmosphere. Such systems are based on eitherferrous carbonate or a mixture of ascorbic acid with sodium bicarbonate.

More effective use of dual-function active packaging inserts has beenforeseen by several companies in this field. The potential for generation ofconditions suitable for growth of anaerobic pathogenic microorganismsexists if oxygen scavengers are used inappropriately. Accordingly, therelease of ethanol or carbon dioxide concurrent with oxygen removal hasbeen cited for this purpose by both Toppan Printing KK (1985) and Leon etah (1987). This performance of commercial dual-function scavengingsachets is discussed in Chapter 6.

The historical development of other forms of emitters of ingredients suchas flavours, antioxidants and antimicrobial agents is dealt with in otherchapters. This applies also to packaging for microwaveable foods wherethere is the potential for one form of active packaging to interact withanother. Thus there is the need to ensure that oxygen-scavenging sachets donot interfere with microwave heating of packages of ready-to-eat foods suchas retorted steamed rice. This product is marketed with an organic reagent,ascorbic acid, in a sachet bonded to the inside of the lid of the tray with a hotmelt adhesive. An additional approach to overcome the same problem, alsoby Mitsubishi Gas and Chemical Co., is the patenting of scavengers basedon boron and its compounds.

1.2.2.3 Horticultural active packaging. Active packaging for horticul-tural produce has evolved via the concept of 'smart films'. It has long beenknown that respiring produce will generate its own modified atmosphere, butin so doing it may reach a stage of ripeness which is not desired. As long agoas the 1970s choices were made between packaging films in an effort toachieve a modified atmosphere which suppressed respiration and lengthenedshelf-life (Prince, 1989). Since there was difficulty in achieving satisfactoryperformance with many produce-film combinations, highly permeableporous patches were introduced. This enabled packers to use film of anycommodity while ensuring that there was adequate exchange of oxygen andcarbon dioxide. This is unsuitable for developing equilibrium modifiedatmospheres close to the optimum for all produce but is useful both inpreventing injury due to excessively high concentrations of CO2 and instopping anaerobic respiration from being initiated. This approach has beendeveloped progressively in the USA with the patches of microporous filmdeveloped by Hercules Freshold Corp. and with subsequent patches, the gaspermeability of which changes with the degree of hydration.

Concurrently, other workers have approached the same problem byseeking to make the entire packaging film more permeable or by seeking todevelop some enhancement of selectivity. This field has developed either byuse of patents or merely by use of advertising claims with little or noscientifically developed results supporting such claims. Thus films have beenmade porous by inclusion of porous powders such as zeolites or volcanicrock, or by inclusion of crushed rocks. More recently, porosity has beenincreased by burning or etching controlled diameter holes to prevent bothcondensation and oxygen depletion. Recently, again, some considerablescientific modelling effort has been focused on the use of a single pore in apackage to achieve the necessary rate of gas exchange (Mannapperuma andSingh, 1994).

During the 1980s, when marketing of films containing inorganic powderswas commenced, widely ranging claims were made for effects such as theemission of infra-red radiation from ceramics. Claimed benefits includepreservation of the produce and absorption or scavenging of ethylene.Evidence for removal of ethylene from package headspaces other than byenhanced permeation through porous solids or porous film does not appearto be substantial.

Some films containing inorganic powders are claimed to offer multiplebenefits for packaging of horticultural produce. Katsura (1989) reports thatFH film containing Oya stone dispersed on the film surface absorbs ethylene,is a freshness preservative, and controls levels of water vapour, oxygen andcarbon dioxide in the pack.

Chemical scavenging of ethylene, however, has been the subject ofconsiderable research since the work of Scott et al. (1970). Their work withpotassium permanganate in porous slabs of vermiculite in bags of bananas

demonstrates the early success of this approach. The detail of the historicaldevelopment of research and commercial development of such processes isdescribed in Chapter 2.

The use of edible or otherwise non-toxic coatings on fruit skins asvehicles for active chemicals has been known for several years. Themodification of the atmosphere close to the skins of fruit has been achievedby shrink wrapping. Both these techniques assist in eliminating the build-upof surface water as a cause of microbial growth on fruit surfaces. The recentintroduction of minimally processed fruits and vegetables in packaged formhas heightened awareness in this area. The importance of hygiene in thepreparation of sliced and diced produce has been discussed by Varoquauxand Wiley (1994). The potential for antimicrobial edible films is reviewed inChapter 5.

1.3 Literature review

Active packaging has developed as a series of topics which are related onlybecause they involve the package influencing the environment of the food.The literature in this field consists very largely of patent applications,technical leaflets and reviews. The latter have often been presented atconferences where specialised audiences have been able to take up the ideaspresented. Reports of academic scientific investigation have been limitedlargely to occasional assessments of the appropriateness of some of thesetechnologies. The literature in this field is therefore discussed in terms of thereviews.

Sneller (1986) reported on the impact of smart films on controlledatmosphere packaging although the first broadly based reviews werepresented in Iceland and Australia in 1987 (Labuza, 1987; Rooney, 1987).The first use of the term active packaging was proposed at that time byLabuza, who defined active packaging as a range of technologies, some ofwhich now represent the borderlines between active, 'intelligent', andmodified-atmosphere packaging (Labuza and Breene, 1989). The essentialfeatures of these 'freshness enhancers' have been summarised in a shortreview by Sacharow (1988).

Katsura (1989) reviewed the range of functional packaging materialswhich had been commercialised with particular reference to Japan. Hedemonstrated the attention that had been paid to freshness preservativepackaging. Wagner (1989) summarised the range of smart packages andemphasised the role of microwaveable-food packaging. Rooney (1989a,b;1990) concentrated on chemical effects, particularly oxygen scavenging. Therole of oxygen scavengers in maintaining the benefits of MAP for processedfoods was reviewed by Smith et al. (1990) following their own research intosuppression of microbial growth (see Chapter 6).

The International Conference on Modified Atmosphere Packaging atStratford-upon-Avon (UK) in 1990 organised by the Campden Food andDrink Research Association included several reviews relating to activepackaging. Louis described several innovative active packages whichgenerated modified atmospheres. Abe gave the first comprehensive quantita-tive assessment of the impact of active packaging. He estimated the marketsize for each of the broad classes of such packaging systems. His reviewreveals that around 6.7 billion oxygen-scavenging sachets and 70 millionethanol-generating sachets were manufactured in Japan in both 1989 and1990. The estimated market for films containing mineral powders was only1000 tonnes in 1989 with 40% of consumption as home use.

The review by Robertson (1991) emphasised the application of activepackaging to processed foods. The emphasis was placed on crown seals forbottled beer, oxygen-scavenging plastics films and microwave susceptors.The use of the term active packaging rather than smart films was noted bythat reviewer and by Sacharow (1991) who also noted the use of sachets ofpotassium permanganate in silica gel for ethylene removal in produce packs.By this time the claimed benefits of freshness preservation technologies forhorticultural products were being examined critically, especially in Japan.Ishitani (1993a,b) surveyed the number of patent applications for thispurpose from 1984 to 1989. Over the first two years the annual rate wasaround 35 applications. This increased to a peak rate of 220 per annum in thesecond half of 1987 before dropping to around 60 per annum in 1989. It wasnoted that initial developments were directed at the needs for low-temperature maintenance and moisture control. The boom in 1987 was theconsequence of the attention being paid to gas composition control andethylene removal. By 1989 gas composition was the main object ofdevelopments but moisture control and coating methods were also impor-tant. Ishitani (1993a) observed two factors that led to much rethinking.These were the lack of data on the requirements of produce and doubts aboutthe capacity of powder-filled plastics to remove enough ethylene. Morerecent developments have been focused on ethylene removal at highhumidities and on matching gas composition and temperature to therequirements of enzymic systems of plants. Several recent books on MAPhave included discussion of the gas-packaging requirements for horticulturalproduce as well those for some processed foods (Ooraikul, 1993; Parry,1993). The environmental aspects of active packaging have not beenconsidered to any great extent in reviews to date. Rooney (1991) addressedsome issues drawing attention to the need to consider the nature of thepackaging which can be replaced by these new technologies.

The current state of development and commercial application of activepackaging has been reviewed in three papers at the symposium Interaction:Foods - Food Packaging Material held in Sweden in June 1994. Miltz et al(1994) reviewed the field in general, Ishitani (1994) concentrated on

Japanese developments, especially antimicrobial films, Day (1994) concen-trated on fresh produce and Guilbert and Gontard (1994) focused on edibleand biodegradable packaging. Several posters described original researchand that of Paik described photoprocessing of a film surface to generateantimicrobial properties. Perdue (1993) has briefly reviewed antimicrobialpackaging from the viewpoint of the Cryovac Division of W.R. GraceCompany and presents a somewhat pessimistic picture.

1.4 Scope for application of active packaging

Active packaging is still developing as a collection of niche markets so it isnot surprising that a diverse range of packages active in the physical andchemical sense are either proposed or commercially available. Early amongthese was the use of the reaction of lime with water to generate heat for self-heating cans of sake (Katsura, 1989). The Verifrais process for meat

Respiring Produce

Delayed ripening

Temperature abuse

Fungal growth

Aseptic

Liquids Oxidation

activepackaging

OxidationHydration

Dry

Foods

PreparedMeals

Microwavecooking

ColourRetention

Chilled

Meats

Mould

Growth

Bakery

ProductsREQUIREMENT FOODCLASS

Figure 1,3 Properties of foods amenable to active packaging.

packaging uses the reaction of organic acid with bicarbonate to producecarbon dioxide in response to meat drip in foam trays. The carbon dioxidereleased helps to suppress microbial growth.

Some properties of foods which can be addressed by active packaging aresummarised in Figure 1.3. These properties are grouped depending uponwhether they are designed to sustain living foods, suppress insect ormicrobial life in any foods, prevent oxidative attack on food constituents,retain flavour, or facilitate serving of the food for consumption. The ways inwhich such packaging performs these roles are elaborated in the remainderof this section.

Active packaging can been seen in one sense as a means of maintainingthe optimum conditions to which a food was exposed at the immediatelypreceding step in its handling or processing. Passive packaging has beenused in an effort to minimise the deleterious effects of a limited number ofexternal variables such as oxygen, water, light, dust microorganisms, rodentsand to some extent, heat. Hence, active packaging has the potential tocontinue some aspects of the processing operation or to maintain chosenvariables at particular levels. This aspect of active packaging is a unifyingtheme and crosses the border between foods such as plant produce, andprocessed foods, including those thermally processed.

A second aspect of active packaging is that it can be involved in thepreparation of the food for consumption. This includes aspects of tem-perature modification either for organoleptic or food safety purposes. Theseproperties therefore include heating, cooling and foaming.

Non-processed, respiring food such as agricultural and horticulturalproduce, fish, crustaceans and other seafood can be stored and/or shippedover long distances provided the respiration requirements are satisfied undercontrolled temperature conditions. Thus if the packaging can regulate thesupply of oxygen to the animal or produce such that a minimum respirationrate can be sustained, an enhanced period of prime-quality life can often beachieved. In plant products the optimum oxygen concentration of theenvironment varies with the species, and levels down to 1% may be possiblewithout inducing anaerobic respiration (Labuza and Breene, 1989). Thegeneration of elevated levels of carbon dioxide to suppress ethylenesynthesis and to suppress microbial action can be achieved by selection ofplastics films of appropriate permeabilities. However, achievement of theoptimum balance of oxygen and carbon dioxide concentrations by use ofplastics films alone is frequently impossible, particularly as allowance mustbe made for temperature abuse.

It is possible to predict the potential packaging requirements forhorticultural produce by modelling the properties of the food and thepackaging film. There have been several reports published on approaches tomodelling such systems, and they have been compared by Solomos (1994),who has tabulated the characteristics provided for in each of the models.

Some of these are quite complicated and they are set out in Chapter 3 toprovide an easy-to-use model. This form of packaging is commonly termedmodified atmosphere packaging (MAP) or more appropriately equilibriummodified atmosphere (EMA) packaging.

EMA packaging involving selection of polymer films is, as mentionedpreviously, the borderline between active and passive packaging. Chapter 3shows how modelling can indicate quickly whether available packagingplastics are going to be suitable for maintaining EMA over the temperaturerange chosen. Several approaches to overcoming the limitations of thesefilms have been reported. One which is still in its infancy is the use of liquid-crystal polymers which undergo a phase change at a characteristictemperature. The permeability of the polymer to oxygen sharply increases asthis temperature is exceeded, thus providing the oxygen necessary to preventpackaged horticultural produce from switching from aerobic to anaerobicrespiration. The present state of the art is not sufficiently advanced to coverEMA films which match produce over a wide temperature range, butresearch has opened up this possibility.

An alternative involves pores in portions of a package which open whenthe temperature exceeds a precisely set value. This has been achieved byfilling pores in a patch on a package with a wax which melts appropriately(Cameron and Patterson, 1992). This wax, when liquid, is drawn away by awick such as a microporous film to leave the pore open to gas exchange.This type of high-temperature emergency valve is applicable to packagesover a wide range of sizes. Microporous patches are already usedcommercially on retail trays of some fruit.

The use of pores in packaging materials to actually balance theatmosphere in packages of respiring fruit has been the subject of someresearch and a large amount of marketing. Several forms of crushed rock,coral and synthetic zeolites have been incorporated into extruded film butthere has been very little disinterested scientific evaluation done. Such filmsextend the range of gas permeability values of the commodity films incurrent use. Some results for P-Plus, a porous film currently manufacturedby Sidlaw Packaging, Bristol (UK), have been presented (Gill, 1990).

Predictive research and some experimental verification of the effects ofsingle pores in produce packages have been reported (Mannapperuma andSingh, 1994). The effects of changes in temperature on gas compositionneed to be evaluated.

Extension of the post-harvest life of fruits and vegetables requires morethan EMA packaging. The water relations between the horticulturalfoodstuff and its atmosphere need to be balanced both to preventdehydration and to avoid condensation induced by temperature abuse. Sincethe RH of such packages exceeds 95%, a temperature drop from 12°C to110C at the pack surface can cause condensation. The visually unpleasantappearance in retail packs is frequently overcome by antifogs in the plastic

and innovative forms of active packaging are described in Chapter 4.Microporous pads containing inorganic salts have been shown to buffer thewater vapour pressure (Shirazi and Cameron, 1992). Some of these are usedcommercially in the USA and Japan but others are close to commercialdevelopment.

There have been some patents directed towards use of combination effectsin active packaging for horticultural produce. Thus there have been patentsof combination CO2 emitter/water vapour absorbers and otherwise similarcompositions but including an oxygen scavenger as well. Yet another isNeupalon, which is described in Chapter 2. This would bring the advantagesof reducing the time the packaged horticultural product is subjected to highoxygen levels and inhibiting the onset of ripening, particularly withclimacteric fruit. The rapid oxygen scavenger films of Rooney (1982) andMaloba (1994) could be suitable for this purpose if they met with regulatoryrequirements.

Other approaches to enhancing the storage life of horticultural producehave been directed towards removing ethylene produced by ripening fruitsand vegetables. Since ethylene is both produced by ripening fruits andtriggers their ripening it is essential to prevent those fruits which are furtheralong the ripening process from triggering ripening of others in the sameenclosed space. Injured fruits are a particular problem in this regard and thisemphasises the need for strict quality control in EMA packaging. Theisolation of packages containing fruit rapidly generating ethylene may be theappropriate target of technologies for ethylene removal. Chapter 2 includesthe approaches taken both commercially and in research reports. Thechallenge appears to be to provide independently verifiable chemicalprocesses which function satisfactorily to remove ethylene at physiologicallysignificant concentrations in packages under conditions of high humidity andpossibly in the presence of condensation. Since the quantities of ethylene aretiny, the cost should not be the major obstacle to commercial development.Produce packages normally have large headspaces so both sachet andpackaging film scavengers should prove acceptable.

Several other 'freshness enhancing' properties have been claimed forsome commercial films but the processes occurring therein have not beenclarified. This matter is dealt with in more detail in Chapter 2.

Besides horticultural produce and living seafood which are meant to bekept alive during transportation, there is the very important field of chilledmeats which retain muscular respiration for some hours or days post-slaughter. While beef, for instance, is capable of oxygen scavenging bymuscle respiration for a few days at meatworks chiller temperatures of-1°Cto 1°C, it is no longer capable of doing so for the remainder of the desiredstorage period, usually 4-8 weeks. Lactic acid bacteria lower the pH andsuppress the growth of Bronchothrix thermosphacta and Pseudomonas spp.and other species. There is scope for oxygen scavenging films in bag

construction to prevent oxygen permeation and for lactic-acid-releasingfilms to enhance this effect in some cases.

The removal of residual oxygen from MAP meat packs by oxygenscavengers would increase security and decrease the need for slow,sophisticated packaging processes in this case. The carbon dioxide levels arenormally very high (> 99%), as in the Captech process (Gill, 1989), sooxygen scavengers would need to operate wet in this environment.

An additional definition of active packaging specific to horticulturalproduce distinguishes between passive and active modified atmospherepackaging (Zagory and Kader, 1988). The passive form which we areconsidering as EMA involves choice of the packaging material for its ratioof permeabilities to O2 and CO2 as well as for their absolute values. ActiveMAP has been defined as gas atmosphere replacement by flushing orevacuation-back flushing, although the option of adding other active agentshas also been considered (Kader, Zagory and Kerbel, 1989).

Modified atmosphere packaging of non-living foods is now a mature areaof research and has resulted in filling significant niche markets, particularlyin the bakery, cheese and fresh pasta areas. Fresh pasta, which has been arecent success internationally, is dependent on MAP (Castelvetri, 1990).

The growth of moulds, while suppressed by elevated carbon dioxideconcentrations, is not uniformly affected across the range of species. Lowlevels of oxygen can in some cases support some species of mould,particularly as carbon dioxide is lost by permeation of packaging films.There is a need to remove most residual oxygen which may reach more than1% when flushing is used without prior evacuation. Oxygen concentrationsbelow 0.1% are desirable especially when cut surfaces are exposed, as inpizza-type cheeses and some MAP meats.

Besides mould growth, chemical effects such as oxidative attack oncolours in preserved meats (Andersen and Rasmussen, 1992), nutrientdegradation such as vitamin C loss which can result in browning products(Waletzko and Labuza, 1976), and rancidity generation in fats and oils(Nakamura and Hoshino, 1983) can be prevented or minimised by use ofoxygen scavengers. The substantial development work aimed at overcomingthese problems is demonstrated by the results shown in the HistoricalDevelopment section of this chapter. One benefit to researchers of oxygen-absorbers is allowing ultimate effects of near-zero oxygen content atmos-pheres to be evaluated so that prediction of shelf-lives under other lessperfect conditions can be more firmly based. Although initial development,and current commercial practice, is based on sachets of scavengers insertedinto packs, much recent research and development has been directed towardsscavenging polymers which can address problems with oxidisable liquidssuch as beer, wines, fruit juices and other beverages. Polymers, because oftheir ease of melt formation, can take the scavenging capacity to localised

areas such as closures and to areas of close contact of product and packageas found with meats, cheeses and wet foods generally.

The ability of polymers to act where there is close contact opens the wayto provide a variety of food additives via a diffusive mechanism. Thisincludes antimicrobial action (Halek and Garg, 1988) or antioxidant (Han etaL, 1987) effects. To date, the use of such packaging has been restricted tocontrolled release of antioxidant into cereal products (Miltz et aL, 1989).The benefit of slow release of antimicrobial agents and antioxidants is thepotential for maintenance of the requisite high concentration at wet foodsurfaces. This applies especially to high-water-content foods in whichdiffusion from the surface into the bulk can deplete surface concentrations(Torres et aL, 1985). This effect has been noted by Smith et aL (1990) whoinvestigated the effectiveness of ethanol-emitting sachets on the growth ofSacchawmyces cerevisiae on apple turnovers. For active packaging to fulfila useful role in this field it will be necessary for it to provide controllable,slow release matched to the needs of the food. Water-triggered sachets ofsilica containing ethanol are very much a first generation approach to thisform of packaging. The role of edible food coatings in release of foodadditives is discussed in Chapter 5.

Besides antimicrobials and antioxidants there is a wide variety of other agentsthat can be added to foods or which can act on them. Thus flavours can beadded to offset degradation on storage, enzymes can remove oxygen or otherundesirable food components, and insecticides can repel insects or kill themwith permitted fumigants. There is a potential ethical dilemma which may arisefrom the application of such approaches to food packaging. There is also thepotential for foods to be self-promoting via the aroma of their packaging. Thus adesirable flavour might be generated by an outer layer of a package to attractcustomers rather than being released from an inner layer to offset scalping orprocessing losses. In an extreme case, supermarkets might become a confusinggarden of unbalanced aromas competing for the organoleptic senses of thecustomer in much the same way as package print attracts the customer visually.At this point the packaging ceases to be active in the sense of the presentdefinition and can be described as intelligent in the definition of the CESTreport (Summers, 1992). Introduction of many of these forms of active or relatedpackaging technologies will necessitate serious consideration of explanatorylabelling. In some cases this may require regulation, as with oxygen-scavengingsachets in Japan, the USA and Australia where the "Do not eat label" is required.In Australia at least, minimum sizes are specified to reduce risk ofingestion.

1.4.1 Do-it-yourself active packaging

Shelf-life extension of foods in the home, particularly following opening ofthe original package, can be seen as a natural extension of the systems used

in previous centuries. For instance the modern bag-in-box systems such asIntasept (Southcorp Flexible Packaging, Australia) is an extension of theSpanish wine-skin concept. Various evaporative coolers for bottled wine orbutter etc. were the predecessors of self-cooling cans frequently invented butless frequently manufactured (Katsura, 1989).

Following the successful introduction of cling-wraps as short-termmoisture barriers in the home, there has been a steady progression ofdevelopments in active packaging for in-home use. Typical applications arethe short-term packaging of unused food portions such as chicken or fishpieces or freshness retention in vegetables. Most of these forms of packagingare directed towards refrigerated items. An exception to this would be adesiccant pack for cookies or biscuits replacing the early canisters withregenerable silica gel desiccant in the twist cap insert in the lid. Suchdesiccant could be regenerated in the oven with gentle heating. It issurprising that such a packaging adjunct is not widely available since biscuitpacks made from oriented polypropylene readily tear and generally are notresealable.

Examples of do-it-yourself active packaging already available are listed inTable 1.2. The potential for the reduction of the surface aw of food pieces byPichit multilayer film is described in more detail in Chapter 4. The sale ofsuch film in domestic packs of 10 sheets, 19 cm x 27 cm, or on a perforatedroll (as with polyethylene bags for plant produce in supermarkets) isparticularly effective. Different approaches to modification of water vapourtransfer from foods such as produce are found in the various perforated bagsavailable in many countries. Perforations in the Ziploc recloseable bags ofDowbrands (USA), are a clear example. Less quantifiable are the effects ofmineral-loaded bags such as are supplied as Natural Radiation Bag(Mitsubishi, Japan). The latter contains zeolite particles with large pores andis available for meat packaging or produce packaging in retail sizes. Suchbags as are made by Mitsubishi and Asahi are claimed to have 'naturalradiation' or 'infra-red radiation' effects, which are discussed in as muchdetail as is possible in Chapters 2 and 3.

ANICO bags have a different type of inclusion in the polymer, consistingof iron and ascorbic acid (Anico, undated). Freshness may be enhanced byrelease of superoxide, although the mechanism has not been demonstrated indetail. Evidence is presented for reduced microbial growth on four foods

Table 1.2 'Do-it-yourself active packaging

Process Example

RH Modification Pichit - humectantsFreshness Asahi - zeoliteFreshness Anico - iron sulfate + ascorbateNatural radiation Ceramic particlesOxygen scavenger Mitsubishi - iron powder

when wrapped in paper impregnated with ANICO. Results presenteddemonstrate the removal of ammonia, hydrogen sulfide and methylmereaptan by the reactive ingredients in solution. The quantity of powderedinclusions is insufficient to cause substantial change in the water vapourtransmission rate.

An entirely different approach has been developed by Mitsubishi in theform of bags with a 'roll-over and clamp' style of removable pressure sealin kit form for use with cookies. The cookies are placed in the oxygen-barrier laminate bag and an oxygen-scavenger sachet is removed from aprotective metallised film sachet and inserted before the pressure seal isapplied. This is particularly effective in removing the oxygen from theheadspace within 10 h as shown in Figure 1.4. This type of domestic activepackaging is very suitable for foods unlikely to be affected by anaerobicpathogens. The regulatory dilemma arises in assessing the likelihood ofdomestic purchasers using such a scavenger system with raw fish orvegetables prone to pathogen infestation.

A partially regenerable system involving use of haemoglobin analogues tobuffer the oxygen concentration at low levels might be useful in someinstances. This might take advantage of the high equilibrium constant forreversible binding of oxygen to a cobalt complex. Some such complexesreversibly bind oxygen until its partial pressure becomes so low that the rateof dissociation equals that of association. The result is buffer action. By

Oxy

gen

(%)

Time (hours)

Figure 1.4 Oxygen scavenging from loosely packed biscuits packs (•) compared to tightly packedpackets (A) (simulating domestic use).

careful choice of the equilibrium constant for binding, an oxygen concentra-tion which is a compromise between protection against oxidation andsuppression of anaerobic organisms could be achieved. It should be notedthat oxygen levels needed for anaerobe suppression are higher than has oftenbeen claimed (Smith et al., 1990).

A metal complex would require a high binding constant for oxygen at 4°Cand a much lower one at 1000C. Accordingly, the oxygen can be desorbedby immersion of the device in boiling water before reuse. This processcould, in principle, be repeated until the accumulation of products of sidereactions resulting in cobalt (3) formation reduced oxygen uptake unac-ceptably. Aquanautics Inc. (USA) investigated the use of such complexes forextraction of oxygen from seawater for submarine use. Other militaryapplications were provision of oxygen for welding on warships and forprovision of life-support oxygen in high-altitude aircraft.

Retail sales of microwave susceptor films have been limited, presumablyas a result of the US FDA concerns about the effect of high temperatures onthe adhesives which bind the polyester layer to any backing. However,susceptor films consisting of paper/adhesive/metallised polyester have beenmarketed in Australia since the mid 1980s.

1.5 Physical and chemical principles applied

The fact that active packaging has developed as a series of responses to theneeds of niche markets should not hide the small number of underlyingprinciples being applied (successfully) thus far. Table 1.3 summarises theuse to which these principles have been applied either commercially or inpatent applications or other publications.

1.5.1.1 Porosity control. Researchers in modified atmosphere packagingof respiring horticultural produce have long sought to generate equilibriummodified atmospheres (EMAs) by use of the permselectivity towards carbondioxide over oxygen of plastics films. Although the ratio of permeability tocarbon dioxide to that of oxygen commonly varies from 3.3 to 8.3 (see Table4.4), this range is insufficient when considered in conjunction with theabsolute values of these properties. Hence research became directed more atmethods of controlling the size and distribution of pores in packagingmaterials. This research has ranged from the modelling of the size of singlepores in a package (Mannapperuma and Singh, 1994) to the incorporation ofcoral sand (Abe, 1990).

The temperature-controlled opening of pores in polymer films is asubstantially more difficult concept to apply in practice. Cameron andPatterson (1992) devised a system that allows sufficient oxygen to preventanaerobic respiration to enter a package if the temperature is raised too high

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careful choice of the equilibrium constant for binding, an oxygen concentra-tion which is a compromise between protection against oxidation andsuppression of anaerobic organisms could be achieved. It should be notedthat oxygen levels needed for anaerobe suppression are higher than has oftenbeen claimed (Smith et al., 1990).

A metal complex would require a high binding constant for oxygen at 4°Cand a much lower one at 1000C. Accordingly, the oxygen can be desorbedby immersion of the device in boiling water before reuse. This processcould, in principle, be repeated until the accumulation of products of sidereactions resulting in cobalt (3) formation reduced oxygen uptake unac-ceptably. Aquanautics Inc. (USA) investigated the use of such complexes forextraction of oxygen from seawater for submarine use. Other militaryapplications were provision of oxygen for welding on warships and forprovision of life-support oxygen in high-altitude aircraft.

Retail sales of microwave susceptor films have been limited, presumablyas a result of the US FDA concerns about the effect of high temperatures onthe adhesives which bind the polyester layer to any backing. However,susceptor films consisting of paper/adhesive/metallised polyester have beenmarketed in Australia since the mid 1980s.

1.5 Physical and chemical principles applied

The fact that active packaging has developed as a series of responses to theneeds of niche markets should not hide the small number of underlyingprinciples being applied (successfully) thus far. Table 1.3 summarises theuse to which these principles have been applied either commercially or inpatent applications or other publications.

1.5.1.1 Porosity control. Researchers in modified atmosphere packagingof respiring horticultural produce have long sought to generate equilibriummodified atmospheres (EMAs) by use of the permselectivity towards carbondioxide over oxygen of plastics films. Although the ratio of permeability tocarbon dioxide to that of oxygen commonly varies from 3.3 to 8.3 (see Table4.4), this range is insufficient when considered in conjunction with theabsolute values of these properties. Hence research became directed more atmethods of controlling the size and distribution of pores in packagingmaterials. This research has ranged from the modelling of the size of singlepores in a package (Mannapperuma and Singh, 1994) to the incorporation ofcoral sand (Abe, 1990).

The temperature-controlled opening of pores in polymer films is asubstantially more difficult concept to apply in practice. Cameron andPatterson (1992) devised a system that allows sufficient oxygen to preventanaerobic respiration to enter a package if the temperature is raised too high

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Principle

Porosity Control

Polymer Permeability

Melting of Waxes

Thermal Expansion

Energy Shielding

Energy Transfer

Inorganic-Organic Oxidation

Enzyme Catalysis

Acid-Base Reaction

Adsorption

Absorption

Hydrolysis

Desorption

Organic Reactions

Application

Gas pressure releaseGas composition balance

Gas composition balanceTemperature compensation

Time-Temperature indicatorsTemperature compensation

Doneness indicators

Microwave shieldingThermal insulationShock absorption

Microwave crispingUV absorption

Oxygen scavengingOxygen permeation barrierOxygen indicatorCarbon dioxide generationEthylene scavengingTaint removal

Oxygen scavengingTime-Temperature indicationLactose removalCholesterol removal

CO2 absorptionCO2 generationOdour absorption

Taint removalOxygen scavengingEthylene scavengingWater removal

Humidity bufferingCondensation controlDrip collection

Sulfur dioxide releaseBenomyl release

Ethanol releaseHinokitiol releaseWater release

Ethylene removalOxveen barrier

Table 1.3 Physical and chemical principles applied in active packaging

during distribution. Their approach was to block a pore in a package with awax which melted at the chosen upper temperature limit. When molten, thewax was drawn away by an absorbent wicking material.

The use of sophisticated pore control is not limited to respiring producepackaging. DRG Flexible Packaging pic introduced the Ventflex® tray inwhich long capillaries formed horizontally in the lid to regulate gas

exchange. Yet another approach is to weld a plastic pressure-release valveinto the wall of packs for freshly roasted coffee beans to release the carbondioxide held in the beans under pressure for some days. This valve has gonethrough at least two designs with the early protrusion from the packagesurface being replaced by a design with the valve flush with the outersurface.

1. 5.1.2 Polymer permeability. Equilibrium modified atmosphere genera-tion is still the major goal of polymer permeability modification. Modellingproduce requirements with film properties is becoming increasingly popular(Chapter 3). A review of this area is also found in Solomos (1994).However, with temperature abuse in produce distribution, the temperaturedependence of film permeability to oxygen and carbon dioxide is quitedifferent from that of the respiration rate. Hence a film which allowsgeneration of a satisfactory EMA at one temperature may cause anaerobicrespiration at another. Use of liquid crystal polymers is an approach tosolving this problem but those currently available do not offer large enoughchanges in permeability to be very significant at present.

Polymer blending has been used to develop sufficiently high permeabilityto water vapour and smoke flavours at 400C while retaining a substantialoxygen barrier at 23°C. The accrued knowledge of polymer permeabilityproperties is already very great and other applications in active packagingmay be expected.

1.5.1.3 Melting of waxes. Use of wax melting for porosity controlledtemperature compensation has been mentioned above. This property wasused for indicating temperature abuse, especially in chilled and frozen foods(see Chapter 10).

1.5.1.4 Thermal expansion. As the package is being used more exten-sively as the vessel in which foods are reheated or cooked, the opportunitiesfor indicating completion of the heating of the food are expanding. To datethe so-called 'doneness' indicators for turkeys are an initial example. Theseplastic inserts in the food expand when the temperature at a predetermineddepth reaches a chosen value. Other forms of time-temperature indicationmay replace these devices but a considerable amount of modelling ofthermal processing may need to be done first.

It is perhaps surprising that thermal expansion has not yet been used intemperature compensating devices for balancing gas compositions in EMApacks of horticultural produce.

1.5.1.5 Energy shielding. The insulation of foods by corrugated fibre-board cartons or polystyrene foam cartons scarcely satisfies the definition ofactive packaging. However, insulation has been brought into retail units in

such novel ways that they deserve attention. Labuza and Breene (1989)reviewed novel two-layer packages both with and without an intermediatelayer such as a chillable gel which imparts an increased thermal load. Theuse of labels which expand to give an insulating foam when plastics lunch-cups are retorted is a novel example.

A higher level of sophistication is found in the inclusion of selectiveshielding of portions of microwaveable meals by appropriate placement ofmetallised strips in the top dome of packs. This makes possible theengineering of food products with mixtures of components with verydifferent heating requirements (see Chapter 9).

The concept of shielding the food from energy applied externally can alsobe applied to shock absorption by layers of foam. One such laminate isdescribed by Yoshizaki (1976). This approach is not substantially differentfrom the application of shrinkable foamed sleeves to glass soft-drinkbottles.

1.5.1.6 Energy conversion. The ease with which energy is convertedfrom one form to another has led to the development of microwavesusceptors (Robertson, 1993). Although the regulation of susceptors iscomplicated by concerns about migration, their potential in the crisping ofpastries and other foods has been widely accepted (Sacharow and Schiffman,1992).

Energy transfer has long been used in protection of plastics for use inexposed conditions (Brydson, 1982). Absorption of this energy has normallybeen followed by energy conversion to heat, either by the process of internalconversion or by energising chemical reactions. Such processes have beenproposed increasingly for food packaging, in the patent literature. Thisfollows the demonstration of the effects of light, both in the near UV andvisible regions, on foods (Andersen and Rasmussen, 1992).

1.5.1.7 Adsorption. The introduction of adsorbents into food packages,both in sachets and dispersed in plastics, has been one of the mainapproaches to active packaging. The major advantage such materials haveover chemical reagents is their absence of migration into the food, or theirinertness. Adsorbents used or described in the literature include activatedcarbon, zeolites, silica gel, wood fibres and other forms of cellulose as wellas various clays and crushed rocks (Labuza and Breene, 1989). Adisadvantage of adsorbents is the often-reversible nature of the bonding andthe limited capacity. They have been used, or proposed for use, forintercepting taints permeating plastics packages (Kiru Kogyo KK, 1994),adsorbing water vapour (Wagner, 1990), releasing water as a reagent inoxygen scavenging (see Chapter 6), adsorbing ethylene to some extent(Louis and de Leiris, 1991), and binding odorous products of some oxygen-

scavenging reactions (Inoue and Komatsu, 1988; Toppan Printing Co.,1992).

1.5.1.8 Absorption. Absorption processes in active packaging havelargely involved control of the availability of water in packages. Thesimplest form of absorption has been removal of weep from meats or fish bymicroporous pads containing superabsorbent polymers. This absorptionprocess has been coupled with carbon dioxide release in the case of theVerifrais process (Sacharow, 1988) and in oxygen or ethylene scavenging inthe TM Corrugated case (Louis and de Leiris, 1991). More complicated, butmeeting a complex need, are the humidity-buffering and condensation-control systems for cartoned horticultural produce described by Patterson,Jobling and Moradi (1993). The humidity-buffering action can also be seenin the carbohydrate and glycol mixture used in Pichit sheet for food-surfacede watering in the home (Labuza and Breene, 1989).

1.5.1.9 Desorption. The use of paniculate solids as vehicles for deliveryof active ingredients has been a commonly used process commercially. Theuse of a variety of porous solids such as vermiculite to hold water foroxygen scavenging has already been mentioned and is described in Chapter6. The desorption of ethanol from dry silica gel is the basis of Ethicapsachets (Freund Co.) which consist of 55% silica gel and 35% ethanol.When these are exposed to water vapour in the headspace of a food pack,they desorb ethanol and a masking flavour. An extension of this is theconcurrent ethanol release and oxygen scavenging by Negamold (FreundCo.). These processes rely on the stronger binding of water to silica geldisplacing the more weakly held ethanol.

Carbohydrates such as dextrins have been the subject of substantialresearch as vehicles for carrying ingredients in the pharmaceutical andflavour industries because of the range of binding forces which can be found.Cyclodextrins in particular are attractive because of their cyclic structure andowing to their favourable regulatory status in some countries. Cyclodextrinsexist in three forms, a, (3 and 7, which consist of ring-shaped molecules with6, 7 and 8 anhydroglucose units joined, respectively. To date they have beenused to release hinokithiol antimicrobial agent from non-woven fabrics andplastics films under the Hosenshi trade name of Seiwa Chemical (Katsura,1989).

1.5.1.10 Hydrolysis. To date, very little use has been made of hydrolyticreactions. The desorption of ethanol mentioned above may involve somehydrolysis. Use has been made of the release of sulfur dioxide from sodiummetabisulfite in quilted pads of microporous material which are placed incartons on top of table grapes. Water vapour from the grapes is absorbed andreleases the gas which is required to act as a fungicide to prevent attack at

the stem junction of the fruit. Thus far, there has been no satisfactory methodof controlling the rate of hydrolysis, with the result that excessive rates ofrelease can cause partial bleaching of the grapes.

An unintended hydrolytic reaction appears to have been found by Halekand Garg (1988) who set out to bind Benomyl fungicide to a carboxylic-acid-containing film, Surlyn (Dupont). The fungal inhibition results obtainedwhen the film was placed in contact with inoculated agar indicateddetachment of the fungicide from the polymer and diffusion into theagar.

1.5.1.11 Acid/base reactions. The abundance of food constituents andpermitted additives which are mildly acidic or basic has facilitated theintroduction of acid/base reactions for altering packaged food environments.The reaction of ascorbic acid with sodium bicarbonate is commonplace inoxygen scavengers which replace oxygen consumption with carbon dioxidegeneration. Conversely, the removal of unwanted carbon dioxide from packsby reaction with lime in coffee packs has also been introduced commercially(Russo, 1986).

The use of food acids dispersed in polymer films for removal ofmalodorous amines from fish has been patented by Hoshino and Osanai(1986). The interest in deodorising food packaging in Japan should result inmore research and development in this area. Katsura (1989) noted thatNippon Unicar has introduced a flavonoid-based compound in low-densitypolyethylene for this purpose also.

Combinations of many of these principles are found in commercialpackages and in proposed systems. The Verifrais system and related systemswhich have followed involve a superabsorbent polymer to absorb water, thelatter dissolving an acid and bicarbonate to release carbon dioxide in acontrolled manner.

1.5.1.12 Inorganic/organic oxidation. Since oxygen scavenging was oneof the earliest forms of active packaging to be introduced the use ofoxidation reactions for several other purposes readily followed. Theoxidation of metals had been limited to headspace oxygen scavenging butrecent developments in bringing water, oxygen and iron together in plasticsfilms has made an oxygen permeation barrier possible by this method (anon.,1994). Oxidation of organic substrates such as polyamides, rubbers, fattyacids and ascorbic acid has been developed for oxygen permeation barriersin plastics films (Chapter 4). Dyes which can be reversibly oxidised havebeen developed as indicators of oxygen concentration by formulation withreducing agents (Nippon Soda KK, 1993).

Carbon dioxide generation is possible as ferrous carbonate is oxidised bymolecular oxygen. This has been patented for applications where packagecollapse on oxygen scavenging cannot be accepted.

Following the early work of Scott et al (1970) on oxidation of ethyleneby potassium permanganate, a variety of these rather non-specific oxidantsachets have become available. Their regulatory status should be examinedbefore they are used. Whereas such non-specific oxidising systems have thepossibility of generating taints, the reaction of ferrous iron with organicacids has been developed in plastics films for oxidative removal of taints(Anico, undated).

7.5.7.75 Organic reactions. Otherwise unclassified organic reactionshave also been proposed in several patent applications. Ethylene removal byreaction with tetrazines has been demonstrated to occur efficiently atphysiologically important concentrations (Holland, 1992). The Maillardreaction seems to be the basis of a patent in which reducing sugars andamino acids are brought together under moist conditions in a plastic film(Goyo Shiko KK, 1993). The products are claimed to reduce oxygenpermeation but may also migrate into packaged liquid food to act asantioxidants.

1.5. L14 Enzymic catalysis. One of the earliest attempts at producing asachet-based oxygen-scavenging system was based on the reaction ofglucose with oxygen, catalysed by glucose oxidase in the presence ofcatalase (Scott and Hammer, 1961). Several systems involving both sachetand in-film chemistry have been developed but have not yet been proven incommercial practice. Kuhn and Kuhn (1991) have claimed spreading eitherthe enzyme or glucose on the packaging material and adding the othercomponent immediately before sealing. Ernst and Vonraffay (1991) andErnst and Ernst (1992) described compositions of silica gel, glucose oxidase,glucose and water. The water can be supplied absorbed in microcrystallinecellulose. Carbon dioxide can be formed as oxygen is removed if an alkalineearth carbonate is included in the composition. The compositions can bepellets, in sachets or between two layers of an oxygen-barrier laminate.Glucose oxidase has been given GRAS status in the USA (Labuza andBreene, 1989) so there is considerable interest in making such systemsfunction efficiently. A range of other applications of enzymes in activepackaging are discussed in Chapter 7.

The enzymatic hydrolysis of a lipid has been introduced as a time-temperature indicator by I-Point Biotechnology in the USA. The resultingchange in pH was detected by means of a dye following mixing of thesubstrate and enzyme.

This listing of principles and applications is not intended to be exhaustivebut rather is aimed at indicating the basis and diversity of active packagingapproaches.

1.6 Implications for other packaging

The decision to address at least one attribute of a food by means of activepackaging can have a significant impact on the choice of the total package.The impact can vary from the choice of resin into which to blend zeolites orceramics for permeability modification to the temperature stability of outerpackaging of microwave-reheatable packs for pastries. Impacts may be suchas to allow the use of cheaper packaging, as with the use of oxygenscavengers in packaging relatively short shelf-life foods. Alternately it mightbe necessary to use more expensive materials in order to gain the benefits ofthe active agents.

Several forms of active packaging require direct contact between the foodand the active components of the package. When multilayer structures areused this implies that the heat-seal layer contains the active components, aswith Zeopac antimicrobial trays offered commercially by Mitsubishi inJapan. In this laminate the cast polypropylene layer containing Zeomiczeolite is closest to the food. Zeomic is a zeolite in which silver ion isbonded in the surface layer of the pores. It is manufactured by ShinanenNew Ceramic Company in association with Mitsubishi (Abe, 1990). Thezeolite particles have a larger diameter than the thickness of this food-contact layer and are thus exposed to any liquid at the food-packageinterface. Other silver zeolites are available, such as Bactekiller manu-factured by Kanebo Zeolite Co. (Louis and de Leiris, 1991).

Hirata (1992) has found that silver zeolite, used at 1% concentration in theheat-seal polyethylene layer of a laminate, can reduce the surface bacterialcount on the plastic from 105-106 cells/ml to 10 cells/ml in 24 hours. Thecells are spread on the surface in aqueous suspension. While there is stilldiscussion as to whether the microorganisms need to come in contact withthe silver surface, it seems far more probable that the silver ion is dissolvedin the liquid by leaching (Ishitani, 1994). This form of packaging has beenused for fresh oysters (Abe, 1990) and oolong tea (Ishitani, 1994).

Halek and Garg (1988) demonstrated the antifungal effect of Benomylbound to the ionomer film Surlyn (Dupont) by coupling with dicyclohexylcarbodiimide. The aim of this work was to determine whether a surface-bound antifungal agent would be effective, thus reducing the chance of theresidues remaining in the food. It was therefore essential that the heat-seallayer should be the Surlyn. In fact, it was demonstrated that some of theBenomyl probably broke free of the ionomer and diffused into the agar testmedium.

Where it is intended that the active agent is to be released into, or onto,the food there may be less restriction on the location of the film layer. Thusthere are opportunities to use slow-release binding agents such as cyclodex-trins (Katsura, 1989) or microencapsulation agents either within the heat-seal or other layers. The only requirement therefore is that the layer(s)

between the active one and the food is sufficiently permeable. This is thebasis of the ethanol-releasing sachets Ethicap and Negamold (both fromFreund Ltd., Tokyo) and of the CO2-releasing sachets manufactured byMitsubishi Gas and Chemical Co., Toppan Printing Co. (both of Tokyo) andMultiform Desiccants Inc. of Buffalo, NY. The permeability of the sachetmaterial to water vapour or ethanol is of critical importance in theapplication of these sachets. Most frequently, microporosity of a hydro-phobic film material has been made use of to achieve this.

Similar considerations to the above apply when active packaging ischosen to remove a food/headspace component such as oxygen, carbondioxide or odours. The need to separate the scavengers from the food by alayer impermeable to the scavenger has thus far been relatively simple, asinorganic substances such as iron powder have been used both commercially(Koyama and Oda, 1992) and in proposed systems (Teijin, 1981; Toyobo,1981). The choice of film materials permeable to oxygen, carbon dioxide ororganic off-flavours has been relatively simple as polyolefins serve thispurpose well. Where concurrent permeation of the barrier layer to water isalso required, problems are encountered with the polyolefins which arehighly permeable to non-polar gases and vapours but are not sufficientlypermeable to water vapour. This problem is overcome in sachet manufactureby use of microporous films which are hydrophobic and resist liquid waterpenetration (Mitsubishi, 1983). Substantial numbers of patents describemethods of achieving the requisite permeabilities to water and oxygen usingthis approach. The careful choice of food contact layer plastics for retortablepackages by Toyo Seikan (Koyama and Oda, 1992) and American CanCompany has resulted in the development of heat-triggered oxygenscavengers which function when the water vapour transmission rate of thelaminate reaches the necessary value. The premature oxidation of thereduced iron scavenger in the Toyo Seikan Oxyguard process at extrusiontemperatures of around 2200C is minimised by keeping the plastics dry.When the container fabricated from these plastics is retorted at 12O0C for 30min, water vapour permeating the container wall is absorbed and takes partin the rusting of the iron. Retention of water by the composition allows theOxyguard to continue to act as a chemical oxygen barrier when the packagereturns to room temperature. This is the opposite situation to that withconventional passive retort package material based on EVOH whereretention of water in the EVOH lowers the oxygen barrier and increasesoxygen availability to the packaged food for many weeks after retorting(Tsai and Wachtel, 1990).

The Oxyguard and other oxygen-scavenging plastics compositions canreduce the cost of the barrier layer by reducing the need for inclusion ofmica platelets in the EVOH or for desiccants in the polypropylene layer ofretortable trays. Another form of active packaging which can also reducecomplexity of oxygen-barrier packaging is ethanol release. Hirata (1992) has

shown that common commodity polymers such as oriented polypropyleneform a sufficient barrier to ethanol permeation to retain this sterilant in thepackage for extended periods of time. Thus in those applications wheresuppression of mould by ethanol is possible, simple packaging materials canbe considered since oxygen control may not be necessary. Currently ethanoluse in this way is not widely permitted, with the result that presentopportunities are limited. Smith et ah (1987) have suggested that the use ofethanol could reach approval status in North America for 'brown and serve'products such as pastries.

1.6.1 Whole packages designed to be active

Some forms of active packaging do not require changes in design of thepackage in order to achieve their effects. Examples of these include theinsertion of the various sachets which modify the gas atmosphere, asdescribed in Chapter 6. Other examples are the various forms of porous ormineral-filled plastics which can be used with respiring horticulturalproduce. In some instances the consumer may not be aware that the packageis different from its passive counterpart as with the oxygen-scavengingclosures for bottled beverages, described in Chapter 8.

Redesign of packages is necessary for some effects to be achieved,particularly when some of the physical principles listed in Table 1.3 areinvolved. This is particularly important where shielding of components inmicrowaveable meal packs is desired, because the location of the shields iscritical (Sacharow and Schiffman, 1992; Robertson, 1993). The use ofsusceptors to achieve surface crisping of foods means that the distancebetween the food surface and the susceptor must be carefully controlled.Amcor Fibre Packaging in Australia has developed a susceptor-coatedcorrugated inner layer of packs for pastries. The exposed corrugatingmedium provides a heating surface claimed to be ideal for such crisping. Acorrugated fibreboard base for a susceptor film is also used to drain fatcoining from foods like fish on heating.

Wagner (1989) reported that in 1988 Japan exported over 30 million self-heating cans for sake alone. Aluminium cans for sake are heated when limeand water in the base are mixed. Steel cans for coffee are also heated usingthe same chemistry. This process also has been used to heat lunch boxes(Katsura, 1989).

Self-cooling cans have also been marketed in Japan for raw sake. Theendothermic dissolution of ammonium nitrate and chloride in water is usedto cool the product. It is necessary to shake the can to ensure the mixing ofthe salts with water.

Packages for chilled foods have also been redesigned to allow for thepresence of an active component. Control of carbon dioxide concentration inpacks for meat or fish under modified atmospheres is the aim of a French

package designed to be active (Louis and de Leiris, 1991). The Verifraispackage manufactured by Codimer consists of a heat sealable lid with a traywith a false bottom which is perforated to allow juice from the product todrain into the base of the tray. A porous sachet containing sodiumbicarbonate and ascorbic acid is in the base. The delay in access of the liquidto the sachet is controlled by an absorbent paper pad located between thesachet and the porous tray. The commencement of dissolution of the acidand bicarbonate can be delayed for up to 24 hours, by which time the carbondioxide concentration can be decreasing due to dissolution in the meat andpermeation of the tray and lid. Release of carbon dioxide from the sachetserves to offset this loss. Meats can have a shelf-life of up to 21 days at2-4°C in this pack compared with the 10-15 days under normal MAP(Louis, 1990). The shelf-life of fish can be extended similarly to 10 daysversus 4-5 days under MAP. A more recent variant on this design has thecarbon-dioxide-generating components concealed in a foam tray withchannels for juice drainage (Leon et ai, 1987). The preferred chemicals aresodium chloride (50%), citric acid (25%), and sodium carbonate (25%). Thechemicals can be in aggregate form rather than held in sachets as withVerifrais. These chemicals help to reduce package collapse owing tooutward permeation of carbon dioxide by generation on a continuous basis.The aim is to prevent growth of anaerobic pathogens on meats and seafoodpacked in low-oxygen-content modified atmospheres. An alternative to theseactive packaging processes is the Valle Spluga process which involvesinsertion of a pellet of solid carbon dioxide into the pack before sealing toprovide sufficient additional gas to retain package shape (Louis, 1990). Thecarbon dioxide is pelletised in-line to a predetermined dosage. Thepackaging material needs to have a strong heat-seal if leakage is to beavoided within the first day of packing.

Although the mineral-filled and other porous horticultural-produce cartonliners differ very little from conventional liners, not all horticultural activepackaging is so simple. The Ventflex laminate pack introduced by DRGPackaging in the UK relies on channels between the layers lidding the filmto allow some regulation of evolution of carbon dioxide by the packagedproduce. Oxygen can enter through the same channels or more slowlythrough the polyester barrier film forming the lid.

The condensation control carton design of Patterson et al (1993) reducessubstantially the chance of produce damage by water condensation causedby temperature abuse. The carton is lined with three additional layers whichprovide in-pack buffering of the water vapour in the carton headspace. Louisand de Leiris (1991) describe the TM corrugated case of the Mantsune Co.which, while superficially similar to the above, has no microporous foodcontact polymer but does contain a ceramic claimed to absorb ethylene. Thiscarton lining is designed to act as a humidity buffer.

Redesign of packages has also been used to address the problems of

packaging of freshly roasted coffee. Taylors of Harrogate, Tea and CoffeeLtd. (UK) pack the beans in foil laminate pouches which resist the pressureof carbon dioxide evolved by the beans. This gas evolution usually occursfor at least the first day after roasting. The Taylors' bag contains a one-wayplastic release valve welded into the face of the bag and this prevents sealrupture. The original valve design had a protruding component and so waswelded into the side gusset. An alternative approach was adopted by GeneralFoods in the USA. A Mitsubishi Ageless sachet which absorbs carbondioxide and oxygen was included with the coffee in their packs (Russo,1986). The General Foods pack did not require redesign to accommodate thesachet. Both active packaging approaches were designed to retain the easilyoxidised fresh coffee flavours normally lost when coffee is allowed to standfor 24 hours to desorb carbon dioxide.

The forms of active packaging which involve minimal changes to the pre-existing package design have been most readily adopted commercially sofar. The widespread use of sachets for headspace atmosphere modificationdemonstrates this point. The number of oxygen-scavenging sachets manu-factured in Japan in 1989 was estimated at 6.7 billion (Abe, 1990). Theexpansion of this market appears to have been limited by the preference offood manufacturers not to include sachets in the packages with the foods.Recent introduction of sachets bonded to the package walls should facilitategreater market penetration (see Chapter 6).

1.7 Limitations of current approaches

The ideal active package would sense the requirements of the food andadjust its properties in order to meet them. At the same time this would becost-effective and have minimal environmental impacts. Current commercialactive packages do not meet these criteria although there are some which aresecond-generation concepts. The latter include meat packs with insertswhich release carbon dioxide in response to absorption of weep or juice(Sacharow, 1988).

Horticultural active packaging is not yet based on films with a sufficientlywide range of CO2-O2 permeability ratios. This ratio would need to betemperature responsive in the same manner as the enclosed fruit orvegetable. The Intellimer side-chain-crystallisable polymers from LandecCorporation (Menlo Park, CA, USA) offer some answers to the temperatureproblem (Stewart et al, 1994). There is a lack of convincing evidence thatseveral of the films designed to scavenge ethylene do in fact worksatisfactorily under actual conditions of use. Industry confidence would bewell served by self-indicating films which change colour on reacting withethylene at physiologically important concentrations. The control of humid-ity in liner bags of fruit and vegetables is still in its early stages of

development. Inserts or films capable of regulating humidity levels arerequired, especially where temperature abuse is likely. The use of sealedplastic bags for equilibrium modified atmosphere generation makes thisrequirement more critical.

Current commercial oxygen scavenging sachets are supplied in a widevariety of forms to suit particular temperature and relative humidityconditions. These designs place limitations on the shelf-life of the sachetsonce the master-packs are opened and before insertion into the foodpackage. An ideal sachet would be useable under a wide variety ofconditions and activated for a particular application by the food packer.Some early steps towards overcoming the shelf-life limitation have beenreported in the patent literature. The use of a peelable barrier on a sachetwith a self-adhesive backing has been proposed.

Attempts to include oxygen scavengers in plastics packaging materialshave not yet been successful commercially except in closure liners forbeverage bottles. Compositions which are able to withstand extrusion andyet be activatable are required.

1.8 Future potential

The future of any innovation in packaging depends upon the extent to whichit can satisfy the requirements of the product packaged. Commercialdevelopment therefore will be driven by needs as perceived in the foodindustry or in other industries with related problems. It is clear from patentsearches that inventors of active packaging frequently see potential applica-tions for their concepts in several industries. This is evident in claims for useof oxygen scavengers in the packaging of clothes, pharmaceuticals, finechemicals such as amines, printing inks, electronic components, metals andmany more areas. Some iron-based oxygen scavengers have been suggestedfor use in hand-warmers for skiers.

If the potential of active packaging technologies is to be realised there willneed to be a recognition that changes in packaging can open up new methodsof presenting foods. The use of oxygen-scavenging plastics as chemicalbarriers to permeation should allow retortable plastics to provide productshelf-lives closer to those found using metal cans. Horticultural produce,such as flowers, should be transportable internationally with reducedlosses.

Acceptance of active packaging solutions to food industry problems willcontinue to depend upon evidence of effectiveness demonstrated byindependent investigators. The lack of hard evidence supporting manyclaimed benefits of some early horticultural produce packages has inhibitedcommercial usage. If the majority of patent claims already made proveuseful and economically viable, active packaging has a bright future.

1.9 Regulatory considerations

At least three types of regulation have an impact on the use of activepackaging in foods. First, any need for food-contact approval should beestablished before any form of active packaging is used. Second, environ-mental regulations of packaging material usage can be expected to increasein the coming years. Third, there may be a need for labelling in cases whereactive packaging can give rise to consumer confusion.

Food-contact approval will often be required because active packagingmay affect foods in two ways. A substance may migrate into the food or maybe removed from it. Migrants may be intended or unintended. The intendedmigrants include food additives which would require regulatory approval interms of their identity and concentration. Unintended additives includeactive substances which achieve their purpose inside the packaging materialand do not need to enter the food. Food additive regulations requireidentification and quantitation of any such migration. The likely introductionof the 'Threshold of Regulation' concept by the US FDA early in 1995, andthe chance of the European Union adopting similar concepts later, mayfacilitate the approval procedures for active packaging. Removal of oxygenfrom packages may lead to the growth of anaerobic pathogens in some cases.The manner in which oxygen scavengers are used may be subject toregulation. Developments in active packaging discussed in this book are notnecessarily commercial and care should be taken in using any of them.

Environmental regulations for packaging materials often refer to recycl-ability or identification to assist in recycling. The effect of active packagingmaterials on recycling may need to be determined on a case-by-case basis.Active packaging is often used currently to allow foods to be packaged withsimpler materials than would otherwise be possible. The environmentalimpact of the food-package combination should be considered.

Labelling is currently required to reduce the risk of ingestion of sachets ofoxygen scavengers, ethanol emitters, etc. Some form of external labellingmay be required when various forms of indicator come into use. Suchindicators would show gas composition, thermal history, or 'done-ness' inthe case of microwaved foods. Some active packages may be expected tolook different from their passive counterparts. It may be advisable to uselabelling to explain this even in the absence of regulation.

References

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2 Ethylene-removing packaging

D. ZAGORY

Ethylene is a chemically simple, ubiquitous chemical that has diverse andprofound effects on the physiology of plants. Ethylene has so many differenteffects on plants, is effective in such low concentrations, and its effects areso dose-dependent, that it has been identified as a plant hormone. Thoughmany of the effects of ethylene on plants are economically positive, such asinduction of flowering in pineapples, de-greening of citrus and ripening oftomatoes, often ethylene has been seen to be detrimental to the quality andlongevity of many horticultural products. For this reason, there has longbeen interest in removing ethylene from the horticultural environment and insuppressing its effects.

Some of the diverse ways in which to absorb, adsorb, counteract orchemically alter ethylene have led to products designed to reduce itsdeleterious effects. This chapter will briefly review the chemistry, physiol-ogy and agricultural effects of ethylene preparatory to describing theresearch and commercial effort undertaken to incorporate ethylene controlagents in packages for horticultural products. Some of this effort has metwith commercial success, but much has not. However, with the rapid growthof packaging of fresh fruits and vegetables, particularly fresh cut salads andfruits, opportunities for such products are bound to increase. Therefore, it istimely to review the basis and activities relating to these products to betterelucidate the possible forms that they can and will take and to point outsome of the advantages and disadvantages of the various approaches likelyto emerge.

2.1 The chemistry of ethylene

The ethylene molecule is of the alkene type, being simply two carbonslinked by a double bond with two hydrogen atoms on each carbon. Such asimple molecule can be synthesized through several different pathways andis subject to many kinds of chemical reaction.

2.1.1 Synthesis

Ethylene can be synthesized both biologically and non-biologically. It is acommon component of smoke and can be found as a product of aerobic

combustion of almost any hydrocarbon. It is thus a common air pollutant, itschief source being automobile engines.

Biological sources of ethylene include higher plant tissues, several speciesof bacteria and fungi, some algae, and some liverworts and mosses. Thebiosynthetic pathways for ethylene are diverse among these differentorganisms. The pathway of synthesis from methionine has been described indetail for higher plants (Yang and Hoffman, 1984). The pathways forsynthesis in bacteria appear to be diverse since any of several carbon sourcesother than methionine will serve as precursors (Sato et al, 1987). Nitrogen-fixing bacteria can reduce acetylene to ethylene (Dillworth, 1966). Approx-imately 25-30% of fungal species tested produce ethylene on appropriatemedia (Fukuda et al, 1984; Hag and Curtis, 1968). The pathways of plantand fungal ethylene synthesis appear to be distinct, as the inhibitorrhizobitoxin blocks synthesis in plants but not in the fungus Penicilliumdigitatum (Owens et al, 1971). The pathway of ethylene synthesis in non-vascular plants may be different from that in vascular plants (Osborne,1989a).

Because this chapter is primarily concerned with methods of eliminatingethylene, not producing it, it is not necessary to go into the details ofproduction by different organisms. This has been reviewed in detailelsewhere (Abeles et al, 1992). The important point is that environmentalethylene can be biologically produced by a wide range of organisms, bothvisible and invisible, and such sources ought to be considered when devisingstrategies to reduce ambient ethylene.

2.1.2 Degradation

Ethylene undergoes several types of degradation reactions. Because of itsdouble bond, ethylene absorbs ultraviolet (UV) radiation at 161, 166 and 175nm (Roberts and Caserio, 1967). Ultraviolet photodecomposition of atmos-pheric ethylene is an important environmental ethylene sink (Scott andWills, 1973) and yields primarily hydrogen, acetylene, n-butane and ethane(Noyes et al, 1964). Soil microorganisms can degrade ethylene and at leastone species, Mycobacterium paraffinicum, is thought to be an efficientoxidizer of ethylene (Abeles et al, 1992).

Ethylene reacts with ozone to yield water, carbon dioxide (CO2), carbonmonoxide (CO), and formaldehyde (Scott et al, 1957). Ultraviolet light willinteract with oxygen (O2) in air to form ozone which breaks down ethylene,but UV light will directly degrade ethylene as well. Thus, UV light willeffectively eliminate ethylene even in low O2 atmospheres (Shorter andScott, 1986). However, the reaction is inefficient at very low ethyleneconcentrations such as those found in fresh produce environments so thecommercial potential of ozone as an ethylene scrubber is limited. Atomicoxygen will also react with ethylene and can form an array of compounds

including ethylene oxide, ethane, CO, propylene, acetaldehyde, propanol,butanol, hydrogen and dioxyketone (Leighton, 1961).

The double bond of ethylene makes it very reactive through a number ofreaction pathways. The double bond will undergo hydrogenation, in thepresence of any of several metal catalysts, to yield ethane (Morrison andBoyd, 1966). Ethylene will react with halogens (chiefly chlorine andbromine) through halogenation and hydrohalogenation reactions to formdihaloalkanes. Thus, ethylene can be eliminated from air by passing it overbrominated activated charcoal to form dibromoethane (Talib, 1983). Bromi-nated charcoal filters are relatively efficient removers of ethylene. Up to90% of the bromine will react with ethylene. However, bromine also reactswith water to form HBr and Br2 gas is released from the carbon filter. Thesecompounds are injurious to plant tissues and corrosive to stainless steel. Inaddition, brominated activated charcoal is hygroscopic and will become wetin humid conditions. Alternatively, ethylene will react with hydrogen halidesto form ethyl halides (Morrison and Boyd, 1966). Ethylene reacts withconcentrated sulfuric acid to form ethyl hydrogen sulfate or with water in thepresence of acids to yield ethanol (Morrison and Boyd, 1966).

Certain oxidizing agents react with ethylene to form glycols. The mostcommon of these oxidizing agents is potassium permanganate (KMnO4)which oxidizes ethylene to ethylene glycol and thence to CO2 and water(Morrison and Boyd, 1966). Potassium permanganate is often adsorbed ontoCelite, vermiculite, silica gel or alumina pellets. Permanganate scrubbers arealso effective in adsorbing air pollutants such as O3, H2S, SO2, NO andNH3.

It is clear that ethylene is a very reactive compound that can be altered ordegraded in many ways. This creates a diversity of opportunities forcommercial methodologies for the removal of ethylene and, in fact, manydifferent methods have been used. However, many of the common reactionsundergone by ethylene require high concentrations of ethylene and/or hightemperatures and pressures. Therefore, many of the processes most com-monly used to modify ethylene in the petrochemical industry are notappropriate for the conditions generally found in a food package environ-ment.

2.13 Adsorption and absorption

In addition to chemical cleavage and modification, ethylene can be absorbedor adsorbed by a number of substances including activated charcoal,molecular sieves of crystalline aluminosilicates, Kieselguhr, bentonite,Fuller's earth, brick dust, silica gel (Kays and Beaudry, 1987) andaluminium oxide (Goodburn and Halligan, 1987). A number of claymaterials have been reported to have ethylene adsorbing capacity. Examplesinclude cristobalite (> 87% SO2, > 5% AlO2, > 1% Fe2O3) (Kader et ai,

1989), Ohya-ishi (Oya stone) and zeolite (Urushizaki, 1986a). Oya stone ismined from the Oya cave in Tochigi Prefecture in Japan. The cave has beenused to store fresh produce and is reputed to confer added storage life. Thesalutary properties of the cave are thought to reside in the largely zeoliticstone interior. To improve its ethylene adsorptive capacity, the Oya stone isfirst finely ground with a small amount of metal oxide. The mixture is thenkneaded and heated to 200-9000C, then oxidized with ozone or electro-magnetic radiation (Urushizaki, 1986b).

Some regenerable adsorbents have been shown to have ethylene adsorbingcapacity and have the benefit of being reusable after purging. Examples ofsuch adsorbents include propylene glycol, hexylene glycol (Rizzolo et al,1987a), squalene, Apiezon M, phenylmethylsilicone, polyethylene andpolystyrene (Rizzolo et a/., 1987b).

Some adsorbents have been combined with catalysts or chemical agentsthat modify or destroy the ethylene after adsorption. For example, activatedcharcoal has been used to adsorb ethylene. In some cases, the activatedcharcoal has been impregnated with bromine or with 15% KBrO3 and 0.5MH2SO4 to eliminate the activity of the ethylene (Osajima et a/., 1983). Anumber of catalytic oxidizers have been combined with adsorbents toremove ethylene from air. Examples include potassium dichromate, KMnO4,iodine pentoxide, and silver nitrate, each respectively on silica gel (Eastwellet al, 1978).

Electron-deficient dienes or trienes, such as benzenes, pyridines, diazines,triazines and tetrazines, having electron-withdrawing substituents such asfluorinated alkyl groups, sulphones and esters (especially dicarboxyoctyl,dicarboxydecyl and dicarboxymethyl ester groups), will react rapidly andirreversibly with ethylene at room temperature and remove ethylene fromthe atmosphere. Such compounds can be embedded in permeable plasticbags or printing inks to remove ethylene from packages of plant produce(Holland, 1992).

Metal catalysts immobilized on absorbents, such as platinized asbestos,cupric oxide-ferric oxide pellets and powdered cupric oxide, will effectivelyoxidize ethylene, but in many cases the reactions require high temperatures( > 1800C). Clearly such systems would be inappropriate for food packagingapplications.

2.2 Deleterious effects of ethylene

Ethylene has long been recognized as a problem in postharvest handling ofhorticultural products. Since the discovery in 1924 that ethylene canaccelerate ripening in fruits (Denny, 1924) it has become clear that ethylenecan be the cause of undesirable ripening of fruits and vegetables. It is nowrecognized that ethylene, in very low amounts, can be responsible for a wide

array of undesirable effects in plants and plant parts. The physiologicaleffects of ethylene are so important, so diverse, and are induced by suchsmall amounts of ethylene that it is considered a plant hormone. The diversephysiological effects of ethylene have been extensively reviewed elsewhere(Abeles et ai, 1992) so only those effects that are deleterious to packagedplant produce will be discussed here.

2.2.1 Respiration

Perishability of produce generally is well correlated with respiration rate.Commodities such as asparagus, broccoli and mushrooms that have veryhigh respiration rates are very perishable, having postharvest lives measuredin days. Those commodities such as nuts, dates, dried fruits, potatoes andonions that have very low respiration rates have postharvest lives measuredin months (Kader, 1985). Reduction of respiration rate increases postharvestlife and elevation of respiration rate generally decreases it. This is one of thereasons why low temperature is so important in postharvest management.Reducing the temperature rapidly reduces the respiration rate of theproduct.

Ethylene accelerates the respiration of fruits, vegetables and ornamentalplants. In the case of climacteric fruit, ethylene can induce a rapid andirreversible elevation in respiration leading directly to maturity andsenescence. In non-climacteric plant organs, ethylene induces a reversibleincrease in respiration. In most cases, exposure to a few parts per million(ppm) of ethylene leads to increased respiration and increased perish-ability.

2.2.2 Fruit ripening and softening

Ethylene has been referred to as a 'ripening' hormone because it canaccelerate softening and ripening of many kinds of fruit. Exposure of maturefruit to ethylene leads to increased respiration, increased production ofendogenous ethylene, and softening of fruit tissues (Abeles et aiy 1992).This is achieved through the direct or indirect stimulation of synthesis andactivity of many ripening enzymes by ethylene.

Some fruits, such as bananas and tomatoes, are often deliberately exposedto high concentrations of ethylene (~ 100 ppm) to induce rapid ripening. Inmost cases, for packaged fruits it would be desirable to prevent exposure toethylene and thereby prevent rapid ripening.

2.2.3 Flower and leaf abscission

Cell wall hydrolysis of specific cells at the base of leaves, petioles, petals,pedicels and fruit leads to abscission of the distal organ (Abeles et al., 1992).

Ethylene has been shown to accelerate abscission for many, though not all,plants and plant parts (Jankiewicz, 1985; Osborne 1989b; Reid, 1985a).Ethylene causes flower and leaf abscission of many potted ornamental plants(Cameron and Reid, 1983).

2.2.4 Chlorophyll breakdown

Ethylene increases the rate of chlorophyll degradation in leaf, fruit andflower tissues (Aharoni, 1989; Knee, 1990; Kusunose and Sawamura, 1980;Makhlouf et al, 1989). This can be of particular concern in the case of leafygreen vegetables such as spinach, immature fruits such as cucumbers andsquash, and flowers such as broccoli (Reid, 1985b). The presence of lowlevels of ethylene can cause yellowing and reduced quality.

2.2.5 Petal inrolling in carnations

Low concentrations of ethylene (< 1 ppm) cause inrolling (or sleepiness)of the flower petals of sensitive carnation varieties accompanied by a loss ofturgor in the petal tissues (Halevy, 1986). Some carnations are so sensitiveto ethylene that they have been used as ethylene bioassays. Such sensitivevarieties are often subjected to a pulse treatment with silver thiosulphate torender them insensitive to the effects of ethylene (Cameron and Reid,1983).

2.2.6 Postharvest disorders

Ethylene can be responsible for a number of specific postharvest disorders offruits and vegetables. Examples include russet spot (small oval brown spots,primarily on the midrib) of crisphead lettuce, formation of bitter-tastingisocoumarins in carrots, sprouting of potatoes, and toughening of asparagus(Reid, 1985b).

2.2.7 Susceptibility to plant pathogens

Many postharvest plant pathogens are opportunistic microorganisms thatthrive on injured or senescent tissues. To the degree that ethylene acceleratessenescence and causes specific physiological disorders, it also enhances theopportunities for pathogenesis. The growth of a number of postharvestpathogens is directly stimulated by ethylene (Barkai-Golan, 1990; Barkai-Golan and Lavy-Meir, 1989; Kepczynska, 1993). In addition, severalpostharvest plant pathogens produce ethylene (Barkai-Golan, 1990) and thisethylene may compromise the natural defences of the plant tissues.

2.3 Interactions of ethylene and other gases

The activity and reactivity of ethylene depends, in part, on the presence ofother atmospheric gases. The user of packaging materials for the removal orinactivation of ethylene should consider the presence and concentrations ofoxygen, carbon dioxide, ozone and ethylene and their interactions with eachother and with plant tissues.

2.3.1 Oxygen

Ethylene production, biosynthesis and explosiveness are all related toambient oxygen concentration. Most pathways of ethylene synthesis,whether biological or chemical, are oxidative conversions or cleavages.

Although rice and some other aquatic plants have been reported tosynthesize ethylene in the absence of O2 (Ku et al, 1970), most plantsrequire O2 for ethylene synthesis. However, the oxygen affinity of ethylene-forming enzyme (EFE) is much less than that for respiratory enzymes. TheK1n for conversion of 1-aminocyclopropane-l-carboxylic acid (ACC) toethylene in apple is about 1.4% O2 (Banks et al., 1984; Bufler and Streif,1986) but K1n values for other plants organs are generally 3-10% (Burg andThimann, 1959; Lieberman et al, 1966). In some cases, reduced O2 in apackage may more effectively reduce ambient ethylene through reducedethylene synthesis than ethylene-adsorbing capacity built into the package.However, reduced O2 apparently slows the conversion of ACC to ethylene,resulting in accumulation of ACC (Burg and Thimann, 1959; Hansen, 1942;Imaseki et al, 1975; Jackson et al, 1978). Upon exposure to higher O2

concentrations, the accumulated ACC will be rapidly converted intoethylene so low O2 must be maintained continuously to maintain lowethylene concentrations.

The combustion of organic materials requires O2 and results in ethylene asone of the combustion products. Ethylene at concentrations between 3.1 and32% by volume, is explosive in air (Reid, 1985b). Neither of theseconditions occurs in packages.

232 Carbon dioxide

Carbon dioxide may stimulate, inhibit or have no effect on ethylenesynthesis, depending on the plant tissue (Abeles et al, 1992) and theconcentration of CO2. More importantly, CO2 renders normally sensitiveplant tissues insensitive to the effects of ethylene, thereby preventingabscission (Wittenbach and Bukovac, 1973), floral senescence (Nichols,1968), chlorophyll loss (Aharoni and Lieberman, 1979) and growth(Chadwick and Burg, 1967).

2.3.3 Ozone

As was mentioned above, ozone oxidizes ethylene to simple breakdownproducts and has been used experimentally to remove ethylene from producestorage areas. However, ozone would not normally be found or introducedinto a food package.

2.4 Ethylene sources in the environment

Ethylene is ubiquitous at low levels in the environment. It is a commonpollutant that can be detected with sensitive instruments. As most methodsof adsorbing or decomposing ethylene have finite capacities for activity, itseems prudent to reduce environmental ethylene to avoid saturating theenvironment of the package with ethylene. Ethylene can come from manysources both within and outside the package.

Although there are no national standards for environmental ethylene,California, USA standards recommend human exposures to no more than0.5 ppm for 1 h or 0.1 ppm for 8 h (Anon., 1962). Such levels are belowdamage thresholds for all but the most sensitive horticultural commod-ities.

2.4.1 Combustion

Ethylene is a common breakdown product of virtually all aerobic combus-tion processes. Burning agricultural wastes, wildfires, diesel- or propane-powered forklifts, cigarette smoke, truck and auto exhaust, and industrialstack emissions are all common sources of ethylene. In addition, the heatgenerated by combustion (from forklifts, for example) can raise thetemperature of the product sufficiently to stimulate production of product-generated ethylene.

Ambient atmospheric levels of ethylene are normally in the range of0.001-0.005 ppm (Abeles et al., 1992), however, urban air levels as high as0.5 ppm have been measured (Scott et al, 1957). Such high levels aresufficient to have physiological effects on some fresh produce. Removingagricultural sources of ethylene and insulating storage rooms from ethyleneair pollution can significantly reduce ambient ethylene.

2.4.2 Plant sources

Growing plants do not normally produce enough ethylene to alter ambientatmospheric levels of the chemical. In closed areas, such as storage rooms,packing houses, shipping containers, greenhouses and warehouses, plant-generated ethylene can be significant (Abeles et al., 1992). Sensitiveproducts should not be held or stored in proximity to ethylene-generatingproducts or product-ripening rooms.

2.4.3 Ripening rooms

Bananas and tomatoes are routinely ripened by exposure to 50-100 ppmethylene in large sealed rooms. When such rooms are vented, the dispersalof ethylene can be significant. When ripening rooms are built into producestorage or distribution warehouses, the ethylene can come in contact withother products being held in the warehouse. If that produce were packed inethylene-adsorbing packaging, the ethylene at such levels might saturate thepackaging and render it ineffective.

2.4.4 Fluorescent ballasts and rubber materials

The ballasts that hold fluorescent lights are sources of ethylene. In addition,rubber materials exposed to heat or UV light can release ethylene (Reid,1985b).

2.4.5 Microorganisms

Although several soilborne microorganisms produce ethylene, others de-grade it. The net effect appears to be that the soil serves primarily as a sinkfor ethylene.

Postharvest plant pathogens growing on stored products in enclosedholding areas can be important sources of ethylene. AU infested foodstuffsshould be immediately discarded.

2.5 Commercial applications in packaging

Several of the technologies described above have been incorporated intopackaging materials that are either commercially available or are likely tobecome available in the near future. As is common in the commercial sector,some of the claims for ethylene ad-/absorbing capacity for these packagingmaterials have been poorly documented and thus the efficacy of thematerials is difficult to substantiate.

Most substances designed to remove ethylene from packages are deliveredeither as sachets that go inside the package or are integrated into thepackaging material, usually a plastic polymer film or the ink used to print onthe package.

2.5.1 Potassium permanganate-based scavengers

Many vendors offer ethylene adsorbers based on KMnO4 immobilized onany of several minerals. These products are available in sachets for packagesand on blankets that can be placed in produce-holding rooms. Potassiumpermanganate is not integrated into food-contact packaging because of its

toxicity. However, sachets could be used inside produce packages and havebeen shown to effectively scavenge ethylene from packages of bananas,persimmons, kiwifruit, avocados (Ben-Arie and Sonego, 1985; Fuchs andTemkin-Gorodeiski, 1971; Hatton and Reeder, 1972; Krishnamurthy andKushalappa, 1985; Liu, 1970; Maotani et al.9 1982; Scott et al9 1970).

Typically, such products contain ~ 4-6% KMnO4 on an inert substratesuch as perlite, alumina, silica gel, vermiculite, activated carbon or celite(Abeles et al, 1992). The performance and useful lives of these scavengersdepends on the substrate surface area and the content of reagent (KMnO4).Formulations differ in density and surface area of substrate and the loadingof reagent.

Some suppliers of KMnO4-based ethylene scavengers are listed in Table2.1. This table is not a complete listing of all companies supplying suchproducts but only those known to the author at the time of writing.

2.5.2 Activated carbon-based scavengers

Various metal catalysts on activated carbon will effectively remove ethylenefrom air passing over the bed of carbon. Commercial units, known asswingtherm ethylene converters, are based on such a system. However, theyrequire heat and movement of gases and so are not applicable to packagedproduce. Activated charcoal impregnated with a palladium catalyst placed inpaper sachets effectively removed ethylene in an experiment on maintainingquality of lightly processed kiwifruit, banana, broccoli and spinach (Abe andWatada, 1991).

The Japanese company Sekisui Jushi has developed a product, Neupalon,that is a sachet containing activated carbon and a water absorbent capable of

Table 2.1 Suppliers (USA) of potassium permanganate ethylene scavengers

Air Repair Products, Inc.PO Box 1006Stafford, TX 77477

Cams Chemical Company, Inc.1001 Boyce Memorial DriveOttawa, IL 61350

Complete ControlPO Box 1006Stafford, TX 77477

DeltaTrak, Inc.PO Box 398Pleasonton, CA 94566

Ethylene Control, Inc.PO Box 571Selma, CA 93662

ExtendaLife SystemsPO Box 55044Hayward, CA 94545-0044

Loomix, Inc.405 E. Branch StreetPO Box 490Arroyo Grande, CA 93420

Purafil, Inc.PO Box 80434Chamblee, Georgia 30366

Purity Corporation9539 Town ParkHouston, TX 77036

Note: Nippon Greener Co. is reported to use potassium permanganate by Abe(1990) in his listing of ethylene absorbers.

absorbing up to 500-1000 times its weight of water. The company providesdata showing that Neupalon adsorbs 40 ml ethylene per m2.

Honshu Paper, also in Japan, has a product called the Hatofresh Systemthat is based on activated carbon impregnated with bromine-type inorganicchemicals. They do not specify which bromine compounds are used. Thecarbon-bromine substance is embedded within a paper bag or corrugatedbox, which is used to hold fresh produce. They claim that the bag willadsorb 20 ml ethylene per g of adsorbent. It is unlikely such bags could beused in most developed countries due to the reaction of bromine compoundswith water, which can release toxic bromine gas.

Mitsubishi Chemical Company of Japan produces a product called Sendo-Mate which is based on palladium catalyst on activated carbon whichadsorbs ethylene and then catalytically breaks it down. The product comes inwoven sachets that can be placed in packages of produce.

2.5.3 Activated earth-type scavengers

In the past several years a number of packaging products have appearedbased on the putative ability of certain finely dispersed minerals to adsorbethylene. Typically these minerals are local kinds of clay that are embeddedin polyethylene bags which are then used to package fresh produce. Many,though not all, of the bags are marketed by Japanese or Korean companies,though some are also sold in the United States and Australia.

The Cho Yang Heung San Co. Ltd. of Korea markets a film bag called theOrega bag, based on the US patent of Dr Mitsuo Matsui (Matsui, 1989). Fineporous material derived from pumice, zeolite, active carbon, cristobalite orclinoptilolite is sintered together with a small amount of metal oxide beforebeing dispersed in a plastic film. Neither plastics containing chlorine such aspolyvinyl chloride or polyvinylidene chloride, nor plasticizers, are appar-ently suitable for these applications (Choi, 1991). The inorganic materialshave pores ranging from 2000 to 2800A and the resulting film is reported tohave the capacity to adsorb at least 0.005 ppm ethylene per hour per m2

(Choi, 1991). Adsorption of this small amount of ethylene may not behelpful for some situations.

Another such film is described in a US Patent assigned to Nissho and Co.Ltd. of Japan (Someya, 1992). This film incorporates finely ground coral(primarily calcium carbonate), having pore sizes in the range of100,000-500,000A. After incorporation in a polyethylene film, the coral isclaimed to absorb ethylene. However, no data have been presented tosupport this claim.

A product called Ethad® has been developed by the Rubber ResearchInstitute of Malaysia; it releases ethylene in order to stimulate the productionof latex by rubber trees. The product is based on powdered zeolite in viscousoil or grease. The zeolite is reported to adsorb 8% ethylene by weight

(Abeles et ah, 1992). Apparently Ethad® has not been used to adsorbethylene in packages.

Evert-Fresh Corporation markets Evert-Fresh bags in the USA. The bagsare, presumably, polyethylene with Japanese Oya stone dispersed within thefilm matrix. Oya stone has putative ethylene-adsorbing capacity. Evert-FreshCorp. offers shelf-life data for several fresh commodities to demonstrate thebenefits of their bags.

A product called BO Film is marketed by the Odja Shoji Co. Ltd. ofJapan. It is a low-density polyethylene film extruded with finely dividedcrysburite ceramic which is claimed to confer ethylene-adsorbing capacity(Joyce, 1988).

There are many other similar bags being sold throughout the world offer-ing improved postharvest life of fresh commodities due to the adsorption ofethylene by the minerals dispersed within the film. The evidence offered insupport of this claim is generally based on shelf-life experiments comparingcommon polyethylene bags with mineralized bags. Such evidence generallyshows an extension of shelf-life and/or a reduction of headspace ethylene.Such data are unconvincing. Although the finely divided minerals mayadsorb ethylene, they will also open pores within the plastic bag and alter thegas-exchange properties of the bag. Because ethylene will diffuse muchmore rapidly through open pore spaces within the plastic than through theplastic itself, one would expect ethylene to diffuse out of these bags fasterthan through pure polyethylene bags. In addition, CO2 will leave these bagsmore readily and O2 enter more readily than is the case for a comparablepolyethylene bag. These effects can improve shelf-life and reduce headspaceethylene concentrations independently of any ethylene adsorption. In fact,almost any powdered mineral can confer such effects without relying onexpensive Oya stone or other speciality minerals. Hercules ChemicalCompany relied on this effect while using calcium carbonate to improve thegas-transmission properties of their Fresh Hold breathable bags withoutmaking any claims regarding ethylene adsorption (Anderson, 1989).

Although the minerals in question may have ethylene-adsorbing capacity,the data supporting the commercial products incorporating these mineralsfail to demonstrate such capacity. Even if they do have ethylene-adsorbingcapacity, it is possible that they will lack significant capacity whileembedded in plastic films. The ethylene would have to diffuse through theplastic matrix before contact with the dispersed mineral, thus greatly slowingany processes of adsorption. Once the ethylene has diffused part-waythrough the plastic film, venting to the outside may be nearly as fast andeffective as adsorption on embedded minerals.

In a study performed in Australia with BO film, the mineral in the bagtook up little ethylene (Joyce, 1988). Furthermore, in studies with puremineral granules of Cera-sutora A, the author found that the ethylene

sorption capacity of the material was only - 170 nmol/g after 15 h at 200C(Joyce, 1988). This amount of ethylene sorption is insignificant.

In studies in the USA, the author tested four proprietary bags from Japan,all containing dispersed minerals and all claiming ethylene-adsorbingcapacity. Weighed samples of each bag were placed in sealed jars withsampling ports attached. A second set of jars were left empty. We injecteda known quantity of ethylene into each jar. Each day for seven days wesampled the ethylene concentrations in each jar. We could detect nodifferences in ethylene concentrations between the jars with film and thosewithout film. Our conclusion was that none of the four films adsorbedmeasurable amounts of ethylene (Zagory et al.9 1988).

In the future, it would be useful if companies claiming ethylene-adsorbingcapacity for their products presented direct evidence for these claims. Shelf-life studies and headspace analysis of ethylene concentrations do not supportclaims of ethylene-adsorbing capacity. Direct measurement of ethylenedepletion in closed systems containing samples of the bags without anyproduce to confound the results would be much more instructive. Fur-thermore, such studies should be done at low temperature and high relativehumidity to mimic the conditions under which they will be expected toperform.

2.5 A New and novel approaches to ethylene-removing packaging

There are some new and unusual approaches to developing ethylene-removing packaging that deserve mention.

Perhaps the most promising new development in ethylene-removingpackaging is the use of electron-deficient dienes or trienes incorporated inethylene-permeable packaging. The preferred diene or triene is a tetrazine.However, since tetrazine is unstable in the presence of water, it must beembedded in a hydrophobic, ethylene-permeable plastic film that does notcontain hydroxyl groups (Holland, 1992). Appropriate films would includesilicone polycarbonates, polystyrenes, polyethylenes and polypropylenes.Approximately 0.01-1.0 M dicarboxyoctyl ester of tetrazine incorporated insuch a film was able to effect a ten-fold reduction in ethylene in sealed jarswithin 24 h and a 100-fold reduction within 48 h (Holland, 1992). Thetetrazine film has a characteristic pink color when it is new and turns brownwhen it becomes saturated with ethylene so it is possible to know when itneeds replacing.

A new product called Frisspack has been developed in Hungary for use instorage of fresh fruits and vegetables. The product consists of a chemi-sorbent of small particle size dispersed among the fibers in the early phaseof paper production. The result is a paper sheet with putative ethylene-adsorbing capacity. The nature of the chemisorbent and data supporting the

claim of ethylene adsorption are not available. No response was receivedfrom the vendor following the author's request for information.

Although there are many packaging products claiming ethylene-removingcapabilities, few of the claims are backed up with reliable data. Standardizedprocedures for demonstrating efficacy would aid the development of thisgrowing industry. In addition, a thorough understanding of the physiologicaleffects of ethylene and its importance in sealed permeable packages shouldprecede any use of these products. In many cases, the elevated carbondioxide levels common in modified atmosphere packages may obviate theneed for ethylene removal. In other cases, with very sensitive commoditiessuch as kiwifruit and carnations, ethylene-adsorbing capability may becrucial in the maintenance of shelf-life and commercial quality.

Acknowledgements

Thanks for literature and helpful discussion are owed to: Linda Dodge,Cheryl Reeves, Michael Reid, Michael Rooney, Mikal Saltveit and KitYam.

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3 Design of modified atmosphere packaging forfresh produce

K. L. YAM and D. S. LEE

3.1 Introduction

Controlled atmosphere (CA) storage and modified atmosphere packaging(MAP) are two useful technologies to extend the shelf-life of freshagricultural and horticultural produce. Simply stated, these technologiesinvolve storing a fruit or vegetable in a modified atmosphere usuallyconsisting of reduced O2 and elevated CO2 concentrations compared to air.The modified atmosphere reduces the rates of respiration and ethyleneproduction, which are often associated with the benefits of retardation ofphysiological, pathological, and physical deteriorative processes occurring inthe product. Aerobic respiration is a complicated process that involves aseries of enzymatic reactions taking place through the metabolic pathways ofglycolysis, the tricarboxylic acid (TCA) cycle, and the associated electrontransport system (Kader, 1987). However, the overall reaction describing therespiration process may be simply expressed as

C6H12O6+ 6 O2 -> 6 CO2 + 6 H2O + heat (3.1)

that involves the oxidation of organic substrates (such as starch, sugars, andorganic acids) to CO2 and H2O along with heat generation. Kinetic theoryand Equation (3.1) suggest that the respiration rate may be reduced bydecreasing the O2 and/or by increasing the CO2 concentration.

There are differences between the ways CA storage and MAP create andmaintain a modified atmosphere. In CA storage, a gas generator is usuallyused to create and control the modified atmosphere in a cold warehousewhere the product is kept. In MAP, the product is kept in a carefullydesigned permeable package, and the modified atmosphere is created andmaintained through an intricate interplay between the respiration of theproduct and the gas permeation of the package. MAP is a more economicaltechnology because an expensive gas generator is not needed; however, it isalso a more difficult technology to implement because of the rathercomplicated interactions between the product and the package. This chapteris focused on the design of MAP for fresh produce.

The modified atmosphere in MAP can be created by either active orpassive modification. In active modification, the modified atmosphere iscreated rapidly by flushing the headspace of the package with a desired gasmixture. In passive modification, the modified atmosphere is created by

allowing the produce to respire inside the package so that an equilibrium isslowly attained. In both cases, once the modified atmosphere is established,it is maintained through a dynamic equilibrium of respiration and per-meation.

Designing MAP for fresh produce is a complicated task, requiring goodunderstanding of the dynamic interactions among the product, the environ-ment, and the package. The food technologist who is asked to design MAPfor fresh produce faces many difficult but practical questions, such aswhether the MAP technology is applicable to the product, what is theoptimum gas composition, what kind of packaging material is needed, and

Determine optimum conditions andtolerant limits

Determine respiration rates

Neither CA storage norMAP is suitable for the

product

Use mathematical model todetermine package requirments

IVerify model predictions with

experiments

Only CA storage is suitablefor the product

Design of MAP possibleNeed to develop new

permeable films for MAP

Figure 3.1 Flow chart for designing MAP for fresh produce.

Conduct literature review

Conduct feasibility study

Could CA storage provide"benefits?

Is suitable permeable filmavailable?

how to protect the product from the potential hazards of a modifiedatmosphere.

Prince (1989) has reported that the majority of modified atmospherepackages are designed by trial-and-error methods, which often lead to poordesigns that are either ineffective or injurious to the product. Althoughnumerous articles have already been written on the various aspects of MAP,almost none of them provides an overview of the design process. To fill thisgap, a simple process for designing MAP of fresh produce is presented inthis chapter. The process is necessarily somewhat simplified because manybiological aspects underlying the effects of modified atmosphere on theshelf-life of plant tissues are still not understood (Solomos, 1994). The stepsinvolved in the design process are outlined in the flow diagram of Figure3.1.

3.2 Literature review

Before designing MAP for a product, the first step is to determine whetherCA storage can indeed provide benefits for the product. Reviewing the

Table 3.1 Recommended optimal modified atmosphere conditions for produce

Commodity

Vegetables

AsparagusBroccoliBrussels sproutsCabbageCauliflowerCorn, sweetCucumberLettuceMushroomPepperSpinachTomato, partly ripe

Fruits

AppleApricotAvocadoBananaBlueberryCherry, sweetGrapefruitPeachPearPersimmonStrawberry

Temperaturerange (0C)

0-50-50-50-50-50-58-120-50-58-120-58-12

0-50-55-1312-150-50-5

10-150-50-50-50-5

Relativehumidity (%)

959595

90-959595

90-959590

90-9595

85-90

9090

85-9085-9590-9590-9585-90

9090-9590-9590-95

Modified atmosphere

O2

air1-21-23-52-52-43-52-5air3-5air3-5

2-32-32-52-5

0-103-103-101-22-3210

CO2

5-105-105-75-72-5

10-2000

10-152-8

10-200

1-22-33-102-5

11-2010-125-10

50-1

815-20

From Labuza and Breene (1989), Powrie and Skura (1991), and Katzyoshi (1992)

literature data is a good start to gather preliminary information about theproduct or similar products. Helpful information that may be available inthe literature is assessment of the potential benefit of CA storage and MAP,optimum storage conditions (such as gas concentration, temperature, andrelative humidity), O2 and CO2 tolerance limits, respiration rate, temperaturebelow which chilling injury of the product occurs, whether the product isclimacteric or nonclimacteric, and so on.

The recommended O2 and CO2 concentrations for some fruits andvegetables are listed in Table 3.1. More data are available elsewhere (Prince,1989; Labuza and Breene, 1989; Singh and Oliveira, 1994). The data mayalso be represented in the form of CO2 versus O2 plots (such as in Figures3.2 and 3.3), in which the windows represent the boundary of recommendedgas concentrations. The size of a window has a practical implication in thatthe smaller it is, the more rigid is the design requirement. However,literature data should be used only as a reference because discrepanciessometimes exist among data from different sources (due to possible reasons

CO

2 C

once

ntra

tion

(%)

Blackberry, Blueberry, Fig,Raspberry, Strawberry

Cherry

Mango, Papaya, Pineapple

Avocado

Persimmon

Banana Grapefruit

OrangeKiwi. Ne< arine, Peach

Air P = 0.8

Cranberry Plum

ApricotGrape LDPEp = Cy

O2 Concentration (%)

Figure 3.2 Recommended gas concentrations for CA storage of fruits. (Redrawn from Singh andOliveira, 1994, with permission.)

such as maturity and cultivar of the product) and the criteria used in makingrecommendations are seldom reported. Therefore conducting a feasibilitystudy is often required, particularly if no literature data are found for theproduct of interest.

3.3 Feasibility study

A simple feasibility study consists in conducting experiments to monitor thequality of the product as a function of time under various modifiedatmospheres. To define quality, a set of instrumental and sensory qualityattributes must be selected. Although the selection procedure is different foreach product, it generally includes assessment of texture, flavor, odor, color,nutritional quality, and microbial growth. The effects of CA storage on thesensory and nutritional quality of fruits and vegetables have been reviewedby Weichmann (1986).

Air P = 0.8

MushroomsAsparagus

CO

2 C

once

ntra

tion

(%)

Leeks

Broccoli

Brussels ! >pr< >uts'BeansCabbage

Parsley

Spinach

LDPE B = 0.8

Cauliflower

TomatoPepperArtichokesRadishLettuce

O2 Concentration (%)

Figure 3.3 Recommended gas concentrations for CA storage of vegetables. (Redrawn from Singhand Oliveira, 1994, with permission.)

Okra

Figure 3.4 Flow-through system (a) and closed system (b).

The modified atmosphere can be created using the flow-through system(Figure 3.4a) that involves storing the product in a glass jar that has an inletport and an outlet port through which a pre-mixed gas (consisting of loweredO2 and elevated CO2) passes. For this feasibility study, the authors suggestthe use of a 3-level factorial experimental design with O2 and CO2

concentrations as independent variables, while keeping temperature andrelative humidity constant. The response or dependable variables are two orthree relevant quality attributes for the product. The O2 and CO2 ranges areto be selected between 2 and 10% and O and 20%, respectively. Thesuggested temperature and relative humidity are 5°C and 90%, respectively.Air (21% O2 and 0% CO2) should always be used as a control.

The purpose of the feasibility study is to determine if CA storage canprovide better storage quality than air storage. If the results are notfavorable, it is likely that MAP is not a suitable technology, and the foodtechnologist should avoid spending more time on designing MAP for thisparticular product.

33.1 Optimum conditions

Further work is justified if the feasibility study confirms the benefit of CAstorage for the product. The question then is whether the same benefit can beachieved by MAP without the use of an expensive gas generator. Since thefeasibility study provides only preliminary data, more experiments areneeded to more closely define the optimum conditions. This should be doneby extending the experimental design to include additional O2 and CO2

concentrations that are expected to give good results. The effects ofadditional temperatures (O and 10°C) and relative humidities (85 and 95%)on storage quality should also be examined.

There are three major design constraints: O2 tolerance limit, CO2 tolerance

(a)Gas In Gas Out

Gas Sampling Port

ProductSamples

limit, and temperature below which chilling injury occurs. Keeping O2

concentration above the O2 tolerance limit is necessary for maintainingaerobic respiration; otherwise, anaerobic respiration will lead to theformation of off-flavor and off-odor inducing compounds such as alcoholsand aldehydes. Keeping CO2 concentration below the CO2 tolerance limit isnecessary for protection of the product from unfavorable physiologicaldisorder such as breakdown of internal tissues. Keeping the product above acertain temperature is necessary for avoiding cell damage leading to loss offlavor and invasion of spoilage organism. Usually the O2 tolerance limitvaries between 1 and 3%, the CO2 tolerance limit varies between 10 and20%, and the chilling temperature varies between O and 15°C, depending onthe product - the actual values can be determined experimentally using theflow-through system illustrated in Figure 3.4a. The O2 tolerance limit maybe determined by monitoring the increase in ethanol content of the tissue.

3.4 Respiration rates

Respiration rate values are required for mathematical modeling and fordefining the package requirements. Respiration is often a good index for thestorage life of fresh produce: the lower the respiration rate, the longer thestorage life (Powrie and Skura, 1991; Lebermann et aL> 1968). As Equation(3.1) shows, respiration involves the rate of O2 consumption (R0) and therate of CO2 evolution (RCo2)- The respiratory quotient (RQ) is a convenientterm, which is defined as the ratio of CO2 evolution to O2 consumption. RQsare reported to range from 0.7 to 1.3, depending upon the metabolicsubstrate (Kader, 1987; Kader et aly 1989).

The respiration rates are known to be affected by several internal andexternal factors (Robertson, 1992). Internal factors include the type ofproduct and cultivar, maturity, resistance of plant tissue to gas diffusion, andwhether the product is climacteric or nonclimacteric. The external factorsinclude temperature, C2H4 concentration, O2 and CO2 concentrations andstress due to physical damage or excessive water loss.

34.1 Temperature effect

Temperature is the most important factor because it affects both therespiration rate and the permeability of the package. In practice, mostproducts experience some temperature fluctuations during storage anddistribution. The Arrhenius model is often used to describe the temperaturedependence of respiration, and the equations for rate of O2 consumption andrate of CO2 evolution are

/?O2 = R°2exp(-£O2/R7^ (3.2)

#co2 = Rco2 exp(-£CO2/R 7^ (3.3)

Another common way to express the temperature dependence is Q10, definedas

Respiration rate at (T + 10)°C10 "~ Respiration rate at rC

which is applicable to either O2 consumption rate or CO2 evolution rate.Typical Q10 values for vegetables are 2.5-4.0 at 0-1O0C, 2.0-2.5 at10-200C, 1.5-2.0 at 20-300C, and 1.5-2.0 at 30-400C (Robertson, 1992).Mathematically, the activation energy is approximately linearly proportionalto Q10 if the temperature range of interest is small (less than 400Cdifference), a condition satisfied by most practical circumstances. Similarly,Arrhenius-type equations can also be used to describe the temperaturedependence of gas permeabilities.

(3.5)

(3.6)

3.5 Measurement of respiration rates

Because respiration rates under modified atmospheres for most fruits andvegetables are not available in the literature, they must be determined byexperiment. There are three methods for measuring respiration rates: theflow-through system, the closed system, and the permeable system (Lee,1987). The flow-through system and the closed system are illustrated inFigure 3.4.

5.5.7 Flow-through system

The experimental setup of the flow-through system is shown in Figure3.4(a). It is important to position the inlet and the outlet tubes sufficiently farapart to ensure thorough mixing of the gas in the jar. The steady-state inletand outlet concentrations are measured with an instrument such as a gaschromatograph. The equations for calculating the respiration rates are

(3.7)

(3.8)

where the subscripts in and out denote the inlet and the outlet concentrations,respectively. The flow-through system has an advantage of being able to

provide more accurate data than the closed system. However, the usefulnessof the flow-through system is limited by the precision of the gaschromatography measurements, because the differences between the inletand outlet concentrations are usually rather small. There are three ways toincrease the concentration differences, as suggested by Equations (3.7) and(3.8): work only with produce of high respiration; reduce the gas flow rate;increase the sample weight. Another drawback of the flow-through system isthat each experiment measures only the respiration rate at a single gasconcentration, and thus much time and labor are required if respiration ratesat many gas concentrations are to be measured.

3.5.2 Closed system method

The closed system method (Figure 4b) is more efficient for measuringrespiration rates as a function of gas concentrations. This method involvesmonitoring the O2 and CO2 concentrations inside a closed jar containingthe product as a function of time (Haggar et al., 1992). The initialgas concentrations inside the jar are usually those of air, but other gasconcentrations may also be used. As the product respires, the gas con-centrations in the jar change with time - from high O2/low CO2 concentra-tions at the beginning to low O2/high CO2 concentrations toward the end.The respiration rates at these O2 and CO2 concentrations may be calculatedusing the equations

(3.9)

(3.10)

The negative sign in Equation (3.9) signifies that the O2 concentration in thejar decreases with time. In order to evaluate the first derivatives, the data ofgas concentration versus time should first be curved fitted. The recom-mended functions for fitting the data are

(3.11)

(3.12)

and their first derivatives are

(3.13)

(3.14)

The validity of Equations (3.11) and (3.12) should be confirmed bycomparing the fitted values with the experiment data. If the comparison ispoor, other forms of functions should be attempted. For convenience, theconversion factor a is omitted in Equations (3.13) and (3.14).

The respiration rates calculated from Equations (3.9) and (3.10) are at O2

and CO2 concentrations unique to a particular closed system experiment. Itis usually difficult to design a priori a closed system experiment to generatecertain desired gas concentrations. The question is how the respiration ratesobtained from closed system experiments can be useful to estimate therespiration rates at other gas concentrations. An answer is to first fit therespiration rate data with a model and then use the model to estimatethe respiration rates at the desired gas concentrations.

The best model presently available for this purpose is the enzyme-kinetictype respiration model proposed by Lee et al. (1991).

(3.15)

The model requires two different sets of adjustable coefficients (Vm, Km, andKj): one for ROi and the other for Rcc>2. The model has been verified quiteextensively using experimental data for a wide variety of products. Since themodel is based on the principle of enzyme kinetics, it requires less adjustablecoefficients and is likely to be more predictive than those purely empiricalmodels (Cameron et a/., 1989; Yang and Chinnan, 1988) used in theliterature. However, the applicability of the model to any new set of datashould always be confirmed by comparing the predicted values with theexperimental data. Overextending the model to predict respiration rates atconcentrations very different from those generated from the closed systemexperiments should be avoided. Table 3.2 lists the model parameter valuesand respiration activation energies for some fruits and vegetables. Theactivation energies are not strong functions of O2 and CO2 concentrations(Haggar et aL, 1992).

3.6 Model equations and package requirements

Mathematical models are useful for defining the package requirements forMAP. Several models (Jurin and Karel, 1963; Veeraju and Karel, 1966;

Table 3.2 Respiration model parameter values and respiration activation engergies for someproducts

Commodity

Blueberry"Coville"3

Broccolib

Cauliflower0

Green pepperd

Temp.(0C)

15

7

13

10

Respirationexpression

O2 consumptionCO2 evolution




Respiration model parameters

v m(mg/kgh)

68.051.0

210.3235.2

133.7134.4

54.331.8

(% O2)

0.40.2

0.61.7

1.71.4

6.02.4

(% CO2)

2.94.9

2.31.93

3.03.1

1.34.3

Activationenergy

(kJ/mol)

147.3163.3

62.766.1

21.2-48.221.2-48.2

48.7-57.348.7-57.3

aSong et al (1992); bHaggar et al. (1992); cYam et al. (1993) and Exama et al. (1993); dExama etal. (1993).

Hayakawa et al., 1975; Deily and Rizvi, 1981) are available in the literature,and some of them have been reviewed by Zagory and Kader (1988).Basically those models use the principles of O2 and CO2 mass balances todescribe the interactions among the respiration of product, the permeabilityof the package, and the environment.

3.6.1 Unsteady-state equations

A simple model based on the principle of mass balance requires that

Rate of O2 or CO2 _ Rate of O2 or CO2 Rate of O2 or CO2

accumulated in package permeated into package generated by respiration

and the mass balance equations for O2 and CO2 are

(3.16)

(3.17)

where the subscripts i and o denote the inside and outside of the package,respectively. Equations (3.16) and (3.17) are first-order linear differentialequations that can be solved quite easily using a computer. They are usefulfor describing the unsteady-state behaviour of the MAP system, such asduring the process of passive modification and during temperature fluctu-ations. The equations can be tailored to fit a particular physical situationthrough the application of initial boundary conditions. For example, theinitial conditions for passive modification are [O2]; = 21 and [CO2]; = O at

t = O. Note that the respiration rates R02 and RCOi are functions of O2 andCO2 concentrations, which can be expressed using the enzyme-kinetic modelof Equation (3.15).

3.6.2 State-state equations

When the accumulated terms are zero, Equations (3.16) and (3.17) arereduced to the steady-state equations

(3.18)

(3.19)

where the subscript s denotes steady-state condition. Equations (3.18) and(3.19) describe the dynamic equilibrium behaviour of the MAP system,when the CO2 evolution rate equals the efflux rate of CO2 through thepackage and the O2 consumption rate equals the influx rate of O2 through thepackage. In most situations, steady-state or dynamic equilibrium is approa-ched within two days. For long storage of the product, the dynamicequilibrium behavior is more important than the unsteady-state behavior.

To use Equations (3.18) and (3.19) as design equations, it is necessary tokeep track of how many independent or design variables are available. Thereare a total of 11 variables: R02, RCOi, and W are associated with the product;P02, PCQ2, S, and L are associated with the package; [O2]o, [O2J1 s, [CO2]O, and[CO2I1 s are associated with the environment. (Although temperature is notexplicitly shown, it is an implicit variable that affects both the respirationrates and the permeabilities to O2 and CO2.) Once the product and thetemperature are selected, six out of the 11 variables are already decided: R02

and RCOi are determined by the flow-through system or the closed systemexperiments; [O2J1 s and [CO2], s are assumed to be the optimum O2 and CO2

concentrations; [O2]o and [CO2J0 are 21 and 0%, respectively. With sixvariables fixed and two equations to satisfy, there are only (11-6-2) = 3design variables. That is, only three out of the remaining five variables (W,5, L, P02 and PCO2) can be specified arbitrarily. For example, if the foodtechnologist chooses to specify the dimensions of W, S, and L (withinpractical limits), the permeabilities P02 and PCO2 must then be determined byEquations (3.18) and (3.19).

The equations also provide a convenient means to reject films not suitablefor a particular application. Dividing Equation (3.19) by Equation (3.18)yields

(3.20)

where [O2]0 and [CO2I0 are assumed to be 21 and 0%, respectively. Further,if RQ is assumed to be 1 and PCQJPOi is defined as (3, Equation (3.20) maybe rewritten as

(3.19)

Equation (3.19) may be represented as a straight line with slope P on a plotof [CO2I1 s versus (21 - [O2]; s). Two such lines (P = 0.8 and (3 = 5) areshown in Figures 3.3 and 3.4. As an example of application, cauliflowerrequires a P = 5 (Figure 3.3), and thus a film with (3 varying considerablyfrom 5 (such as 2) should be rejected for packaging cauliflower. However,there is no guarantee that a film with (3 = 5 will work well for cauliflowerbecause, in addition to (3, the individual F02 and PCOi must be alsodetermined by solving Equations (3.17) and (3.18) simultaneously. Satisfy-ing Equation (3.19) is a necessary but non-sufficient requirement forselecting a suitable polymeric film.

There are on-going research efforts being made to develop moresophisticated models for more accurate prediction, since none of the existingmodels considers every factor of the MAP system. A more complete modelshould include the generation of H2O and heat and the effects of N2 andC2H4 in addition to balancing the O2 consumption rate and CO2 evolutionrate. In the meantime, the simple model described above can be used toprovide helpful information for preliminary design of MAP.

3.7 Polymeric films for MAP applications

Since there are many varieties of produce, a wide range of permeabilities isrequired. High permeabilities are needed for rapidly respiring produce, lowpermeabilities for slowly respiring produce. Table 3.3 lists the per-meabilities, (3 values, and permeability activation energies of some commonfood packaging polymeric films. Among them, low-density polyethylene andpolyvinyl chloride are most widely used for packaging fruits and vegetables(Zagory and Kader, 1988).

A fortunate situation occurs when the desired P02 and PCOl requirementsare met by one or more existing commercial films. If this is the case, a goodchance exists for a successful design. Unfortunately, this is not often thecase because the choices of suitable commercial polymeric films are ratherlimited. The problem can be appreciated by examining Table 3.3, whichreveals that the (3 values for most films fall within a rather narrow range

al (1993).Table 3.3 Permeabilities at 100C and permeability activation energies for polymeric film

Permeabilities (ml mil/m2h PCO2 Activation energies (kJ/mol)atm)

Polymeric films PQ2 PQ2 PQ2 Ep.o2 E

P,co;

Polybutadiene 1118 9892 8.8 29.7 21.8Low-density 110 366 3.3 30.2 31.1polyethyleneCeramic-filled LDPE 199 882 4.4 36.8 28.4Linear low-density 257 1002 3.9polyethyleneHigh-density 2.1 9.8 4.6 35.1 30.1polyethyleneCast polypropylene 53 151 2.9Oriented polypropylene 34 105 3.1 - -Polyethylene 1.8 6.1 3.3 26.8 25.9terephthalateNylon laminated 1.7 6.0 3.5 52.6 50.0multilayer filmEthylene vinyl acetate 166 985 5.9 48.4 37.0Ceramic-filled 116 630 5.4 34.5 26.2polystyreneSilicone rubber 11170 71300 6.4 8.4 0.0Perforation (air) 2.44 X 109 1.89 X 109 0.8 3.6 3.6Microporous film 3.81 X 107 3.81 X 107 1.0 13.0 3.7

From Exama et al (1993); Lee et al. (1992); Lee et al. (1994); Ohta et al (1991); Mannapperumaand Singh (1990); Anderson (1989); and Shelekshin et al (1992).

between 3 and 6; however, Figures 3.3 and 3.4 show that many fruits andvegetables require P values outside this narrow range. This problem has alsobeen recently investigated by Exama et al. (1933), who conclude most filmsdo not satisfy both the gas flow and selectivity requirements for many fruitsand vegetables packaged in typical MAP configurations.

There are at least two possible solutions for this problem. The firstsolution is to compensate the inadequacy of the films with techniques suchas placing oxygen absorbers in the package or using two different films toselectively control the permeability. The second solution is to look for newand better films - some recent advances in the development of polymericfilms suitable for fresh produce are discussed below.

3.7.1 Perforation and microporous films

A major challenge is to develop films that have greater permeability andhave a wider range of p values than existing types. Films of enhancedpermeability are necessary for packaging high respiration rate products andfor preventing the development of anaerobiosis. A wider range of (3 values,especially those below 3, is necessary to better match the respirationbehavior of many products.

The use of either perforation systems or microporous films is a possiblesolution to meet these two requirements. These systems and films have

permeabilities many orders of magnitude higher than those of non-perforatedpolymeric films, as well as (3 values between 0.8 and 1 (Anderson, 1989).The uses of perforation systems or microporous films in MAP are currentlybeing studied in several laboratories.

Emond et al (1991) have studied gas exchange through perforationsystems. They developed empirical equations to predict the effectivepermeabilities to O2 and CO2 for various diameters, thicknesses, andtemperatures. Their computer simulations showed that neither a siliconemembrane alone nor a perforation system alone could provide a satisfactorygas concentration for broccoli. However, a combined system, consisting ofsilicone membrane with area of 0.0061 m2 and perforations of 0.006 mdiameter and 0.0127 m thickness, could provide favorable conditions forbroccoli. Their other computer simulations also showed that while nopolymeric film or silicone membrane could provide satisfactory conditionsfor strawberries, a perforation system (with perforations 0.008 m in diameterand 0.00159 m thick) could provide an effective solution. However,experiments are required to confirm these computer predictions.

Meyers (1985) described the use of perforations in MAP of fruits. Thetechnique involved placing the product in a bag (or on a tray) constructed ofa high-barrier film such as polyvinylidene chloride. The bag was flushedwith N2 or CO2 as a preservative gas before sealing. After sealing, the filmwas perforated to assure gas outflow from the bag, to prevent distortion andto provide a gas pressure within the bag sufficient to inhibit air inflow intothe container. The packaging of strawberries and nectarines using thistechnique was described.

Mizutani et al (1993) reported that microporous polypropylene sheetscould be prepared by biaxially stretching filler-containing polypropylenesheets. Examples of fillers were CaCO3 and SiO2. The gas permeabilities ofthose sheets were controllable by adjusting filler content, particle size offiller, and degree of stretching. The average pore size ranged between 0.14and 1.4 juum.

Anderson (1989) has described the use of microporous films for MAP offruits and vegetables. The package was constructed of a gas-impermeablematerial having a microporous membrane panel to provide controlled flowsof O2 and CO2 through its walls. The microporous membrane was a biaxiallyoriented film composed of a blend of propylene homopolymer and apropylene-ethylene copoylmer having an ethylene-moiety concentration of2-5% by weight. The film was filled with 40-60% CaCO3 based on the totalweight of the film. Depending on the loading of CaCO3, the permeance(defined as permeability per unit thickness) of the film ranged between77,500 and 465,000,000 ml/m2 day atm. Good results were reported forstrawberries, mushrooms, and broccoli florets with the proper selection ofpermeance.

3.7.2 Temperature compensating films

Another challenge is to develop films that can tolerate temperaturefluctuation during storage and distribution. The problem of developing suchfilms is the mismatch of the activation energies for respiration andpermeation: respiration rates of produce are strongly affected by tem-perature, but the permeabilities of existing packaging films are only slightlyaffected by temperature. In some cases, even a small temperature increasewill cause rapid accumulation of CO2 and depletion of O2 in the package, asituation that may damage the product. Presently, research is being done ondeveloping a new class of polymeric films with permeation activationenergies more closely matching the respiration active energies of freshproduce. This class of polymeric films exhibits dramatic changes inpermeability by transforming the polymer matrix reversibly from a crystal-line state to an amorphous state as temperature is increased above a switchtemperature. This switch temperature can be controlled within ± 2°C bychanging the polymer side-chains.

3.7.3 Ceramic-filled films

In recent years, commercial ceramic-filled polymer films have beenintroduced in Japan and Korea for packaging fruits. The films usuallycontain about 5% of very fine ceramic powder, and the manufacturers claimthat these films emit far-infrared radiation or absorb C2H4 that can help toextend the shelf-life of the fruits. Although some workers (Isaka, 1988;Joyce, 1988) have reported that these films seem to improve the storagequality (especially color) of fresh produce, the benefit of using such filmshas not been reported in other laboratories. Lee et al (1992) have reportedthat the O2, CO2, and C2H4 permeabilities of ceramic-filled LDPE films arehigher than those of plain LDPE film, and that the temperature dependenceof the permeabilities follows the Arrhenius relationship. The higherpermeabilities make these films more suitable for packaging high respirationrate products. Since ceramic is a filler, it is expected that higher loadings ofceramic filler should yield higher permeabilities.

3.8 Concluding remarks

This chapter provides some practical suggestions for designing MAP offresh produce. The model equations are a time-saving tool to reduce thenumber of experiments and to answer many 'what-if questions. Asmentioned before, the model equations are oversimplified because they donot include many factors such as transpiration of the product, diffusivity ofskin and flesh to O2 and CO2, effect of C2H4, etc. Thus the model predictionsshould be used with an understanding of their limitations, and must alwaysbe verified with experimental data.

Nomenclature

[CO2] % CO2 concentration[CO2J1 % CO2 concentration inside the package at any time[CO2J1 s % CO2 concentration inside the package at steady state[CO2] in Inlet CO2 concentration in flow-through system (%)[CO2]out Outlet % CO2 concentration in flow-through system[CO2]O CO2 concentration outside the package (%); 0% for air[O2] O2 concentration (%)[O2J1 O2 concentration inside the package at any time (%)[O2]in Inlet O2 concentration in flow-through system (%)[C^lout Outlet O2 concentration in flow-through system (%)[Cy^s Steady-state O2 concentration inside the package (%)[O2]o O2 concentration outside the package (%); 2 1 % for aira Conversion factor (1 hr"1)(S Permeabi l i ty ratio, PCQJPO2 (dimensionless)a t , a2 Coefficients ( I r 1 )b j , b 2 , C1, c2 Coefficients (dimensionless)JE1CO2, E02 Activat ion energies for respiration (J /mole)EpCO2, EpOi Activat ion energies for permeabi l i ty (J /mole)F Gas flow rate (ml/h)K1n Michae l i s -Men ten constant (% O2)K1 Inhibit ion constant (%• CO 2 )L Thickness of film (mm)MC O 2 Molecular weight of C O 2 (0.044 kg/mole)M 0 2 Molecular weight of O 2 (0.032 kg/mole)Pa tm Pressure of 1 atmosphere (atm)^co2> ^ 2 Pre-exponential factors for permeabil i ty (mg mil /m 2 h atm)PCO2 Permeability to CO2 (mg mil/m2 h atm)P 0 2 Permeability to O2 (mg mil/m2 h atm)P Pressure in the package or the jar (Pa)R Gas constant (8.314 J/mol K)^Co2' R o 2 Pre-exponential factors for respiration (mg/kg h)RCo2 Rate of CO2 evolution (mg/kg h)RQ2 Rate of O2 consumption (mg/kg h)S Package surface areas (m2)t Time (h)T Absolute temperature (0K)V Free volume in package or in jar (ml)Vm Maximum respiration rate (mg/kg h)W Product weight (kg)

References

Anderson, H.S. (1989) Controlled atmosphere package. US Patent 4842875.Cameron, A.C., Boylan-Pett, W. and Lee, J. (1989) Design of modified atmosphere packaging

systems: modeling oxygen concentrations within sealed packages of tomato fruits. J. FoodScL, 54, 1413-16, 1421.

Deily, K.R. and Rizvi, S.S.H. (1981) Optimization of parameters for packaging of freshpeaches in polymeric films. /. Food Processing, 5(1), 23-41.

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Lee, D.S., Haggar, P.E. and Yam, K.L. (1992) Application of ceramic-filled polymeric filmsfor packaging fresh produce. Packaging Technology and Science, 5, 27-30.

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4 Active packaging in polymer films

M.L. ROONEY

4.1 Introduction

Polymers constitute either all or part of most primary packages for foods andbeverages and a great deal of research has been devoted to the introductionof active packaging processes into plastics. Plastics are thermoplasticpolymers containing additional components such as antioxidants andprocessing aids. Most forms of active packaging involve an intimateinteraction between the food and its package so it is the layer closest to thefood that is often chosen to be active. Thus polymer films potentiallyconstitute the position of choice for incorporation of ingredients that areactive chemically or physically. These polymer films might be used asclosure wads, lacquers or enamels in cans and as the waterproof layer inliquid cartonboard, or as packages in their own right.

The commercial development of active packaging plastics has notoccurred evenly across the range of possible applications. Physical processessuch as microwave heating by use of susceptor films and the generation ofan equilibrium modified atmosphere (EMA) by modification of plasticsfilms have been available for several years. Research continues to be popularin both these areas. Chemical processes such as oxygen scavenging havebeen adopted more rapidly in sachet form rather than in plastics. Oxygenscavenging sachets were introduced to the Japanese market in 1978 (Abeand Kondoh, 1989) whereas the first oxygen-scavenging beer bottle closureswere used in 1989 (see Chapter 8). The development of plastics activepackaging systems has been more closely tied to the requirements ofparticular food types or food processes than has sachet development.

This chapter surveys the range of polymer-based active packagingprocesses that have been reported, their chemical or physical basis and theirapplication to foods and beverages. Attention is given to opportunities for,and obstacles to, either commercialisation or extension of the current rangeof application. Some processes which have become economically importantare treated individually in other chapters.

4.2 Oxygen scavenging

The removal of oxygen from package headspaces and from solution in foodsand beverages has long been a target of the food technologist. Introductionof vacuum packaging and inert-gas flushing has provided solutions for some

Table 4.1 Food characteristics influenced by oxygen scavengers

Characteristic Targets

Microbiological status Moulds, aerobic bacteriaInfestation Insects, larvae, eggsChemical degradation Rancidity, pigment/nutrient loss, browningPhysiological changes Respiration

of the problems of distribution of oxygen-sensitive foods as described byBrody (1989). However, the opportunity to improve on the benefits gainedby application of those technologies, as well as the chance to treat theproblems of distribution of foods individually, has led to the current interestin oxygen-scavenging plastics.

The properties of foods that can be influenced by the presence of oxygenscavengers are shown in Table 4.1. The growth of moulds is particularlyimportant in dairy products such as cheese and in bakery products. Oxygenlevels of 0.1% or lower are required to prevent the growth of many moulds.Bacterial growth and the growth of yeasts can be a problem in high water-activity foods including meats and prepared dishes, as well as in juices.Oxygen scavengers can prevent oxidative damage to flavour and colour in awide range of foods. Likewise, they can maintain atmospheres with oxygenconcentrations too low for insect survival in agricultural and horticulturalproducts. The list in Table 4.2 is indicative of the range of foods whichcould benefit from oxygen scavenging, their type of packaging and the formsin which the scavenger might be applied. The package converter can decidethe nature and quantity of active components used in plastics packaging.This allows the opportunity for tailoring the packaging to the requirementsof the product.

Table 4.2 Potential applications of oxygen scavenging plastics

Product Packaging Component

Aseptic liquids Cartonboard, bag-in-box film, coating, adhesive, inkBakery products Flexibles film, etc.Beverages Flexibles, bag-in-box film, etc.Beer Crown seal liners Resin, organosolCheese Flexibles film, etc.Coffee, tea Closures, flexibles film, etc., resinCereals Flexibles film, etc.Dried Fruit Flexibles film, etc.Dried Foods/Nuts Flexibles, closures film, etc., resinFried snacks Flexibles film, etc.Fruit/Vegetables Flexibles film, etc.Milk powder Flexibles film, etc.Meat - fresh Flexibles film, etc.

- processed Flexibles film, etc.

Pasteurised liquids Closures, bottles resin

Retorted foods Can lacquers, trays, lidding resin, film, etc.

Wagner (1990) lists a wide range of oxygen-sensitive prepared foodswhich are of increasing importance in consumer societies. Some are suitablypackaged using existing processes. However, quality can often be retainedlonger if residual oxygen is removed. This would allow use of differentpackaging materials and distribution systems. Some foods, and particularlybeverages, cannot be stabilised adequately with existing packaging technolo-gies in order to allow use of the full range of desired distribution systems.This is particularly important when reduced levels of additives have beenchosen for regulatory or marketing reasons.

Koros (1990) has set out the maximum quantity of oxygen which ageneralised range of foods can take up and still have a shelf-life of one year.These quantities are generally in the 1-200 mg/kg range. Abe and Kondoh(1989) have shown the need for oxygen removal by in-pack systems whenthe economic limit of around 0.5% is reached in the general case. This figurecan vary in practice as residue levels of around 2% are often encounteredwhen form-fill-seal (ffs) gas flushing is used commercially. Alternatively,less than 0.1% oxygen can be found in vacuum packs of beef primals wheremuscule respiration and bacterial action scavenge oxygen.

The most appropriate method of removal of oxygen from a food packagedepends on the nature of the food, its prior processing and the packagingmachinery and the way it is distributed. The factors which may need to beconsidered, and estimates of efficiency when sachets are used, aresummarised in Table 4.3 which is based on a similar table devised by Hirata(1992), who compared sachet technologies with vacuum and gas-flushpackaging systems. The expected benefits of use of oxygen-scavengingplastics are to minimise the materials cost by matching the quantity ofscavenger to the need, and to keep the filling speed high.

4.2,1 Forms of oxygen-scavenging packaging

In-pack oxygen scavenging involves use of a variety of forms of scavenger.Sachets merely inserted into the food package constitute most of the presentsystems in commerce. Alternatively, the scavenger can be hot-melt bondedto the inner wall of the package. This is done using the Mitsubishi Agelessscavenger sachet attached to the lid of the steamed rice packages

Table 4.3 Comparison of headspace oxygen removal systems

Residual Capital Film FillingSystem O2 kPa investment cost speed

a Vacuum < 1.5 High High Lowb N2 Flush 1-2 Medium High Mediumc a + b < 1.0 High High Lowd Scavenger <0.1 Low Medium Highe b + d <0.1 Medium Medium Medium

manufactured in Japan by Ajinimoto under the Take Out brand name. Theself-adhesive label concept of US company Multiform Desiccants is used inthe Marks & Spencer retail chain's preserved meat packs in the UK.

Beyond the concept of adhesion of sachets to the package lies a widerange of possibilities and it is to this area that much of the recent researchand development has been directed. Alternatives include package inserts inthe form of cards or sheets, or layers coated onto the inner wall of thepackage. An example is the beverage crown seal liner currently used in thebeer industry. This liner was developed by Advanced Oxygen TechnologiesInc. and Zapata Industries Inc. under the name SmartCap and independentlyby W.R. Grace and Company and under the name Daraform. Bothcompanies manufacture in the USA. Alternatively an iron-based system hasbeen developed under the name Oxyguard by Toyo Seikan Kaisha Ltd inJapan. The industrial development and history of oxygen-scavengingclosures is discussed in detail in Chapter 8.

Oxygen-scavenging films or other plastics packaging materials arealternatives to sachets. Low molecular weight ingredients may be dissolvedor dispersed in a packaging plastic or the plastic may be made from apolymeric scavenger. The scavenger may be incorporated into a solid whichis dispersed in the plastic or may be introduced into various layers of thepackage such as in the form of adhesive, printing ink, lacquer or enamel,such as found in cans.

Sachets can be a highly efficient form of oxygen scavenger but theirnature does not favour contact with liquid foods or where cling of thepackage to the film may isolate the sachet from areas of oxygen entrapmentor ingress. In such situations it would be preferable to have the scavenger inthe packaging material. This allows all exposed surfaces of the food to bedeoxygenated or protected from oxygen ingress by permeation.

4.2.2 Plastics packaging as media for oxygen scavenging

Oxygen-scavenging packaging has to date been applied only in packs whichhave inherently a substantial barrier to oxygen ingress by permeation orleakage. Abe and Kondoh (1989) recommend that Ageless sachets should beused in packages with an oxygen transmission rate less than 20 ml/m2/atm/day. Such requirements therefore rule out the common heat-seal polymersand thin layers of the more mediocre barriers like polyester (PET) and nylon6. However, there can be situations such as in transportation of beef primalcuts in which a shorter period of absolute barrier to oxygen is desirable andthese plastics might be used as the barrier. The patent of Speer and Roberts(1993) describes a system involving oxidation of poly(l,2-butadiene) andappears to be directed at that market.

The commonly found oxygen barriers may be in the form of a single-layerpackage, as in the case of the PET or PVC bottle, or jar or cup, but are more

Table 4.4 Permeability coefficients P X 1010 (cm3 mm cm"2 sec"1 cm 1 Hg) @ 300C

Film material N2 O2 CO2

Polyvinylidene chloride 0.0094 O053 029Polyester (Mylar A) 0.05 0.22 1.53Polyamide (Nylon 6) 0.10 0.38 1.6Polyethylene (d = 0.960) 2.7 10.6 35Polyethylene (d = 0.922) 19 55 352Polystyrene 2.9 11.0 88Polyvinylchloride 0.40 1.2 10

(Reproduced from Paine, 1962, with permission.)

likely be part of a multilayer package. Table 4.4 shows the oxygenpermeability of a range of polymers. A more extensive tabulation of filmpermeability values can be found in the review by Bixler (1971).

Oxygen-scavenging compounds can be dispersed or blended in high-permeability films such as plasticised PVC and polyethylene. Where theoxygen scavenger is molecularly dispersed in the polymer it is available tooxygen in its entirety, unlike there case where solid particles are used. Theparticular advantage of such polymers is that they allow rapid diffusion ofoxygen and water (at elevated temperatures) from the headspace or food into the reactive ingredients. Such a characteristic of the polymer helps offsetthe disadvantage of plastics-film-based compositions in comparison withsachets where the scavenging powder has a large reactive surface areaexposed. Polyethylene and polyvinyl chloride (plasticised) are nominated asthe reaction medium in crown closure liners for beverage bottles such as forbeer, as described in detail in Chapter 8.

The use of plastics as media in which to disperse or dissolve oxygenscavengers places a severe limitation on the number of reactions which canbe involved in the scavenging process. Whereas molecules of the size ofoxygen or water can diffuse at an adequate rate, larger molecules behave asif they were immobilised. The fact that a molecule can migrate fast enoughto fail food-contact regulatory tests does not indicate that it can be used inan oxygen-scavenging system. Thus the use of iron oxidation in polymershas been a challenge for some years requiring all reagents to be intimatelymixed. A further challenge is to establish whether the breakdown of ironparticles on oxidation occurs as freely in a polymer matrix as it does inpowder form in sachets. Labuza and Breene (1989) note that virtually all ofthe iron in commercial sachets is available for oxygen removal.

The high gas and vapour permeability of the common heat-seal polymersallows them to be used as reaction media for oxygen scavenging inlaminates. Indeed this has been proposed in a large number of patents suchas those of Hofeldt and White (1989) and Farrell and Tsai (1985). Thepresence of the oxygen scavenger in the heat-seal layer allows maximumadvantage to be taken of the full thickness of the physical barrier to oxygenpermeation of the barrier layer. However, if the scavenger is incorporated

into the barrier layer as in the Ox-bar bottle of CMB Technologies pic(Folland, 1990) that portion closer to the outside becomes exposed torelatively large amounts of oxygen.

The permeability of the polymer medium in which the scavengingreaction occurs need not be the limiting variable determining scavengingrate. Depending upon the chemistry involved, the oxidation reaction can beinherently slow, as with crystalline sulfites at room temperature or thereaction can require the presence of an additional species as with the rustingof iron. The permeability of the heat-seal layer to both oxygen and watervapour can be limiting as in the mixed sulfite/acetate layer in retort pouchespatented by Farrell and Tsai (1985). In this case the water is needed insubstantially larger quantity than the oxygen in order to dissolve thedeliquescent potassium acetate in which solution the potassium sulfitedissolves and reacts with oxygen.

An approach to overcoming any limitation on scavenging rate by the heat-seal layer's permeability has been to use microporous polymers such as non-woven polyolefins. Several recent patent applications describe claims ofenhanced availability to the package atmosphere of granular reagents, as inthat of Mitsubishi Gas Chemical Industry Co. Various sulfites can be held ina fibrous layer sandwiched between, for example, one layer of foil, and asecond of a plastic or paper with an oxygen permeability greater than 7000ml/m2/day atm. A second patent describes a package consisting of threelayers. The outer layer is a plastics film, the middle is a perforated or non-woven layer containing an oxygen scavenger and the inner layer of thepackage is a microporous film which resists the flow of liquids through itspores. This is claimed to be useful for packaging liquid foods (Ohtsuka etal, 1984). Such films cannot be used where transparency is required but mayhave application in many forms of packaging.

Traditionally, poly(vinylidene chloride) copolymers have provided awater-insensitive oxygen barrier when used as a layer in laminates, coatedfilms or coextrusions. The introduction of ethylene-vinyl alcohol copoly-mers (EVOH) and poly(vinyl alcohol) means that, together with traditionalnylons, most oxygen barriers are now water sensitive. The permeability tooxygen of EVOH copolymers increases approximately 10-fold whenexposed to a relative humidity change from about 40-100%. Such a changemay render a material which is suitable for packaging an oxygen-sensitivefood at low relative humidity into one which is most unsuitable at highrelative humidity.

The plasticising effect of water on the barrier properties of EVOH (orother hydrophilic barriers) is time dependent, especially if the hydrophiliclayer is protected by a water-barrier layer such as polypropylene as in thecase of retortable lunch-cups. When such retortable packs containing a wetfood such as an entree are subjected to steam retorting, water is absorbed bythe EVOH in such large quantities that the barrier layer becomes quite

permeable to oxygen. The rate of water release through the outerpolypropylene layer becomes very slow on cooling, so the oxygenpermeability can remain elevated for many weeks (Tsai and Wachtel, 1990).Although addition of desiccants to the polypropylene (Tsai and Wachtel,1990) and mica platelets to the EVOH (Bissot, 1990) is used to reduce thisimpact there is an opportunity here to include an oxygen-scavenger layer inthe coextrusion to absorb the oxygen, particularly during the period ofenhanced permeability. In fact there has been a recent news report of theintroduction of an oxygen scavenger into such packages by Toyo SeikanKaisha Ltd. Thus active packaging has the potential to contribute to solvingthe permeability problem in two ways, providing: desiccants which absorbwater in polypropylene; and oxygen scavengers which remove oxygen whenit does pass through the hydrated barrier layer.

Whereas elevated temperatures and high humidity have been used toadvantage in the research of Farrell and Tsai (1985), the effect oftemperature on the performance of oxygen scavengers in polymer-basedfilms has been reported in only rare cases. W.R. Grace (1994) haveinvestigated the effect of low temperatures on their (optionally photo-sensitised) metal-catalysed oxidation of syndiotactic poly(l, 2-butadiene).They have foumd that this polymer, and certain other low crystallinitypolymers with glass transition temperatures below -15°C, scavenge at least10 ml/m2/day O2 at 100C or lower. The photoinitiated system described byRooney (1994) has been shown to function at 00C.

4.2.3 Brief history of oxygen-scavenging films

The initial oxygen-scavenging packaging film was a multilayer described byKuhn et al (1970) and by Warmbier and Wolf (1976). These systems werebased on the earlier work of King (1955) and Abbott et al (1961) whoapplied palladium metal to the inside surface of can lids. The cans wereflushed with mixtures of hydrogen (8%) in nitrogen to give a mixture inwhich the residual oxygen could react with hydrogen to form water on thepalladium surface.

The reaction of hydrogen with residual oxygen on palladium has beentaken further by Hayashi et al (1986) who vacuum-metallised polyester filmat 2.5 x 10" moles of Pd/m2 and laminated this to high density polyethylene.When a bag made from this laminate was filled with 500 ml of a mixture ofhydrogen-nitrogen 8:92% by volume, this layer was found to be effective incatalysing the conversion of oxygen to water, reducing the oxygen contentfrom 3-4 to 0.4% in 1 day. Due to the expense of this process it would besuitable for packaging high-valued items such as probes for an oxygenanalyser, although production of a thinner metal layer may change futureeconomics.

The earliest investigation involving reaction of oxygen in homogeneous

polymer films was described by Rooney and Holland (1979), Rooney,Holland and Shorter (1981) and by Rooney (1981, 1982). This techniquetakes advantage of the ease of excitation of oxygen from its ground state toits first singlet excited state and has shown that use of polymers asscavengers or reaction media need not inhibit rapid oxygen removal.

Very little investigation of the chemistry of oxygen-scavenging films hasbeen published in peer-reviewed journals. However, numerous patents andsome conference proceedings give sufficient detail to allow a comparison ofthe systems reported. The most evident trend in oxygen-scavenging systemdevelopment during the past 20 years has been the increasing importance ofpatent applications for compositions and designs based on plastics. Very fewof these have involved actual polymer oxidation but rather have required thereactive ingredients to be dispersed within the polymer matrix or to besandwiched between film layers. An examination of patent applicationsworldwide gives the results shown in Figure 4.1 which shows the numbersof initial applications for new compositions or designs without considerationof whether additional applications for the same idea have been lodged inother countries. The histogram shows that whereas initially only sachettechnologies were considered, there was a slow growth in the number ofplastics-based systems devised until the numbers were equal for both typesof system in 1993-94.

The increasing number of plastics-based systems results from a substantialoverall increase in the interest in oxygen-scavenging systems, not from adecrease in the numbers of sachet technology applications. This has resultedfrom a more lateral approach to potential reactions coupled with approaches

Appl

icat

ions

Sachet Plastics

Years

Figure 4.1 Patent applications for oxygen scavengers involving sachets and plastics.

to overcoming previous deficiencies. This is particularly evident in patentapplications for systems involving oxidation of carbon-carbon double bondsin small molecules like squalene and fatty acids or in polymers like rubbers.In each case the transition-metal catalysed oxidation results in developmentof odorous compounds such as low molecular weight aldehydes which areadsorbed by zeolites, carbon or other adsorbents. In one example linseed oil,iron oleate, calcium carbonate and active clay are mixed with activatedcarbon to give a solid oxygen scavenger (Toppan, 1992). The advantage ofthis system is that water is not needed as a reagent. Some compositions, suchas that of W.R. Grace and Co. for oxidation of squalene with a transitionmetal catalyst (Ebner et al9 1992), appear not to include an adsorbent forodorous products. Some patents involve claims of conventional antioxidantsas oxygen scavengers. The use of bifunctional antioxidants at up to 2% inrigid poly(vinyl chloride) was claimed to reduce the permeability of thatpolymer 20-fold although the period of effectiveness was not reported(Wijbrief, 1971). These antioxidants are normally associated with reactionsof primary products of reaction of molecular oxygen with polymers, so themode of action of this process is still uncertain.

W. R. Grace and Co. has applied for patents for the use of ascorbic aciddispersed into plastics such as the common heat-seal plastics or closureliners (Hofeldt and White, 1989). This process relies on the presence ofwater from the food or beverage as well as the presence of isoascorbic acidor a metallic sulfite such as sodium sulfite. The process is now commercialand follows the patent of Farrell and Tsai (1985) who patented thesandwiching of either a sulfite alone, or one mixed with potassium acetate,between the layers of a retortable pouch structure. Several other applicationsfor oxygen-scavenging plastics containing ascorbic acid have subsequentlybeen lodged. One combines oxygen scavenging with the antimicrobial actionof a silver zeolite (Shimagawa Nenryo KK, 1992).

CMB Foodcan pic has developed a novel system for use in blow-mouldedbottles made from PET into which up to 7% MXD-6 nylon had beenblended (Cochran et ai, 1991). An additional catalyst in the form of apolymer-soluble cobalt salt such as cobalt stearate was necessary to causethe nylon to react with oxygen.

An example of the performance of a bottle made from the Ox-Bar(Trademark of CMB Technologies pic) has been given by Folland (1990).The results suggest that beer held in such a bottle would be saved fromoxygen ingress via the bottle wall for at least 12 months thus providingoxygen-barrier performance functionally equivalent to that of glass. As withany form of active packaging, oxygen-scavenging plastics are designed toachieve a specific effect - in this case protection of a packaged food fromoxygen. As it turns out, the shelf-life of carbonated beverages like beer islimited by the loss of carbon dioxide by permeation. Thus the effect of theoxygen-scavenging bottle would be to change the nature of the limiting

variable if used in those countries where beer has a high level ofcarbonation.

This process has not reached the market place since the reaction productsneed to be defined in greater detail. What is particularly interesting in thiscase is that the use of a cobalt salt catalyst in PET appears to have satisfiedsome national regulatory authorities.

42.4 Chemistry of oxygen scavenging

Unlike most other forms of active packaging, oxygen-scavenging films mustbe stable in the oxygen-rich environment of air prior to use. This haspresented a problem to chemists formulating such systems and surprisinglyfew methods of activation appear either in the patent literature or incommercial practice.

In some instances activation might not be a necessary consideration if thepackage is prepared from all constituents immediately before filling. Suchpackaging systems would include blow-moulding of beverage bottles whichoften occurs on the premises of the beverage filler. Thus the catalyst and theoxidisable substrate can be kept apart until the bottle is blown, as in the Ox-Bar process developed by CMB Technologies pic.

A further factor is the nature of the packaging material carrying theoxygen scavenger. In the case of the PET bottle, containing MXD-6 nylonblended into the PET for instance, the oxygen permeability of the bottle is solow that a delay between blowing and closure is quite reasonable. A generalguide to such circumstances is that if the plastic material actually carryingthe oxygen scavenger is exposed to the outside air during its life as apackage the system can probably be chosen not to have an additionalactivation step.

The systems which have been developed for use (or prospective use) incoatings, laminations or other plastics layers with high oxygen permeabilityare normally activated by one of the following mechanisms.

• Supplying a reagent on package filling, viz. hydrogen or water• Supplying water as a solvent or swelling agent on filling.• Continuous exposure to light as energy source.• Brief exposure to light for activation of

o chain reactions, e.g. autoxidationo rapid photoreduction of scavenger precursor

In each of the above processes several variants exist. For instance, the watercan come from the food itself as described in the Oxyguard process of ToyoSeikan Kaisha Ltd, or in the variety of patents referred to in Chapter 1. Thisrequirement is elaborated in the discussion on sachet technologies in Chapter6.

Water needed as a reaction solvent or to burst micro-capsules can besupplied from the retorting steam in the case of retortable plastics packageswhich are processed at around 1200C. In this case water vapour permeationcan be sufficient from the outside of the pack as well as from the food itselfand here heat is also a trigger for commencement of the process. Tsai andWachtel (1990) have shown that EVOH trays can take up as much as 2.8%extra water on retorting, and this diffuses out slowly over a period of someweeks.

4.2.4.1 Supplying a reagent. The oxidation of hydrogen by molecularoxygen requires metal catalysis at room temperature. Indeed such a processhas long been used by microbiologists to generate low-oxygen-contentenvironments for cultivation of anaerobic microorganisms. Conventionally amixture of hydrogen (8%) in nitrogen is either flushed into the cultivationchamber or hydrogen is generated in a sealed anaerobic jar in the presenceof an air headspace. The surface of palladium metal either as mesh ordeposited on porous alumina catalyses the reaction to form water.

The application of this process to packaging by King (1955) and Abbottet al. (1961) was particularly successful for removing oxygen which desorbsfrom canned spray-dried milk powder. The hydrogen content of the flushinggas is limited to 7% or less to avoid the risk of explosion. It should be notedthat an explosion limit of 6.5% hydrogen is specified in plants for productionof chlorine by electrolysis.

The laminate bag described by Warmbier and Wolf (1976) consists of apolyester outer layer bonded to aluminium foil then Surlyn. Between thisSurlyn and the heat-seal layer, also of Surlyn, is sandwiched a layer ofpowdered alumina upon which palladium metal has been deposited. The bagwas flushed with the above-mentioned H2ZN2 mixture before heat sealing.On storage the milk desorbed oxygen which diffused, together with thehydrogen, through the Surlyn to react on the catalyst surface. The quantity ofwater which was formed was calculated by the authors to be insufficient toaffect the food. There appears to have been very little use of this system,marketed as Maraflex 7F by American Can Company, with foods althoughit was used in the US space programme.

The oxidation of iron in the presence of electrolytes has been wellestablished in the sachet technologies reviewed in Chapter 6. Where patentsdescribe incorporation of treated iron powder into plastics, water ingress isgenerally needed. Some specify the need for such materials to be used underretorting conditions. This is probably for the purpose of introducingsufficient water on to the iron surface to carry out the reactions described inChapter 6. It is doubtful whether an adequate rate of oxygen scavenging canbe achieved when the polyolefin heat-seal plastics are used as the reactionmedium at room temperature.

4.2.4.2 Supplying water as a solvent or swelling agent. Some packagingapplications potentially permit the use of scavenging components asheterogeneous additives to the packaging material. Particularly wherereagents are in the crystalline form or coated with a low-permeabilitysubstance, the rate of reaction with atmospheric oxygen can be acceptablylow for incorporation into packaging. When the package is exposed to highhumidity and elevated temperatures the rate of transmission of water fromeither the food or the outside (e.g. as steam) can be sufficient to dissolve thereagent thus enabling oxidation to proceed rapidly.

An early example of the dissolution of a salt was patented by Farrell andTsai (1985). They incorporated a mixture of potassium acetate and sodiumsulfite crystals (or potassium sulfite alone) between the barrier and heat-seallayers of a retort pouch laminate which had five layers overall. Thepotassium acetate is deliquescent and absorbs sufficient water to dissolvewhen the food-filled pouch is retorted. The sodium sulfite dissolves in thepotassium acetate solution and reacts with oxygen diffusing into thissolution from the food and especially from the retort atmosphere. The watervapour permeability of the laminate can increase 1000-fold from 210C to121°C. The sulphite reacts with oxygen as in Equation 4.1.

2Na2SO3 + O2 -> 2Na2SO4 (4.1)

Although this process can result in the presence of an aqueous solution inthe laminate, it should be possible to consider use of some form of binder forthe solution after retorting. A process such as this which requires both heatand high humidity is really limited to applications which involve retorting orsubstantial heat-treatment of the packaged food.

A recent patent describes the incorporation of a metallic salt and acomplexation agent in separate microcapsules within a polymer film (Zenneret al.9 1992). On exposure of the film to an atmosphere of high relativehumidity, as in the headspace of some food packs, the microcapsules absorbwater and swell. This results in bursting of the microcapsules and mixing ofthe complexing agent and the salt under moist conditions. The resultantmetal chelate forms an oxygen adduct with a binding constant greater than105M'1. The presence of such a strong adduct is effectively equivalent tooxygen removal by means of an irreversible reaction.

The reaction of oxygen with a metal complex can also involve a weakcoordinate bond without converting the metal ion to a higher oxidation state.This type of reaction is the basis of blood oxygenation and can frequently bereversed, at least to some extent (see Chapter 1). This type of oxygenabsorber was the subject of extensive research by Aquanautics Inc in theUSA, the predecessor of Advanced Oxygen Technologies Inc.

Ascorbic acid and isoascorbic acid react readily with oxygen, morerapidly at high pH. The use of metallic salts of these acids is described in

patents from many sources. The chemistry of oxidation of ascorbic acid iscomplex, consisting of a series of consecutive and concurrent reactions,depending upon the conditions. The steps involved in the commerciallyaccepted scavenging compositions have not been described in the scientificliterature. It has been observed in the author's laboratory that celluloseacetate films containing dissolved ascorbic acid turn light-brown in colourafter standing in darkness under ambient temperature and humidity condi-tions for one to two years.

The steps involved in ascorbic acid oxidation in solution have beenreviewed by Tannenbaum (1976) who summarised the reactions involved inthe oxidation of ascorbic acid. A simpler summary is shown in Figure 4.2.The initial step is the formation of dehydroascorbic acid and this step isstrongly dependent upon pH. Ascorbic acid can be regenerated by reactionwith mild reducing agents such as metallic sulfites. It is particularlyinteresting that the patent of Hofeldt and White (1989) describes the optionaluse of sodium sulfite in combination with ascorbic acid (and its isomers) intheir oxygen-scavenging closure liner for use with beverages such as beer.There is thus the potential for use of ascorbic acid as an intermediate in the

HOHC

CH2OH

2NaOH

HOHC

BROWNPRODUCTS

Figure 4.2 Oxidation of ascorbic acid.

oxidation of sodium sulfite to the sulfate. Whether this occurs in the ambientor low-temperature storage of beer has not been described in the literature.Given the heterogeneous nature of the reaction mixture it is doubtfulwhether this desirable regeneration occurs to a very great extent incommercial practice without the formation of an aqueous solution asdescribed by Farrell and Tsai (1985).

As can be seen from the summary in Figure 4.2 one of the reactionproducts of oxidation of ascorbic acid to dehydroascorbic acid is hydrogenperoxide or initially the hydroperoxy free radical. Further hydrogenabstraction by this radical forms hydrogen peroxide. The use of sodiumsulfite, or another reducing agent, would therefore seem to be desirable if theultimate re-introduction of an oxidising agent is to be avoided. The role ofascorbic acid as a promoter of browning when oxidised in foods is alreadyknown (Tannenbaum, 1976).

The use of ascorbic acid or its isomers or salts is the basis of the W.R.Grace and Co. and Zapata Inc oxygen-scavenging closures in common usein some beer bottles.

4.2.4.3 Continuous exposure to light as an energy source. Some oxida-tion reactions which do not occur when oxygen is in its unexcited (ground)state can be brought about by the process of photosensitisation, whichinvolves transfer of visible light energy to oxygen via the intermediacy of adye. A polymer film can be the medium for these photosensitised oxygen-scavenging reactions. Such a film must contain a photosensitising dye and anelectron-rich oxidisable compound termed a singlet oxygen acceptor. Theoxygen-scavenging process occurs when the film is illuminated with UV,visible or near infra-red irradiation of appropriate wavelengths. The steps inthe process are shown below, and these occur within the matrix of thepolymer film.

(4.2)

(4.3)

(4.4)

(4.5)

(4.6)

(4.7)

When a suitable photosensitiser, D, absorbs light (hx>) it is excited to ashort-lived higher energy singlet state, *D, (Equation 4.2) which largelyconverts to the longer-lived triplet excited state, 3D, (Equation 4.3). In thisform the dye can pass the excitation to oxygen by the process of triplet-

triplet energy transfer (Equation 4.4). The oxygen diffusing into the polymerfrom the package headspace (for instance) needs to come very close to theexcited, immobile dye molecules during the triplet lifetime of the latter. Thisis of the order of 10-1000 microseconds. Thus, for such a process to beuseful the polymer matrix (such as an inner layer of a laminate) would needto be very permeable to oxygen.

Once excited to its singlet state the oxygen can react with any electron-rich acceptors, A, present in the polymer matrix (Equation 4.5) providedthey are within the distance the singlet oxygen can diffuse before it isquenched back to the ground state (Equation 4.6). The process occurs onlywithin the lifetimes of the excited sensitiser (Equation 4.7) and singletoxygen and so requires continuous illumination. The distance singlet oxygencan diffuse before quenching in a polymer matrix is of the order of IOOAdepending on the polymer permeability and other factors (Turro et a/.,1981).

This chemistry has been used in the laboratory as an oxygen-scavengingprocess where the polymer matrix is, for instance, cellulose acetate or ethylcellulose (Rooney et al., 1981; Rooney, 1982). The sensitisers includeerythrosine or me^o-tetraphenylporphine and the acceptors are bis(fur-furylidene)pentaerythritol and ascorbic acid. It was found that the permeabil-ity of the polymer film is an important determinant of scavenging rate, withthe ethyl cellulose being a better matrix for rapid scavenging than celluloseacetate. Ethyl cellulose has an oxygen permeability coefficient at 25°C of 7Barrers (cm3 (STP) cm/cm-2 s"1 cm"1 Hg x 10~10) compared with that of 0.7Barters for cellulose acetate (Bixler, 1971). It was also found that the rate ofscavenging of oxygen from a pouch by the film is limited by light intensityinitially but becomes diffusion limited when the oxygen partial pressurereaches low values (Rooney et al.9 1981). The light intensity used was 2 x105-7 x 105 Lux. It was subsequently shown that the rate of oxygenscavenging reaches a maximum at a concentration of tetraphenylporphineabove 10~3 M but less than 10~2 M. When the scavenger film was used as aroll it was found that the rate was dependent upon the length of film in theroll consistent with increased access of oxygen to the film.

The singlet oxygen acceptor does not need to be a small moleculedissolved in the polymer. It has been shown the double bond of naturalrubber can be photo-oxidised at a rate sufficient to bring about rapid oxygenscavenging from the headspace of a package. Figure 4.3 shows the rate ofscavenging of oxygen from air, 20 ml, in a 10 cm x 10 cm pouch coated onthe inside with either natural rubber dyed with tetraphenylporphine, 10~3 M,or ethyl cellulose 0.6 M with respect to PEF and 10~3 M with respect totetraphenylporphine. The rubber scavenged oxygen substantially faster thanthe PEF in the ethyl cellulose. This was interpreted as being due to thehigher concentration of double bond (13.5 equivalentsflitre) in the rubberthan the concentration of furan rings (1.2 equivalents/litre of film) in the

(bifunctional) PEF, although the higher permeability of rubber towardsoxygen (24 Barrers) than that of ethyl cellulose (7 Barrers) is also likely tocontribute. It is significant that the reaction occurring in natural rubberoccurs on the polymer chain and therefore is not dependent on the freerotation of the acceptor. The photo-oxidation of the tacky natural rubberlayer resulted in rapid crosslinking as indicated by the loss of tack withinminutes. An unpleasant odour was also generated. The oxidation of therubber continued on dark storage resulting in formation of a brittle powderyfilm.

The potential for use of synthetic rubbers in place of natural rubber wasinvestigated in order to determine whether the nature of the rubbermonomers affects the rate of scavenging. The results in Figure 4.4 show therate of oxygen scavenging by poly(dimethylbutadiene) (PDMB), cis-polybutadiene (PB) and ra-polyisoprene (PI). Based on the reactivity ofsimple low-molecular weight analogues of these polymers, it would beexpected that the more highly methylated rubber poly(dimethylbutadiene)would react more rapidly with singlet oxygen. However, the inverserelationship is observed, presumably due to the low oxygen permeability ofPDMB of 2.1 Barrers compared with values of 20 Barrers and 24 Barrers forPB and PI. Thus permeability rather than reactivity can be the important

% O

xyge

n

Time (min)

Figure 4.3 Oxygen scavenging by natural rubber • , and PEF 0.5M in ethyl cellulose a, both dyedwith tetraphenylporphine, 103M. Volume of air, 20ml, pouch area 100cm2. (Reprinted from

Rooney, 1982b, with permission.)

time (minutes)

Figure 4.4 Oxygen scavenging by polydienes, poly(dimethylbutadiene) +, cis-polyisoprene O, cis-polybutadiene A. Coatings on inner surface of pouches of area 100 cm2, volume of air 20 ml.

variable in an oxygen-scavenging polymer system as was found to be thecase with PEF dissolved in cellulose acetate and ethyl cellulose.

The use of multiple dyes in oxygen-scavenging films has been investi-gated, since illumination with white light of polymer films containing onlyone dye is wasteful of potentially usable energy (Rooney et ai, 1994). It hasbeen found that even dyes which are not photosensitisers can harvest energyand transfer this energy to a photosensitiser also in the film. Thus curcumine,a natural food colour and a poor photosensitiser (Chignell et ai, 1994), wasshown to enhance the rate of oxygen scavenging photosensitised by eosinein ethyl cellulose. The effectiveness of such a film in suppressing rancidityin sunflower oil has been investigated and found to be substantial at 23 0Cand 37°C (Maloba et al., 1994).

oxy

gen

(%

)

4.2.4.4 Brief exposure to light for activation.

4.2.4.4.1 Chain reactions. Continuous exposure of a package to lightplaces an intolerable restriction on the way in which most foods would needto be stored and distributed. It is not surprising therefore that the use of lightto activate reactive polymer-based systems has been investigated. Theproblem, inherent in all oxygen-scavenging systems, of using compoundsreactive towards oxygen but stable towards thermal processing of thepackaging material needs to be addressed.

The use of autoxidation of fatty acids for oxygen scavenging has alreadybeen claimed but this process seems likely to result in formation of the samevolatile oxidation products which are undesirable in foods (Frankel, 1982).Some sachet patent applications describe the use of adsorbents in the sachetsto bind volatile off-flavours generated in this way. Mitsubishi Gas andChemical Company has claimed that activated alumina, silica gel or charcoalabsorbs any odour formed on the oxidation of fatty acids or oils catalysed bytransition metal compounds in the presence of alkaline earth bases (Inoueand Komatsu, 1988). Toppan Printing Company (1992) claimed a similarcomposition involving addition of activated clay. The benefit of suchsystems is independence from water for the reaction. The research effort putinto development of photodegradable plastics packaging over the past twodecades has provided a background to one approach to this problem

Rabek and Ranby (1975) show how a transition-metal metal salt and aphotosensitiser dispersed in a plastic can cause it to degrade in darknessonce exposed to sunlight for some days. This form of photodegradationinvolves a substantial extent of hydrogen abstraction from the polymerbackbone coupled with hydroperoxide formation, particularly on tertiarycarbon atoms. Subsequent breakage of the hydroperoxy bond leads toformation of either keto or aldehyde groups which become additionalphotosensitisers or result in polymer chain scission. It is in the prevention ofthe latter reaction that the opportunity for development of oxygen-scavenging polymers exists.

The scavenging of oxygen including oxidation of hydrocarbon polymersimpregnated with salts of transition metals has been described by Speer et al(1993). The novelty of their method is that the oxidation process is activatedby brief exposure to light of wavelengths less than 750 nm. The optional useof a photosensitiser is reported to increase the rate of activation, particularlyin the presence of an antioxidant.

As with the photodegradable plastics processes, the chemistry involvesinitiation of the free radical process on the polymer chains by changes in theoxidation state of the transition metal ion, preferably cobalt. The redoxreaction on the polymer chain involves either hydrogen abstraction inpolypropylene or polyethylene or oxygen attack on the double bond in thecase of poly(l,2-butadiene).

The hydroperoxide formed by reaction of either the polymer free radicalor by direct attack on the polymer decomposes to form ketones, aldehydes,peroxides or carboxylic acids, in some cases with chain scission. In the caseof poly(l,2-butadiene) it is the aim to localise the oxidation to the side chaincontaining the double bond. Speer et al. (1993) report that this polymerretains its physical properties even when most or all of its oxidisable groupshave been reacted. Since this is a typical autoxidation that is also thermallyactivated, the polymer needs to be stabilised against premature initiation inthe extruder or moulding machine in package fabrication. Antioxidantsprovide this protection. The quantity of antioxidant is chosen both to providethis protection and to retard the autoxidation chain reaction for a selectedperiod of time between activation and package filling. Removal ofantioxidant from the polymer appears likely to increase the likelihood oftaint formation due to unrestricted polymer oxidation. Attention shouldprobably be given to inclusion of odour absorbents in the composition asproposed by Toppan (1992).

The manner in which such a system could be used is particularly broadbut one suggested is to laminate or coextrude the reactive layer with anoxygen-barrier layer. Such an application would be appropriate for manypackaging processes such as in the storage of cheese, nuts and meats such asbeef primal cuts.

The use of such compositions at the low temperatures required for storageof meats and cheese requires that the reactive layer be readily permeable tooxygen. Speer and Roberts (1993) have specified that the compositionscontaining their oxidisable components should be largely amorphous andhave a glass transition temperature below -15°C.

4.2.4.4.2 Rapid photoreduction of a scavenger precursor. Researchinto systems involving this form of triggering indicate that this approachovercomes some of the limitations of other methods already proposed(Rooney, unpublished results).

4.2.5 Chemical barrier to oxygen permeation

Implicit in the use of oxygen scavenging inserts (sachets) or closures is theeffect these have on the consumption of oxygen as it enters the productduring its storage life. The results in Table 4.5 show the calculated impact of

Table 4.5 Calculated life of chemical oxygen barriers

OTR Barrier lifeFilm cm3 m~2 day"1 atm~l (days)_ _ _ _ _ _ _

PET 25 nm/LDPE 25 \x.m 63 56PVDC-coated OPP 25 jjum 9 362Metallised PET 12 ^m/LDPE 25 |xm 0.5 7250

the oxygen transmission rate of packaging materials on the period ofeffective oxygen scavenging by a commercial oxygen-scavenging sachetwith an absorption capacity of 50 ml of oxygen. The packages are 10 cm x20 cm and initially contain 100 ml air. LDPE is low-density polyethylene,OPP is oriented polypropylene, PET is polyester and PVDC is poly-(vinylidene chloride). However, in a food-packaging situation where thefood is tightly packed, the sachet can be expected to deoxygenate only asmall portion of the headspace and combat permeation of oxygen that isaccessible to it.

Close-fitting packages such as vacuum packs for block cheese and meatsor aseptic cartons of beverages are examples where the headspace is verysmall and oxygen permeation is the prime cause of quality loss. It is in suchcircumstances that oxygen-scavenging plastics films are particularly needed.The use of the oxygen-scavenging reaction to intercept oxygen diffusingthrough the package wall is an example of a chemical barrier as distinct fromthe physical barrier normally provided by aluminium foil and vacuummetallising or crystalline polymers such as poly(vinylidene chloride).

The use of a chemical oxygen barrier offers the opportunity to cheapenbarrier packaging for relatively short shelf-life products such as wholesale orexport units or for some fresh foods. This can be achieved by using a barrierfilm of intermediate performance coupled with a chemical barrier. Thiscombination was investigated by Rooney and Holland (1979) and the resultsin Figure 4.5 demonstrate a period of total barrier to oxygen permeationfrom air at 25°C. A laminate of polyethylene/nylon 6/cellulose acetatecontaining oxygen-scavenging reagents was used to separate the twocompartments of a glass permeability cell. When the ethyl cellulose layercontained 0.5 M fe-furfurylidenepentaerythritol, illumination with fluores-cent light resulting in a barrier life of around 30 days was followed by aperiod of oxygen transmission at a rate less than that found in darkness. Inthis case the chemistry used was a singlet oxygen reaction which requiresconstant illumination to photo-excite the oxygen. Other chemistries can beused to achieve similar reactions in darkness and Speer et al (1993)identified this application for their photoinitiated chain reactions in rubbers,and Cochran et al (1990) devised the Ox-Bar process largely for thispurpose.

The use of oxygen-scavenging sachets to allow the use of a cheaperpackaging material has been described by Alarcon and Hotchkiss (1993) andtheir work is reviewed in Chapter 6.

Although use of multilayers of plastics films appears the most appropriateapproach to solving permeability problems, there have been early attempts tocarry out scavenging reactions in liquid layers. Oxygen scavenging in liquidlayers was one of the earliest processes described in the patent literature.Cook (1969) claimed that bilayers of plastics films separated by a layer of asolution of one of several antioxidants in high boiling point solvents

Time (days)

Figure 4.5 Barrier to oxygen permeation of polyethylene/Nylon 6/cellulose acetate laminate.Cellulose acetate 26 |xm thick containing PEF (0.5 M) and erythrosine, 5 X 10~3 M. Oxygenconcentration measured in a cell on the cellulose acetate side of the laminate, air on the other

side.

demonstrated a reduced oxygen permeability. Whereas the mechanism wasunknown, all of the common antioxidant classes were claimed to beeffective.

Use of an aqueous solution of an oxygen-scavenging reducing sulfite as alayer between two polymeric layers has been claimed by Scholle (1976). Aswith the organic system of Cook (1969), the purpose was to reduce theoxygen permeability of the laminate. In the case of Cook's system the roleappears to have been to improve shrinkable packaging, as for meat, whereasthe Scholle system was particularly suitable for bag-in-box liners since thepackages consist of, at least, a collapsible duplex which is sealed togetheronly at the edges where the liner forms a bag (and around the tap fitting).Such a system requires an oxygen scavenger for oxygen-sensitive productsdue to the gas space between the two film layers.

4.3 Moisture control films

Moisture affects the gas and vapour permeability of hydrophilic plasticspackaging films (Davis, 1966). Fruit cakes packed in films with anappropriate water vapour permeability can have long shelf-lives because thesurface dries somewhat. This creates conditions unfavourable to mouldgrowth. Nitrocellulose-coated regenerated cellulose has been used because it

in darkness

illuminated

% O

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Time (days)

Figure 4.5 Barrier to oxygen permeation of polyethylene/Nylon 6/cellulose acetate laminate.Cellulose acetate 26 |xm thick containing PEF (0.5 M) and erythrosine, 5 X 10~3 M. Oxygenconcentration measured in a cell on the cellulose acetate side of the laminate, air on the other

side.

demonstrated a reduced oxygen permeability. Whereas the mechanism wasunknown, all of the common antioxidant classes were claimed to beeffective.

Use of an aqueous solution of an oxygen-scavenging reducing sulfite as alayer between two polymeric layers has been claimed by Scholle (1976). Aswith the organic system of Cook (1969), the purpose was to reduce theoxygen permeability of the laminate. In the case of Cook's system the roleappears to have been to improve shrinkable packaging, as for meat, whereasthe Scholle system was particularly suitable for bag-in-box liners since thepackages consist of, at least, a collapsible duplex which is sealed togetheronly at the edges where the liner forms a bag (and around the tap fitting).Such a system requires an oxygen scavenger for oxygen-sensitive productsdue to the gas space between the two film layers.

4.3 Moisture control films

Moisture affects the gas and vapour permeability of hydrophilic plasticspackaging films (Davis, 1966). Fruit cakes packed in films with anappropriate water vapour permeability can have long shelf-lives because thesurface dries somewhat. This creates conditions unfavourable to mouldgrowth. Nitrocellulose-coated regenerated cellulose has been used because it

in darkness

illuminated

% O

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prevents contamination with moulds and allows substantial water vapourpermeation.

There are, however, several sets of circumstances in food packagingwhere greater selectivity in control of liquid and gaseous water is required.These include:

• Transpiration of horticultural produce.• Melting of ice, e.g. in fish transportation.• Temperature fluctuation in high erh food packs.• Drip of tissue fluid from cut meats and produce.

The problems which arise from these circumstances are the result ofbuild-up of liquid water. Alternatively, the natural water level in the tissuecan be undesirably high near the surface for microbiological stability underthe chosen storage conditions. The results of the presence of water inunwanted quantities include bacterial (yeast) and mould growth as well asfogging of films and mobile blood and tissue fluid. Moisture migration viathe vapour phase can result in transient formation of regions withoutadequate preservative or where the aw is high even though the food is packedat lower aw. Some of the ways of addressing these problems via packagingare considered in detail in terms of either liquid water control or humiditybuffering.

4.3.1 Liquid water control

Temperature cycling of high erh foods has led most film manufacturers touse heat-seal plastics with anti-fog additive. These additives are blendedwith the resin before extrusion and migrate to the surface after filmformation. The additives are chosen for their amphiphilic nature with thenon-polar chain in the plastic and the polar end group at the interface. Theresult is a lowering of the interfacial tension between water condensate andthe plastic film. The fog droplets therefore coalesce and form a transparentfilm on the plastic. This film may even flow on sloped surfaces and gather atthe bottom of the pack in extreme circumstances.

Anti-fog treatments are a cosmetic form of active packaging, assisting thecustomer to see the packaged food clearly. There is no change in theavailability of liquid water in the package and this has the potential to causeproduce spoilage unless managed by one of the processes described below.Other active packaging systems go further in removing liquid water fromfood contact.

Several companies manufacture drip-absorbent sheets, which may have avariety of other names. Basically they consist of two layers of a microporousor non-woven polymer, such as polyethylene or polypropylene, betweenwhich is placed a superabsorbent polymer in the form of free-flowinggranules. The duplex sheet is sealed at the edges and is normally quilted to

allow the water absorbent to be held in place rather than aggregating towardsone edge of the sheet when tilted towards one edge.

Such sheets are then used either as pads under whole chickens or chickenpieces to absorb drip, thereby preventing discoloration of either the meat orthe white foam tray. Given the interest in reducing the volume of solid wasteby decreasing foam polystyrene usage, the use of drip-absorbers maybecome even more necessary to prevent water damage to the morehydrophilic alternatives.

Large sheets are used for absorption of melted ice in the packaging ofseafood for air transportation. The superabsorbent polymer used in theThermarite® Pty Ltd sheet manufactured in Australia is capable ofabsorbing at least 100 times and possibly as much as 500 times its ownweight of liquid water depending dramatically upon salinity (Malouthi et al.,1994). Another product of this type is Toppan Sheet manufactured in Japan.A recent patent application in Japan describes a water-absorbent sheet whichhas a high barrier to oxygen for use with fresh meat (Showa Denko KK,1990), This would necessitate use of a sealed outer pack.

The preferred polymers used to absorb the water are polyacrylate salts,although graft copolymers of starch can also be used. Such copolymersconsist of a polysaccharide backbone with synthetic polymer chainsradiating from the starch backbone. The strong association of the poly-saccharide chains is disrupted allowing the starch to exercise its affinity forwater by hydrogen bonding. Such polymers tend to become slimy whenswollen with large amounts of water.

The swelling of the polymer on hydration results in substantial distortionof the duplex sheet, an effect that is controlled somewhat by the quilted sealpattern. This effect can be controlled by choice of the amount and thecapacity of the superabsorbent polymer.

The significance of such active packaging in seafood shipment by air liesin the removal of the potential for spillage of salt water from cartons inaircraft holds. The aluminium construction of aircraft makes them easilysusceptible to corrosion damage, at great cost.

4.3.2 Humidity buffering

An entirely different approach to the control of excess moisture in foodpackages is to intercept the moisture in the vapour phase. This approachallows the food packer or even the householder to reduce the surfaceconcentration of water in a food by reducing the in-pack relative humidity.This can be done by placing one or more humectants between two layers ofa plastic film which is highly permeable to water vapour. An example of thistype of product is Pichit manufactured by Showa Denko in Japan. The filmduplex is described as containing an alcohol, described by Labuza (1989) aspropylene glycol and a carbohydrate, both of which are humectants.

Table 4.6 Water vapour uptake by Pichit and Toppan sheet

YimQ Weight loss from petri dish (%)

(h) Pichit Toppan sheet__ -_ _

3 2.4 0.74 3.6 1.2

24 15.2 8.426 22.0 8.8

The effectiveness of such a combination material in comparison with asuperabsorbent polymer absorbent for liquid water is shown in Table 4.6.Distilled water 5 g was placed in each of two petri dishes (with lids) andthese were enclosed with either Pichit or Toppan Sheet (450 cm2) in a high-barrier package. The packages were opened at intervals and the petri disheswere weighed to give the per cent loss of weight of water, shown in Table4.6. Thus, whereas the superabsorbent polymer has a far greater capacity forliquid water uptake, the humectant-based film more rapidly absorbs watervapour.

Pichit is marketed for home use in roll or single sheet form for wrappingpieces of flesh food such as fish or chicken to reduce the aw proximate to thefood. At present there is a lack of experimental verification of thesignificance of this effect. Louis and de Leiris (1991) suggest two to threedays of fresh storage is possible. It has been suggested by Labuza (1989)that the permeability of poly(vinyl alcohol) to propylene glycol may besufficient to allow some of the latter to diffuse to the food surface. If this isthe case, some antimicrobial action might be expected. Propylene glycol isa GRAS substance in the USA.

Louis (1991) reports the availability of an additional moisture control filmwhich plays the same role as Pichit in the Japanese market. The potential foruse of such materials for quality retention in the home appears to beconsiderable and this type of film should be subjected to detailed objectiveevaluation for use in the domestic situation where excess portions of fleshfoods need refrigerated storage for some days. The potential for washing thesurfaces of sheets like Pichit, followed by re-use, is an attractive incentivefor active packaging use in the home provided absence of microbialcontamination can be demonstrated.

A different approach to humidity buffering has been under investigationfor use in the distribution of horticultural produce. Currently the packagesare wholesale fibreboard cartons of produce, usually with a polyethyleneliner or consisting of the very expensive waxed fibreboard without a liner. Arecent development has been the water-barrier coating of the inside offibreboard cartons to allow moist produce to be placed directly into thecarton. Besides the introduction of liquid water with the produce, packinginto closed spaces allows the build-up of water vapour. The produce

continues to lose water by evaporation during distribution and the relativehumidity in the lined carton can reach close to 100%. Since temperaturecycling is very difficult to avoid during handling, there is every likelihood ofcondensation, and, with this, the growth of microorganisms on fruits andvegetables.

Two widely different approaches have been taken to buffering thehumidity in the cartons in order to prevent condensation while notconcurrently causing desiccation of the produce. One is to includemicroporous bags or pads of inorganic salts and the other is to line the cartonwith a protected layer of a solid polymeric humectant.

The approach of using microporous sachets of inorganic salts has beenused in the US tomato market in recent years. This was proposed by Shiraziand Cameron (1992) who showed that an equilibrium relative humidity ofaround 95% above produce can be reduced to around 80% by use of sachetsof desiccant salt such as sodium chloride. The application was extended byHudson (1991). Both these studies involved use of heat-sealed liners whichwere concurrently being evaluated for their performance to maintainequilibrium modified atmospheres (EMA) generated by the fruit (seeChapter 3). The control of moisture is one of the main impediments to theintroduction of EMA packaging. Indeed it is common to package potatoes inperforated sacks to prevent unwanted build-up of condensation.

The most recent alternative involves the use of the carton as the activepackage rather than an insert. This approach lends itself to combination withEMA generation less readily as the humidity is buffered at the interface withthe fibreboard. The designs of Patterson and Joyce (1993) involve: anintegral water vapour barrier layer on the inner surface of the fibreboard; apaper-like material bonded to the barrier and which acts as a wick; and alayer highly permeable to water vapour but unwettable next to the fruit orvegetable. The latter layer is spot welded to the layer underneath.Accordingly, the multilayer of material on the inside of the carton is able totake up water in the vapour state when the temperature drops and the RHrises. When the temperature rises the multilayer releases water vapour backinto the carton in response to a lowering of the RH. The condensationcontrol system therefore acts as an internal water buffer. The criticalcharacteristic of the system is the capacity of the wick layer for water.

The performance of this system is demonstrated in Table 4.7 taken fromPatterson et al. (1993) which shows the results of a comparison of the free

Table 4.7 Free water in carrot packs on cooling

Cooled from Cooled fromPackaging 10 to 3°C 22 to 3°C

Polyethylene liner 10.1 (0.6)* 22.9 (0.7)*Condensation control carton 0.0 (0.0)* 0.2 (0.2)*

•Standard error of the mean (n = 5)

water found in cartons of carrots with a conventional liner with thecondensation control multilayer. The cartons containing carrots, 6 kg, werecooled to 3°C from either 100C or 22°C and after 3 days the amount of freewater in the boxes and on the carrots was measured. The results demonstratethe capacity of such a system for water uptake under circumstances likely tobe found in commerce.

Shrink wrapping is an alternative approach to the use of active packagingsystems for control of condensation on spherical fruits such as citrus. Ben-Yehoshua (1989) has reviewed this field, to which he has made such a largecontribution.

4.4 Removal of taints and food constituents

The interaction of packaging plastics with food aroma has long beenrecognised, especially through the flavour 'scalping' which is of consider-able commercial interest. Hirose et al. (1989) demonstrated the impact of thenature of the metal ion in Surlyn film layers in aseptic brick-packs on thescalping of limonene from orange juice. In periods as short as 2 weeks at24°C, almost 30% of the limonene was found in the Surlyn-1601 and 20%in the polyethylene layer in brick packs. Commercially, plastics packaginghas not been used to remove selectively components of the flavour or aromaof foods which are considered undesirable, but a potential opportunity hasbeen available for over a decade.

Some varieties of orange, such as the Navel, contain a tetraterpenoid ofthe formula C26H30O8 which is initially present in the albedo but which isextracted into the juice on standing or heating. Thus the juice of suchoranges becomes bitter on pasteurisation when this compound, limonin,reaches concentrations exceeding 8-12 mg/kg (Chandler et al, 1968).Processes have been developed for debittering such juices by passing themthrough columns packed with cellulose triacetate or nylon beads (Chandlerand Johnson, 1979).

A simple active packaging process was suggested by the same authors,who proposed that since the juice extracts the bitter principle on standing for24 h, inclusion of the absorbent in the packaging might remove it as it isextracted. To this end they proposed using their absorbents in film form suchas cellulose triacetate or as acetylated paper. They showed that a 1-litreplastic bottle coated internally with cellulose acetate-butyrate reduced thelimonin content of 500 ml of juice from 42 to 11 mg/kg after 3 days'refrigeration. Similarly, when cans lined with acetylated filter papercontaining juice with 14.9 mg/kg limonin were spin cooked and allowed tostand, the juice was only slightly bitter after 4-13 days.

It appears that this process has not been taken up commercially althoughit offers considerable potential for freshly squeezed Navel juice marketing

(Johnson, R.L., private communication). On the other hand, the sorption oflimonene oil from packaged juices by heat-seal layers has been the subjectof several studies (Mannheim et al, 1989; Hirose et al, 1989). Thusopportunities for active packaging can be closely related to problems offood-package interactions.

A closely related goal had long been achievable commercially in thetinplate canning of foods in which protein degradation resulted in the releaseof sulfur compounds from the food. These sulfur compounds cause thephenomenon of 'sulfur staining' on the tinplate and so it has been thepractice to disperse zinc oxide in tinplate lacquers to intercept suchcompounds reacting with them before they can diffuse to the tinplatesurface.

The remaining methods described in the literature to date for removal oftaints or off-flavours have largely involved incorporation of ingredients witha specific interaction or reaction with a functional group known to be presentin the taint or undesirable food component. Two types of taints amenable toremoval by active packaging have been identified by researchers responsiblefor current commercial products. These are amines resulting from proteinbreakdown in fish muscle and aldehydes formed from the breakdown ofperoxides which result from the initial stages of autoxidation of fats and oils.The formation of aldehydes can make a wide variety of oil-containing foodsorganoleptically unacceptable well before there is significant damage to thenutritional or functional properties of the food. Examples of such productswould be fried snackfoods such as potato crisps, biscuits and cerealproducts. Early developments occurred in Japan where there was seen to bea need to remove amine smells from fish which was stored in domesticrefrigerators. The amines formed in fish muscle degradation include stronglybasic compounds and thus are potentially strong in their interaction withacidic compounds such as citric or other food acids. Hence the earliest workinvolved incorporation of such acids in heat-seal polymers such aspolyethylene and extruding them as layers in packaging (Hoshino andOsanai, 1986).

A later approach to removal of amines odours has been provided by theANICO Company Ltd in Japan under the trade name ANICO BAG. Bagsmade from film containing ferrous salt and an organic acid such as citric orascorbic acid are claimed to oxidise the amine or other oxidisable compoundas it is absorbed by the polymer.

If these materials can be shown to be effective there is an opportunity todetermine which variables optimise their rate and extent of reaction.Questions which would need to be answered are What is the nature of theproducts of such reaction and What is their fate. Questions such as these areof particular interest to regulatory authorities, and the potential for severalactive packaging systems to generate mobile reaction products is consideredin Chapter 11.

Removal of aldehydes such as hexanal and heptanal from packageheadspaces is claimed by Dupont Polymers, Packaging Division, for theirrecently introduced tie layer Bynel IXPlOl which is a high-densitypolyethylene (HDPE) resin masterbatch. This masterbatch is blended(2.5-12%) with unmodified HDPE or other linear polyethylenes to form anintermediate layer in coextrusions. It is specified that the heat-seal layershould not be a 'good to excellent gas barrier' (Dupont, 1993). It isinteresting to note that the use of a form of active packaging can placerestrictions on other components of the packaging. The restriction in thiscase, and many others, is that the extrusion temperature should not exceed2200C to avoid fuming. The chemistry of the process is not described butsuch a process would require the reaction with the aldehyde to be effectivelyirreversible at least over the temperature range the package is likely toencounter. One such reaction would be the formation of a Schiff base byreaction of the aldehyde with an amino group. The amino group would needto be rather stable to heat and oxygen in order to remain unaffected after theextrusion at temperatures up to 2200C in an air atmosphere.

There may well be a wide range of food constituents which can beremoved by making use of specific interactions with selected packagingcomponents or by chemical reaction with them. A fertile research fieldwould seem to be open especially with liquid foods since solubility anddiffusion of food constituents in the packaging can be utilised so that theremoval process is not limited to compounds with a significant vapourpressure at distribution temperatures. It will be necessary for industry andregulators to ensure that such processes are not used to conceal themarketing of sub-standard or even dangerous products if for instancemicrobial odours were to be scavenged.

The tainting of foods by compounds originating either in the packagingmaterial itself (e.g. monomers) or outside but permeating the packagingmaterial continues to be a source of problems for the food industry. Theapproaches described above may contribute to their solution but additionalapproaches have been described in the patent literature. These are formationof chemical barriers as distinct from physical barriers like aluminium foil orcrystalline polymers.

Myrcene (7-methyl-3-methylene-l,6-octadiene) has been found to reactwith traces of styrene or acrylonitrile when the latter are present inacrylonitrile-butadiene-styrene copolymers or blends being extruded(Tokas, 1979). Addition of myrcene is claimed to introduce no new taintwhile reducing residual levels of the monomers. In another patent, foodspackaged in plastics consisting of more than one layer are claimed to beprotected against taints from the outside by inclusion of the appropriateadsorbent in the outer layer (Kiru Kogyo KK, 1994). The adsorbent ischosen for expected taints and is kneaded onto the outer layer, apparentlyretaining the taints there. The period of this type of equilibrium adsorption

needs to be established, especially when a package is subjected totemperature changes. This may have application in areas where products areto be stored or shipped together with odorous products which areinadequately packaged.

A compound which while not a taint is often found undesirable inpackages of respiring horticultural produce is the gaseous auxin ethylene.The potential benefits resulting from such removal are particularly great andare discussed separately in Chapter 2.

4*5 Ingredient release

Active packaging materials considered so far have exerted their action on thepackaged food by removing unwanted components of either the food or ofthe headspace enclosed with the food. Another form of interaction is byrelease of desirable ingredients into the food from the packaging materials orfrom inserts packaged with the food. Some substances released commer-cially or which have been the subject of investigation are listed in Table 4.8.Most are used for their antimicrobial activity, although sulfur dioxide alsoserves as a chemical stabiliser of colour and flavour by preventing progressof the Maillard reaction which causes non-enzymic browning of productssuch as dried fruit and wines (Davis, 1975; Davis et aL9 1978). Hinokitiol,also known as p-thujapricin (Hirata, 1992), derived from cypress bark, is anadditional antimicrobial compound specific to the Japan market.

Processing of foods often results in loss of flavour by degradation orevaporation. Another mechanism of flavour loss is the scalping of someflavour components by plastics used in packaging (Mannheim et aL, 1989).There is therefore the opportunity to replace these lost food constituents bydiffusion from the packaging, especially where scalping or flavour degrada-tion occurs after packaging. There is, however, the question of whether afood is being sold as fresh when this is not so. This is more a legal matterof consumer protection than a technical one as the question arises of when

Table 4.8 Substances emitted by Active Packaging

Substance Purpose Source Reference

Carbon dioxide antimicrobial film Rooney, 1989sachet Abe, 1990

Ethanol antimicrobial sachet Abe, 1990Silver ion antimicrobial film Hirata, 1992Organic acids antimicrobial film Hotchkiss, 1993Sulfur dioxide antimicrobial sachet ICI AustraliaBenomyl antimicrobial film Halek and Garg, 1988Flavours fortification film Venator, 1986Hinokitiol antimicrobial film Abe, 1990BHT antioxidant film Han et a/., 1987Enzymes various film Budny, 1990

flavour addition to a food fabricated from many ingredients becomescontrary to consumer interests.

Release of very few flavours has been investigated to date, although theease of oxidation of many flavours suggests an opportunity to provide slow-release flavour precursors in the packaging material. The manufacture offlavour concentrates in the common commodity plastics has been describedby Venator (1986). Master batches of plastics with concentrations of up to40% of the flavour have been marketed with a view to obviate the effects offlavour scalping.

4.5.1 Antioxidant release from plastics

Two factors acting concurrently are likely to influence the use of packagingmaterials as sources of antioxidants in some foods. The first of these is theneed of the industry to respond to pressure by some consumer advocates forreduced use of food additives (Smith, 1993). The second is the renewedinterest of plastics resin manufacturers in using natural, or other approvedfood antioxidants in polymer stabilisation replacing some of those developedspecifically for plastics. Dilaurylthiodipropionate and its base acid, thiodi-propionic acid, are approved food additives in some countries and are usedas stabilisers in food-grade polyethylene (Anon., 1992).

The potential for evaporative migration of antioxidants into foods fromtheir packaging plastics has been studied by CaIvert and Billingham (1979)who developed a theoretical model. This work has been taken further by Hanet al. (1987) who determined the effect of temperature on both the diffusioncoefficient of butylated hydroxytoluene (BHT) in HDPE and the rate of itsevaporation into the package of oat flakes. It was found that at 39°C only55% of the original BHT remained in the film after 1 week. The loss byoutward migration was 70%, and 25% of the BHT was found in the cereal.After 6 weeks the HDPE film was free of antioxidant and 19% of thatoriginally in the film remained in the cereal. The workers compared theimpact of two starting levels in the film on oxidation of the cereal oil andfound that with an initial 0.32% BHT there was less oxidation than with0.02%.

These results of Han et al (1987) demonstrate the potential for release ofantioxidant into foods provided the rate of diffusion can be matched to thefood's needs. The outward loss can be controlled by use of a layer of filmwith low permeability to the antioxidant or by use in a closure. In the caseof liquid foods or solids with close-fitting packaging the process could bebased on diffusion alone and not require the antioxidant to be able toevaporate.

Commercial use of this approach to antioxidant release has been reportedby Labuza and Breene (1989) who noted that waxed paper has sometimes

been used as a reservoir for antioxidant release by the US cerealindustry.

Other antioxidants might also be used in this way. Lignert and Eriksson(1980) found that Maillard reaction products have a strong antioxidantfunction. This work has been extended by Anese et al (1993). The potentialfor applying such antioxidants to foods via packaging materials has beensuggested (Eriksson, private communication).

The patent of Goyo Shiko KK (1993) describes application of amino acidsand saccharides which produce reducing sugars on decomposition andperform as oxygen scavengers when used in coating or lamination ofpackaging films. These compositions would be expected to undergo theMaillard reactions but their rate would depend upon the thermal conditionsused in preparing the packaging and any thermal processing of the packagedproduct. The inventors nominate liquid foods in cans as a target product areaand, given the hydrophilic nature of the polymers involved, it appears likelythat Maillard reaction products could be extracted into the food to act asantioxidants as well as scavengers.

It has recently been shown that polyethylene bottles stabilised withvitamin E cause less noticeable flavour in distilled water than bottlesstabilised with either BHT or Irganox (a hindered amine antioxidant forpolymers). There is an opportunity to investigate whether oils can bestabilised by diffusive addition of antioxidant at sustained low levels fromthe packaging.

The observation of Han et al (1987) that BHT was lost outwards fromHDPE film packs points to an otherwise unrelated opportunity for activepackaging. There has been constant pressure from importers of grains andother agricultural products for reduced levels of pesticide residues in theseproducts at the time of delivery. Packages such as sacks and lined cartonscontaining these products are often attacked by insects during warehousingand transport. The inclusion of low-toxicity fumigants such as pyrethrins inan outer layer of packaging material offers the opportunity to achievesustained insecticidal activity without substantial addition of fumigant to thefood. Highland and Cline (1986) found that polypropylene containing203 mg m~2 of permethrin provided rather similar resistance to attack byburrowing insects to that provided by polyester which has a harder, slipperysurface. Their work involved exposing: polyethylene; polypropylene/poly-propylene/polyethylene laminate, the latter containing permethrin in theouter layer; and polyester/polyethylene pouches of many foods, to fourinsect populations. The permethrin-treated pouches were resistant to twoinsect species for 24 months. The effect was not entirely consistent since thetreated pouches failed at 24 months with one insect species and were betterthan polyester with one of the remaining species and worse than polyesterwith the other. The authors concluded that the order of descending resistanceof films to the insects was permethrin-treated film, polyester film, polyethyl-

ene film and worst, polypropylene film. This work might be developed toprovide a low-cost answer to some of the major problems of fumigation,including the cost of repeated fumigations with methyl bromide approx-imately bi-monthly, as well as reducing the exposure of staff to fumigantapplication conditions. The permethrin or alternative treatments would needto be submitted for regulatory approval.

It has been suggested recently that enzymes might be released into foodsfrom packaging materials, probably to achieve effects such as antimicrobialaction. Labuza and Breene (1989) have reviewed the potential for release ofbound enzymes into foods. Enzyme inhibitors might also be bound to a filmsurface. An example would be the binding of the inhibitor of methyl esteraseto the package surface to bind methyl esterase. The result might beprevention of cloud drop in fresh orange juice (R.L. Johnson, privatecommunication). The subject of enzymes in active packaging is discussed inChapter 7. The use of enzymes in edible coatings is discussed in Chapter5.

4.6 Permeability modification

There are a number of circumstances in which it is desirable for thepackaging material to be more permeable to one substance than to another.The importance of predicting the requirements of plastic film's permeabilityto carbon dioxide and oxygen in packaging of fresh horticultural produce hasbeen discussed in Chapter 3. It is possible to modify the permeability of awindow in a package and to control gas exchange through this limited area.A variety of semipermeable patches were initially developed by the Herculescompany in the USA. Subsequently the effect of sorbed water on cellulosicpatches has been claimed to give selectivity in gas permeability matched tothe respiration rates of some produce.

There are other substances which need to be selected for entry intoplastics packaging. Of particular interest is the ability to selectively transmitsmoke flavours through films which are useful as skins for ham and otherpreserved meats. These are films of polyamide alloy which are highlypermeable to water and oxygen under conditions of high temperature andhigh humidity and highly impermeable to oxygen at room temperature underdry conditions (Nishini and Yoshii, 1988). Hirata (1992) described one suchtype of film as having an oxygen transmission rate of 8 ml/m2/day at 200C,60% RH and a water vapour transmission rate of (WVTR) 60 g/m2/day at400C, 90% RH. The low oxygen permeability is necessary for colourretention of preserved meats. This high WVTR should favour rapidtransmission of polar flavours. Kureha Chemical Co. (1986) described apolyamide film for this purpose with an OTR in the range 50-300 ml/m2/dayfor film thicknesses of 5-50 |xm.

4.7 Current use commercially

The commercial development of plastics-based active packaging has notoccurred evenly either geographically or in terms of their field ofapplication. The major field to date has been horticulture in which severalforms of enhanced permeability films have been commercialised for bothtrade and home use. This success, in spite of a lack of soundly basedevidence of effectiveness in many cases, owes much to substantial priorscientific research. This research into equilibrium modified atmospherepackaging has been based on the work of Kader (1980) and earlier workersincluding Jurin and Karel (1963). Since some of the active packagingtechnologies currently commercial are scarcely improvements on existingtechnologies there will probably be significant realignment in the market-place. The need for active packaging solutions to problems in the storageand distribution of horticultural produce may well ensure that the develop-ment of the most soundly based technologies is commercialised. Ethylenescavenging, condensation control and equilibrium modified atmospherepackaging will continue to be emphasised.

Active packaging for processed foods is still based largely on sachettechnologies, with the exception of moisture control packaging. It is in theprocessed food field that commercial development of plastics-based systemscan be expected to be substantial over the next few years. Perhaps ToyoSeikan Kaisha's planned manufacture of an oxygen-scavenging laminate forthe semi-aseptically packaged boiled-rice market by Sato Food Industry Co.Ltd is the first example. This tray has been described as 'epoch making',causing, as is planned, a 100 000 meals/day operation to be more costeffective (Anon., 1994).

The substantial impact on the marketplace of oxygen-scavenging crownseals for beverages, metallised polyester microwave susceptors and time-temperature indicator strips is discussed in other chapters.

4.8 Regulatory and environmental impacts

Since plastics-based active packaging involves not only changing currentmaterials somewhat but also inclusion of reactive components in some cases,regulatory authorities must become involved in many developments. Indeedregulatory considerations appear to have caused the delay in the introductionof a chemical barrier to oxygen into PET beverage bottles.

The effect of environmental considerations on plastics-based activepackaging will vary with the nature of the product/package combination.Commodity films used for produce packaging may become the object ofrecycling schemes, especially in the European Union, and so additionalingredients will need to be evaluated for their impact. Barrier packages used

for processed foods at present are generally not amenable to economicrecycling. The benefits of active packaging in terms of food quality, safetyand shelf-life extension will need to be considered in a holistic approach toenvironmental impact assessment. A study by Kooijman (1994) of foodpackaging in the Netherlands demonstrated that 'the packaging sub-systemcannot be studied or optimised in isolation'. Accordingly, active packagingplastics should be judged on the basis of their contribution to the quality andsafety of food.

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Wagner, B.F. (1990) Getting to Know the Packaging Activists - A Comprehensive View ofAbsorbers, Getters, and Emitters, and their Kin for Food Preservation. Proceedings PackAlimentaire '90. Innovative Expositions, Princeton, New Jersey. Section B-2.

Warmbier, H.C. and Wolf, MJ. (1976) Modern Packaging, 38.Zenner, B., Decastro, E. and Ciccone, J. (1992) Ligand extracting composition - containing

chelated transition metal immobilised on solid phase, especially for scavenging oxygen incontainers. US Patent 5096724.

5 Edible films and coatings as active layers

B. CUQ, N. GONTARD and S. GUILBERT

5.1 Introduction

Edible films and coatings are traditionally used to improve food appearanceand conservation. The most common examples are wax coatings for fruit(used in China since the 12th century), chocolate coatings for confectionery,lipid films to protect meat products, and soy milk-based lipoprotein films toimprove the appearance and preservation of certain foods in Asia.

Formulations for edible films or coatings must include at least onecomponent able to form a suitably cohesive and continuous matrix. Thebasic materials can be classified in three categories: polysaccharides,proteins and lipidic compounds. Polysaccharides (vegetable and microbialgums, starches, celluloses and derivatives, etc.) have good film-formingproperties. Films formed from these hydrophilic compounds provideefficient barriers against oils and lipids (Murray et al., 1972), but theirmoisture barrier properties are poor. Although not as extensively studied,protein-based films have highly interesting properties. Many protein materi-als have been tested: collagen, zein, wheat gluten, ovalbumin, soybean,casein, etc. (Guilbert and Biquet, 1989). The mechanical and barrierproperties of these films are generally better than those of polysaccharide-based films; this is due to the fact that, contrary to polysaccharides which aremonotonous polymers, proteins have a specific structure which conferslarger potential functional properties (Guilbert and Graille, 1994). Manylipidic compounds, such as animal and vegetable fats (natural waxes andderivatives, acetoglycerides, surface-active agents, etc.), have been used tomake edible films and coatings (Guilbert and Biquet, 1989; Kester andFennema, 1986). They are generally used for their excellent moisture barrierproperties, but there can be problems concerning stability (particularlyoxidation), texture and organoleptic quality (opacity, waxy taste).

Edible films and coatings formed with several compounds (compositefilms) have been developed to take advantage of the complementaryfunctional properties of these different constitutive materials and toovercome their respective drawbacks. Most composite films studied to datecombine a lipidic compound and a hydrocolloid-based structural matrix(Cole, 1969; Daniels, 1973; Gontard et al.9 1994a; Guilbert, 1986; Kamperand Fennema, 1984a, b).

Coatings are formed directly on the food product using either liquid film-forming solutions (or dispersions) or molten compounds (e.g. lipids). They

can be applied by different methods: with a paint brush or by spraying,dipping-dripping, fluidizing, etc. Films are preformed separately from thefood product. They can be produced, for instance, by drying a film-formingsolution on a drum-drier, by cooling a molten compound, or throughstandard techniques used to form synthetic packagings, e.g. thermoformingor extrusion techniques for thermoplastic materials (Guilbert and Biquet,1989; Kester and Fennema, 1986).

The production process from a film-forming solution generally includes afirst step with macromolecule solubilization in a solvent medium (oftenwater-, ethanol- or acetic acid-based) which can contain several additives(plasticizers, crosslinking agents, solutes, etc.). The film-forming solution isspread in a thin layer, usually followed by a drying treatment. The functionalproperties of the film are dependent on a number of parameters (Gontard,1991; Gontard et al.9 1992): formulation (characteristics and concentrationof the basic and secondary components, pH, denaturing conditions, etc.),film-forming conditions (type of surface upon which the film-formingsolution is spread, drying conditions) and conditions in which the film isused (temperature, relative humidity).

The degree of cohesiveness of the matrix is a critical parameter affectingthe functional properties of edible films (Banker, 1966). It is sometimesdifficult to obtain adequate adhesion of the film to the food product, forinstance when a hydrophobic film-forming material is used to protect ahydrophilic food product. In such cases, surface-active agents can be coatedon the food or added to the film-forming solution, or a material capable ofadhering to both components can be applied as an intermediate precoating(e.g. precoating with cocoa before sugar-coating peanuts).

An edible film is an integral part of the food product it encloses andtherefore must have neutral sensorial properties (or compatible with productnature) so as not to be detected during consumption. Application of an ediblebarrier layer is an easy means to structurally strengthen certain foods, toreduce particle clustering and to improve the visual and tactile features onthe surface of the product. For example, Allen et al. (1963b) used alginateand cornstarch coatings to improve texture juiciness, general appearance,surface texture and color of beef steaks and pork chops. Edible films canalso be used to package components or additives that are to be dissolved inhot water or food mixes (Daniels, 1973; Kroger and Igoe, 1971).

Edible films or coatings act as an additional parameter for improvingoverall food quality and stability. They represent one way to apply hurdletechnology to solid foods without affecting their structural integrity(Guilbert, 1994; Guilbert et aL, 1995). Many functions of edible films arethe same as those of synthetic packaging (water, gas and solute barriers,mechanical properties, opacity, etc.). However, they must be chosenaccording to their specific application (i.e. type of food product and maindeterioration mechanisms).

Films with substantial gas and moisture barrier properties are required formany applications: to control gas exchange for fresh foods and oxygenexchange for oxidizable foods and to reduce moisture exchange with theexternal atmosphere (Guilbert and Biquet, 1989). Retention of specificadditives in edible films can lead to a functional response generally confinedto the surface of the product (modification and control of surfaceconditions), (De Savoye et ai9 1994; Guilbert, 1988; Torres et aL, 1985a, b;Torres and Karel, 1985). Oil and solute penetration into foods duringprocessing can also be limited by edible coatings (Daniels, 1973; Guilbertand Biquet, 1989).

In this chapter, we will expand on the use of edible films and coatings asactive layers, i.e when the edible film contributes by itself to thepreservation. Figure 5.1 gives a schematic representation of food preserva-tion with edible films and coatings as active layers when the first mode ofdeterioration results from respiration, from dehydration or moisture uptake,or from surface microbial development or oxidation. The protective featuresof edible films and coatings are dependent on gas and water vapor barrier

Food additives (e.g. antioxygenand antifungic agents)

Diffusionof food additives Gas Transfers

Storage

Water Transfer

Food additives in SURFACE RETENTIONof food additives CONTROL

of Gas TransfersAOffiWOLM

Storage

CONTROLof Water Transfer

Figure 5.1 Schematic representation of food preservation with (top) or without (bottom) ediblefilms and coatings as active layers, when the first mode of deterioration results from respiration (a),

from dehydration or moisture uptake (b), or from microbial development or oxidation (c).

properties, on modification of surface conditions and on their ownantimicrobial properties.

5.2 Use of edible active layers to control water vapor transfer

Moisture transfers due to water vapor pressure or concentration gradientshave major effects on the organoleptic and microbiological qualities of foodproducts. For example, products such as raisins (Bolin, 1976; Kochnar andRossell, 1982; Lowe et a/., 1963; Marston, 1983; Watters and Brekke,1961), candies and chocolate-coated products (Andres, 1984; Barron, 1977;Cosier, 1957; Feuge, 1970; Jokay et al., 1967), dry crackers, biscuits, pizzacrust, and filled bakery crust (Dhale, 1983; Heiss, 1968; Katz and Labuza,1981; Labuza, 1985) may become unsatisfactory as a result of moisture gainor loss. Moisture exchanges are difficult to control in multicomponent foodssuch as mixtures of dehydrated foods, jelly-filled cookies, pies, and pizzas.Moisture transfer can be limited by reducing the vapor pressure gradientbetween components, e.g. using aw lowering agents (salts, sugars, polyols,etc.). However, this is not always possible and may result in drasticmodifications of the sensory and physiochemical characteristics of theproduct (Guilbert, 1984, 1985; Karel, 1976). Another solution consists inusing edible films or coatings with good moisture barrier properties toseparate compartments and thus control further moisture transfer (Guilbert,1986; Kamper and Fennema, 1985). Surface drying on some fresh andfrozen foods or, inversely, moisture uptake in dry or semi-moist foods, canalso be hindered by using films with a low water permeability (asschematized in Figure 5.1).

Permeability is defined as a state which permits the transmission ofpermeants through materials (Mannheim and Passy, 1985). When there areno pores, faults or membrane punctures, permeability P, is equal to theproduct of the diffusion coefficient D, representing the mobility of permeantmolecules in the polymer, and the solubility coefficient S, representing thepermeant concentration in the film in equilibrium with the externalpressure:

P = D x S

In practice, water, gas or solute permeability, P, through a membrane isdetermined by steady-state measurements:

AWxP =

AtAApwhere AW is the permeant weight that passes through a film of thickness xand area A; where At is the time and Ap is the differential partial pressureacross the film. The diffusion coefficient can be obtained by taking

measurements before the steady state is reached. The solubility coefficientcan either be calculated from P and D, or measured in a separate experiment(sorption isotherms).

The diffusion and solubility of permeants are affected by temperature andby the size, shape and polarity of the diffused molecule. Moreover, these twoparameters depend on film characteristics, including the type of forcesinfluencing molecules of the film matrix, the degree of cross-linking betweenmolecules, the crystallinity, the presence of plasticizers or additives, etc. (DeLeiris, 1985; Gontard et ah, 1993; Kumins, 1965; Pascat, 1985; Schwartz-berg, 1985).

Permeability is only a general feature of films or coatings when thediffusion and solubility coefficients are not influenced by permeant content,i.e. when Fick's and Henry's laws apply. In practice, for most edible films,the permeant interacts with the film and the D and S coefficients aredependent on the difference in partial pressure. For instance, in relation tothe water vapor permeability of hydrophilic polymer films, the watersolubility and diffusion coefficients increase when the water vapor differ-ential partial pressure increases because of the moisture affinity of the film(nonlinear sorption isotherm) and because of increased plasticization of thefilm due to water absorption (Gontard et ai, 1993; Schwartzberg, 1985). Thefilm thickness can also influence permeability when using film-formingmaterials that do not behave ideally. The permeability of an edible film isthus defined as a property of the film-permeant complex, under specifiedtemperature and water activity conditions.

Water vapor permeabilities of some edible and synthetic films are given inTable 5.1. Permeability is clearly high in edible films formed fromhydrophilic materials. These films can only be used as protective barrierlayers to limit moisture exchange for short-term applications or in low-moisture foods such as dried fruits (Forkner, 1958; Swenson et aLy

1953).Lipidic compounds are often used to make moisture barrier films and

coatings (Table 5.1). For instance, coating fresh fruit and vegetables withwax reduces desiccation-induced weight loss during storage by 40-75%(Kaplan, 1986). Water is not very soluble or mobile in lipid-based filmsbecause of the low polarity and dense, well-structured molecular matrixesthat can be formed by these compounds.

Moisture resistance of lipid films is inversely related to polarity of thelipids. Hydrophobic alkanes and waxes, such as paraffin wax and beeswax,are the most effective barriers. More hydrophilic lipids, such as fatty acids,are less resistant to water vapor transmission since their polar groups attractmigrating water molecules and thereby facilitate water transport. Themoisture barrier capacities of different films can be classified in increasingorder of efficiency, as follows: liquid oils < solid fats < waxes (Gontard etah, 1994a; Kamper and Fennema, 1984b). This efficiency order has been

Table 5.1 Water vapor permeability of various films

Water vaporpermeability T Thickness RH %

Film (X 10'2 mol m m~2 s"1 Pa"1) (0C) (X 103m) conditions

Starch, cellulose acetate(1) 142 38 1.190 100-30Wheat gluten(3) 69.7 26 - 100-50Casein-gelatin(20) 34.3 30 0.250 60-22Wheat gluten(l5) 34.2 21 0.400 85-00Sodium caseinate(2) 24.7 25 - 100-00Cornzein(33> 22.8 26 - 100-50HPC and PEG(I8) 13.7 21 0.130 85-00MC and PEG(I8) 13.6 21 0.100 85-00MC and PEG(5) 7.78 25 0.025 52-00Corn zein(18) 6.45 21 0.200 85-00HPMC(13) 5.96 27 0.019 85-00Glycerol monostearate(16) 5.85 21 1.750 100-75MC(I6) 5.23 30 0.075 11-00Wheat gluten and glycerol(9) 5.08 30 0.050 100-00Wheat gluten and oleic acid(10) 4.15 30 0.050 100-00Wheat gluten and carnauba wax(l0) 3.91 30 0.050 100-00Wheat gluten(8) 3.11 23 0.127 11-00HPC(I9) 2.89 30 0.075 11-00Wheat gluten and soy protein(7) 2.84 23 0.075 11-00Wheat gluten and mineral oil(8) 2.28 23 0.125 11-00Corn zein and oleic acid(12) 1.48 38 0.040 95-00HPMC and palmitic acid(l4) 1.22 25 0.040 97-67Dark chocolate(4) 0.707 20 0.610 81-00MC and beeswax bilayer<n) 0.199 25 - 100-00LDPE(17) 0.0482 38 0.025 95-00HPMC/MC and beeswax bilayer(15) 0.0360 25 0.051 97-00Beeswax(6) 0.0320 25 0.100 100-00Wheat gluten-beeswax bilayer<9) 0.0230 30 0.090 100-00Carnauba wax(6) 0.0185 25 0.100 100-00HDPE(!2) 0.0122 38 0.025 97-00Beeswax(16) 0.0122 25 0.120 97-00Aluminium foil(17) 0.000289 38 0.025 95-00

(According to Allen et al, 1963a(1); Avena-Bustillos and Krochta, 1993(2); Aydt et al, 1991(3);Biquet and Labuza, 1988(4); Donhowe and Fennema, 1993a(5); Donhowe and Fennema, 1993b(6);Gennadios et al, 199(F; Gennadios et al, 1993a(8); Gontard, 1991(9); Gontard et al, 1994a(10);Greener and Fennema, 1989a(ll); Guilbert and Biquet, 1989(l2); Hagenmaier and Shaw, 1990(13);Kamper and Fennema, 1984a(l4); Kester and Fennema, 1989a(15); Landman et al, 1960(16); Myers etal, 1961(17); Park and Chinnan, 1990(l8); Park et al, 1991(19); Schultz et al, 1949(20).)(HPC = hydroxypropylcellulose; HDPE = high-density polyethylene; HPMC = hydroxypropylmethylcellulose; LDPE = low-density polyethylene; MC = methylcellulose; PEG = polyethyleneglycol; RH = relative humidity; T = temperature).

confirmed by Kester and Fennema (1989a) in a study on the resistance ofvarious lipids, heated and adsorbed into filter papers, to water vaportransmission. Composition, fusion and solidification ranges, lipid crystallinestructure, in addition to the interaction with water, oxygen and othercomponents of the food product influence the physico-chemical, functionaland organoleptic properties of lipid-based films (Fennema et al, 1994;Kamper and Fennema, 1984a, b; Kester and Fennema, 1989a).

In general, the rate of transmission of water through a lipid film increasesas the length of the lipid hydrocarbon chain is decreased and the degree ofunsaturation or branching of acyl chains is increased (Archer and Lamer,1955; Fettiplace, 1978; Fettiplace and Haydon, 1980; Kamper and Fennema,1984b and 1985). This is a consequence of enhanced mobility ofhydrocarbon chains and less efficient lateral packing of acyl chains causedby a reduction of interchain van der Walls' interaction (Jain, 1972; Taylor etal., 1975).

However, specific information on the water vapor barrier properties offilms of more hydrophobic lipids is lacking, and almost all the informationavailable has been gathered using a 100-0% RH gradient which is notcommonly encountered during storage of foods under commercial condi-tions.

The water vapor barrier properties of a lipid-hydrocolloid composite filmis generally determined by the potentials of its component parts (Table 5.1).The coating operation used, i.e. emulsion (suspension or dispersion of non-miscible compounds), successive layers (multilayered films) or solutionswith a common solvent, affects the water vapor barrier properties of thesefilms (Table 5.2).

According to Schultz et al (1949), Martin-Polo et al (1992), Debeaufortet al (1993) and Gontard et al (1994a), who investigated the moisturepermeability of multicomponent films composed of methylcellulose, pectin-ate or gluten and various lipids (waxes, fatty acids, etc.), it is better to formtwo successive layers than to apply a dispersion in solvent. Kamper andFennema (1984a, b) carried out detailed studies of films, composed ofsoluble cellulose esters and a mixture of palmitic and stearic acids, and

Table 5.2 Effect of coating operation on water vapor permeance of edible multicomponent filmscomposed of MC and paraffin wax (weight ratio 1:1); gluten and beeswax (at 2.4 mg/cm2); andHPMC and stearic-palmitic acids (at 9 mg/cm2)

Water vaporpermeance T Thickness RH %

Film (X 108 mol m~2 s'1 Pa'1) (0C) (X 103 m) conditions_ _ _ _ _ _ 8 4 _ 2 2

MC and paraffin wax (emulsion)(1) 7.39 25 0.090 84-22MC and paraffin wax (bilayer)(1) 0.251 25 0.130 84-22

Glutten(2) 10.1 30 0.050 100-00Gluten and beeswax (emulsion)(2) 0.727 30 0.120 100-00Gluten and beeswax (bilayer)(2) 0.0257 30 0.090 100-00

HPMC(3) 8.02 25 0.040 85-00HPMC and fatty acids (emulsion)(3) 0.00643 25 0.040 85-00HPMC and fatty acids (bilayer)(3) 1£7 25 0.125 85-00

(According to Debeaufort et al., 1993(l); Gontard et aLy 1994a(2); Kamper and Fennema,1984b(3).)(HPMC = hydroxypropylmethylcellulose; MC = methylcellulose; RH = relative humidity; T =temperature.)

demonstrated that application of emulsions resulted in reducing moisturepermeability by 10-fold relative to bilayer systems. Variations in homogen-eity and/or structure (size, form and orientation of the crystals) of the lipidlayer related to the film-forming process and to coating operation, couldexplain this discrepancy.

Permeability of composite films decreases substantially when the propor-tion of lipids increases. For instance, an increase of glyceride sucro ester(beeswax, stearic acid or monoglycerides) contents in gluten- (caseinate- orhydroxypropyl methylcellulose-) based composite films, causes a substantialreduction in water vapor permeability (Figure 5.2), (Avena-Bustillos andKrochta, 1993; Gontard et al, 1994a; Hagenmaier and Shaw, 1990). Thenumber of hydrophobic residues (from lipid derivatives) in the matrix affectsthe water interaction potential, and consequently the barrier properties.Solidification of lipids (especially saturated) in a densely organizedcrystalline structure results in a very significant reduction in moisturepermeability (Kamper and Fennema, 1984b; Kester and Fennema, 1989b;Landman et ai, 1960; Watters and Brekke, 1961).

Water vapor permeability variations relative to aw reveal the non-Fickianbehaviour of biological materials. At low aw, water diffusion, and especiallythe water solubility coefficient, remains relatively low: film permeability isminimal at this point. As expected in a hydrophilic film (Barrie, 1968;Biquet and Labuza, 1988; Kamper and Fennema, 1984a; Crank, 1975; DeLeiris, 1985; Pascat, 1986; Schwartzberg, 1985), increasing aw leads to an

Water Vapor Permeability (i ltf2 mol. m. m*. s l . Pa *)

Lipid Materials Content (%w/w)

Figure 5.10 Effect of reduced surface pH on the microbiological quality of an intermediate moisturecheese analog coated with a carrageenan and agarose film; challenged with Staphylococcus aureusS-6 (aw = 0.88 and 35°C) (after Torres and Karel, 1985).

increase in film moisture content (rise in the sorption isotherm) and soinduces an increase in water vapor permeability. At high aw, extensiveswelling of the protein network with water probably enhances watermolecule diffusion and such films would clearly not be efficient water vaporbarriers (Figure 5.3).

Many authors have studied the effect of temperature on water vaportransfer using synthetic, simple edible films (protein or cellulose-based) orcomposite edible films (cellulose derivatives and lipids). A rise in tem-perature causes an increase in water vapor permeability (Figure 5.4). This isoften characterized by Arrhenius-type representations (Donhowe andFennema, 1993b; Higuchi and Aguiar, 1959; Kester and Fennema, 1989a, c).In films formed with hydrophilic materials, temperature-dependent varia-tions in barrier properties are affected by the moisture level (see Figure 5.3for gluten films). However, it was difficult to interpret the temperaturedependence of the effect of hydration on water vapor permeability of glutenfilm in terms of disruptive water-polymer hydrogen bonding in a polymerhydrogen-bonded network. The critical role of water as plasticizer of glutenfilm appeared to be highly temperature dependent. According to Levine andSlade (1987) and Slade et al (1989), the structure/property relationships ofhydrated proteins, and particularly of gluten, could be better understoodthrough the theories of glass transition used in polymer science, in terms ofcritical variables of time, temperature and moisture content. Glass transition

Water Vapor Transmission Rate (WVT), (x 108 mol. m. iff2. r1)

Water activity

Figure 5.3 Effect of average water activity on water vapor transmission rate of edible filmscomposed of methylcellulose and palmitic-stearic acids (at 0.76 mg/cm2) at 25°C and 32% RHgradient (according to Kamper and Fennema, 1984a), and of edible wheat gluten films at 5, 30 and500C, and 10% RH gradient (according to Gontard et al., 1993). •, MC and C 1 8 -C 1 6 at 25°C

(WVT x 0.1); D, gluten at 500C; D, gluten at 300C; D, gluten at 5°C

hydrated proteins, and particularly of gluten, could be better understoodthrough the theories of glass transition used in polymer science, in terms ofcritical variables of time, temperature and moisture content. Glass transitionis typically described as a transition from a brittle glass to a highly viscousor rubbery solid. It is well established that plasticization by water affects theglass transition temperature of amorphous or partially crystalline proteinssuch as gluten (Gontard et al., 1993; Hoseney et al., 1986), gelatin (Marshalland Petrie, 1980; Yannas, 1972), collagen (Batzer and Kreibich, 1981) andelastin (Kakivaya and Hoeve, 1975; Scandola et al., 1981), thus resulting ina drop in the glass transition temperature. The anomalous diffusion behaviorof glassy polymers may be directly related to the influence of the changingpolymer structure on solubility of the penetrant and diffusional mobility ofthe penetrant (Crank, 1975).

The water vapor barrier properties of edible active films and coatings havebeen used in several food systems. For example, meat coatings composed ofcorn starch and alginate were used by Allen et al. (1963a) to reduce moistureloss by 40-48%. Avena-Bustillos et al. (1993) found that sodium caseinateand stearic acid emulsion coatings improved the storage stability andreduced water loss of peeled carrots. The use of sucrose esters of fatty acids,mono- and diglycerides, and the sodium salt of carboxymethylcellulosecoatings allowed the increase of water vapor resistance (up to 75%) inzucchini fruit (Avena-Bustillos et al., 1994). Wax is commercially applied to

Water Vapor Permeance (WVP), (x 10* mol.m .m 2 . s l )

Temperature (0C)

Figure 5.4 Effect of temperature on water vapor permeance of edible wheat gluten films, at RHconditions 90-80% (according to Gontard et al., 1993); of edible glycerol monostearate films, at RHconditions 100-0% (according to Higuchi and Aguiar, 1959); and of edible films composed ofmethylcellulose and beeswax, at RH conditions 97-0% (according to Kester and Fennema, 1989b).

a, Gluten (WVP X 0.1); A, MC and beeswax; • , monostearate.

many fruits and vegetables to reduce dehydration and improve consumerappeal (Hall, 1981), e.g. to oranges (Albrigo and Brown, 1970; Bursewitzand Singh, 1985; Eaks and Ludi, 1970), to apples (Hall, 1966; Trout et al,1953), to prune plums (Bain and McBean, 1967), and to sweet cherries(Drake et aL, 1988; Lidster, 1981). Ukai et al (1976) used hydrophobicemulsions to coat fruits and vegetables such as orange, peas, apples, greenbeans, tomatoes, pears, and peaches. Lidster (1981) suggested the use ofxanthan gum applied to cherries as a postharvest dip to prevent water loss.El Ghaouth et al. (1991) used edible chitosan coating to reduce water loss ofcucumber and bell pepper fruits.

The use of edible films to lessen internal migration of moisture in foodshas been studied by several investigators, and some of these films appearpromising for commercial use (Fennema et al, 1993; Greener and Fennema,1989b; Kamper and Fennema, 1985; Kester and Fennema, 1989d; Rico-Penaand Torres, 1990). One standard example of moisture transfer control inheterogeneous food is the wafer-ice cream system of ice cream cones. Instandard conditions, the wafer loses its crispness after three months' storageat -23°C. This qualitative degradation is due to moisture transfer betweenthe ice cream and the wafer. Rico-Pena and Torres (1990) tested compositefilms (methylcellulose and chocolate and methylcellulose, chocolate andpalmitic acid) to separate the two components. Films can limit moisturetransfer during ice cream cone storage at -230C or -12°C and preserve wafercrispness. Films based on methylcellulose, chocolate and palmitic acid arethe most efficient. In addition, several studies have confirmed the efficiencyof chocolate and cocoa butter coatings as moisture barriers (Biquet andLabuza, 1988; Kempf, 1967; Landman et al, 1960; Neidiek, 1981; Sobolevaand Chizhikova, 1978; Tiemstra and Tiemstra, 1974).

These examples show that water vapor transfer in food products can becontrolled using edible films and coatings. The knowledge of the influenceof parameters such as structure and composition of film-forming materials,allows modulation of the water barrier properties of these edible activelayers either on the food surface or in multicomponent foods.

5.3 Use of edible active layers to control gas exchange

The gas barrier properties of edible films and coatings are potentially ofgreat interest. For instance, edible oxygen barrier films can be used toprotect foods that are susceptible to oxidation (rancidity, loss of oxidizablevitamins, etc.). In contrast, a relatively high gas permeability is necessary forfresh fruit and vegetable coatings (especially carbon dioxide permeability).The development of edible films with selective gas permeability (oxygen,carbon dioxide, ethylene) allows the control of respiration exchange andmicrobial development and seems very promising for achieving a * modified

atmosphere' effect in fresh fruit and for improving the storage potential ofthese products (Smith et a/., 1987; Trout et al., 1953), as schematized inFigure 5.1.

Gas permeability can be measured using air porosimeters and specificpermeability cells. As in the case with moisture barrier properties, theformulation, manufacturing and environmental parameters have an impacton a film's response to gases. The oxygen and carbon dioxide permeabilityvalues of various edible films and synthetic films are given in Table 5.3.

Films formed with hydrocolloids (proteins, polysaccharides) generallyhave good oxygen barrier properties, particularly under low-moistureconditions. The oxygen permeability of hydrocolloid-based films (at 0%relative humidity) is often lower than that of common synthetic films such aspolyethylene and non-plasticized PVCs. For example, the oxygen permeabil-ity of wheat gluten film was 800 times lower than that of low-densitypolyethylene and twice as low as that of polyamide 6, a well-known highoxygen barrier polymer.

Films formed with lipid derivatives have suitable oxygen barrier proper-ties. For example, the oxygen permeability value for beeswax film liesbetween those of low-density and high-density polyethylene (Table 5.3).According to Blank (1962 and 1972), lipids with the best oxygen barrierproperties are those formed with straight-chain and saturated fatty acids.Increased unsaturation (or branching) and a reduction in the length of thecarbon chain result in decreased oxygen permeability. The following barrierefficiency order was observed by Kester and Fennema (1989c): stearicalcohol > tristearine > beeswax > acetylated monoglycerides > stearicacid > alkanes. These differences can be explained by the presence of poresor cracks, and by the homogeneity of the composition density of the network(Kester and Fennema, 1989b, c). The network density is dependent on thepolymorphic shape and orientation of the chains and morphologicaldifferences in the lipid layers.

As previously mentioned for water vapor permeability, formulation ofcomposite films allows advantage to be taken of the complementary barrierproperties of each component. At high aw, where hydrophilic materials arenot effective as gas barriers (see below), the addition of lipidic compoundsresults in a decrease in the gas permeability of the film. For example, at aw

= 0.91, the oxygen permeability is reduced by about 30% for a compositegluten and beeswax film (Table 5.3).

The effect of temperature on gas permeability is similar to that reportedfor water vapor permeability (Donhowe and Fennema, 1993b; Gennadios etal.9 1993). These variations can be characterized by Arrhenius-typerepresentations. But, as far as gas solubility decreases with temperatureincrease, the increase of gas permeability with temperature is lower than forwater vapor permeability (Gontard et a/., 1994b).

High aw conditions cause an increase in gas permeability in hydrophilic

Table 5.3 Oxygen and carbon dioxide permeabilities of various films

O2 Permeability CO2 Permeability TFilm (X 1O18HIoImIn-2S-1Pa-1) (X 1018 mol m m 2 s 1 Pa1) (0C) aw

_ _ _ _ _ _ _ __ 0 0

HDPE(3) 285 972 23 0.0Rigid PVC(II) 16.0 - 23 0.0PET<3) 11.9 37.6 23 0.0Polyamide 6(3) 11.6 19.5 23 0.0PVDC(3) 1.86 7.65 23 0.0Cellophane(3) 1.34 - 23 0.0EVOH(H) 0.155 - 23 0.0

Polybutadiene<3) 19400 - 23 0.50Polypropylene(3) 1170 - 23 0.50LDPE(l3) 961 - 23 0.50Flexible PVC film(3) 682 - 23 0.50Uncoated cellulose0} 406 - 23 i.00HDPE(3) 227 703 23 0.50HDPE(I) 224 - 23 1.00Cellophane(H) 130 - 23 0.95Polyvinyl alcohol(1) 50.8 - 23 1.00PET(3) 24.3 - 23 0.50PET™ 12.2 - 23 1.00Nylon 6(I) 10.2 - 23 1.00Cellophane(l4) 8.27 - 23 0.50EVOH(I3) 6.20 - 23 0.95

MCandPEG ( I O ) 522 - 30 0.0MCandPEG ( 1 0 ) - 29900 21 0.0MC and beeswax<9) 499 - 25 0.0Beeswax<5) 480 - 25 0.0HPC and PEG(IO) 470 - 30 0.0HPC and PEG(IO) - 28900 21 0.0MC and palmitic 190 - 24 0.0

acid(l2)

Carnauba wax(5) 81.1 - 25 0.0Corn zein(2) 34.8 - 38 0.0Cornzein(2) - 216 23 0.0Wheat gluten and 19.6 - 23 0.0

glycerin(6)

Cornzein ( l0) 16.1 - 30 0.0Wheat gluten 8.92 - 30 0.0

protein(l0)

Wheat gluten - 119 21 0.0protein(l0)

Corn zein(10) - 95.0 21 0.0Wheat gluten protein(2) 3.50 - 38 0.0Soy protein(6) 2.30 - 23 0.0Wheat gluten and 1.70 - 23 0.0

mineral oil(7)

Wheat gluten(8) 1.24 - 25 0.0Wheat gluten and soy 1.19 - 25 0.0

protein(4)

Wheat gluten protein(2) - 1.20 23 0.0Pectin<8) 1340 21300 25 0.96Wheat gluten(8) 1290 36700 25 0.95Wheat gluten(8) 982 24500 25 0.91

Table 5.3 Continued

O2 Permeability CO2 Permeability TFilm (X 10 l 8molm HT2S-1 Pa"1) (X 1018 mol m m"2 s"1 Pa"1) (0C) aw

Wheat gluten and 687 6614 25 0.91beeswax(8)

Chitosan(8) 472 8010 25 0.93MC and palmitic 407 - 24 0.79

acid(12>

(According to Ashley, 1985(l); Aydt et al., 1991(2); Bakker, 1986(3); Brandeburg et al, 1993(4);Donhowe and Fennema, 1993b(5); Gennadios et al, 1990(6); Gennadios et al, 1993a(7); Gontard etal., 1994b(8); Greener and Fennema, 1989a(9); Park et Chinnan, 1990(l0); Poyet, 1993(n); Rico-Penaand Torres, 1990(l2); Salame, 1986(l3); Taylor, 1986(l4).)

(aw = water activity; EVOH = ethylene-vinyl alcohol; HDPE = high-density polyethylene;LDPE = low-density polyethylene; MC = methylcellulose; PEG = polyethylene glycol; PET =polyester; PVDC = poly(vinylidene chloride); PVC = poly(vinyl chloride); T = temperature.)

films but generally not in synthetic hydrophobic films which are not watersensitive (Figure 5.5). In these hydrophilic films, increased aw promotes bothgas diffusivity (due to the increased mobility of hydrophobic macromoleculechains) and gas solubility (due to the water swelling of the matrix), leadingto a sharp increase in gas permeability (Kumins, 1965). The effects of aw ongas barrier properties of composite films (methylcellulose and palmitic acid)and of gluten protein-based films was studied by Rico-Pena and Torres(1990) and by Gontard et al. (1994b) respectively, and are compared inFigure 5.5 with oxygen permeabilities of synthetic hydrophilic films.

Carbon dioxide permeability in hydrocolloid-based films is often much

Oxygen Permeability (x 10* mol. m. m 2 . s 1 . Pa ! )

Water activity

Figure 5.5 Effect of water activity on oxygen permeability of edible and synthetic films at 25°C(according to Gontard et al, 1994b (o, gluten); Poyet, 1993 (A, EVOH; •, Nylon 6); Rico-Pena andTorres, 1990 (D, MC and palmitic acids, at weight ratio 3:1); Rigg, 1979 (•, cellophane)). (MC is

methylcellulose; EVOH is ethylene-vinyl alcohol).

higher than oxygen permeability (Table 5.3). The effect of film aw on carbondioxide permeability is similar to that on oxygen permeability, but the sharpincrease of permeability is more important. This could be explained by thedifferences in water solubility of these gases (Schwartzberg, 1985), i.e.carbon dioxide is very soluble (carbon dioxide solubility in water =34.5mmol/l at 25°C and 105 Pa; oxygen solubility = 1.25mmol/l at 25°Cand 105 Pa).

At high aw, the addition of lipidic components to gluten film results in ahigh decrease of carbon dioxide permeability. For example, at aw = 0.91,the carbon dioxide permeability is reduced by about 75% for a compositegluten and beeswax film (Table 5.3). This could be related to thehydrophobic characteristics of these components which for the same aw

reduce the amount of water available for solubilisation of carbon dioxide.The selectivity coefficient between carbon dioxide and oxygen is defined

as the ratio of the respective permeabilities of both gases. In hydrophilicmaterials, the effect of an aw increase on permeability is greater for carbondioxide than for oxygen. The selectivity of these materials is thus sensitiveto moisture variations (for example, the selective coefficient of edible glutenfilms varies from 4.0 at aw = 0.30, to 25 at aw = 0.95), (Gontard et aL,1994b), whereas the selectivity coefficient for synthetic polymers remainsrelatively constant, at 4 to 5 (Table 5.4).

Edible films with selective gas permeability can be applied to reducedegradation of some fresh fruits and vegetables. In fact, the diffusion of

Table 5.4 Gas selectivity coefficient of various films (CO2/O2)

Gas selectivity TFilm coefficient (CO2/O2) (0C) aw

PP(2) 4.0 23 0.0PA(2) 4.2 23 0.0PVC(2) 5.8 23 0.0Wheat gluten(3) 7.5 25 0.0Corn zein(3) 9.5 25 0.0HPC and fatty acid(3) 23.7 25 0.0MC and PEG(3) 31.6 25 0.0HPC and PEG(3) 40.6 25 0.0

Gluten(I) 4.0 25 0.35Gluten(1) 4.5 25 0.62MC and beeswax(l) 6.7 25 0.56Gluten and beeswax0} 9.6 25 0.91Pectin(l) 16.0 25 0.96Chitosan(1) 17.0 25 0.93Gluten(l) 25.0 25 0.91Gluten*'> 2$A 25 0.95

(According to Gontard et aL, 1994b(1); Lefaux, 1972(2); Park and Chinnan,1990(3).)(aw = water activity; HPC = hydroxypropylcellulose; MC = methylcellulose; PA =polyamide; PEG = polyethylene glycol; PP = polypropylene; PVC = poly(vinylchloride).)

oxygen and carbon dioxide between fruit or vegetables and the environmentis a basic element in the post-harvest physiology of fruit (Burton, 1974,1978; Cameron and Reid, 1982). The type of coating material applied altersthe relative effects on the skin permeability to oxygen and carbon dioxide(Trout et a/., 1953). For example, waxing increased the carbon dioxidecontent and decreased the oxygen content of the orange's internal atmos-phere (Eaks and Ludi, 1960). The coating of peach surfaces with beeswax-coconut oil emulsion decreased oxygen gas transfer (Erbil and Muftugil,1986). Chitosan coatings retarded ripening and prolonged storage life oftomatoes, cucumber and bell pepper fruit without affecting their ripeningcharacteristics (El Ghaouth et a/., 1990, 1991). Lowings and Cutts (1982)reported that an edible composite coating (carboxymethylcellulose, sucrosefatty acid esters and mono- and diglycerides) would produce a semi-permeable modified atmosphere within fresh fruit after application. Thepost-storage uses of these coatings were investigated on apples (Chu, 1985;Drake et a/., 1987; Elson et al., 1985; Smith and Stow, 1984), on pears(Elson et al.y 1985), on bananas (Banks, 1983, 1984; Lowings and Cutts,1982), on limes (Motlagh and Quantick, 1988) and on mangoes (Dhalla andHanson, 1988). For example, the selective gas permeability properties ofthese films applied on bananas can reduce oxygen flux fivefold, while carbondioxide flux is only decreased by about half (Banks, 1984). Other examplesof gas permeability control by edible active layers are gelatin films used toprotect frozen meats from rancidity (Klose et ah, 1952), to coat candiesand dried products (Grouber, 1983) and to microencapsulate flavors(Anandaraman and Reineccius, 1980).

The development of edible active films with selective gas permeabilityseems have considerable potential applications for achieving a modifiedatmosphere effect for fruits and vegetables.

5.4 Modification of surface conditions with edible active layers

Edible active films and coatings can be applied on foods to modify andcontrol surface conditions. They can be used as surface retention agents tolimit food additive diffusion in the food core. The improvement of foodmicrobial stability can also be obtained by using edible active layers whichhave specific antimicrobial and pH lowering properties.

Surface microbial growth is a main cause of spoilage for many foodproducts (Gill, 1979; Maxci, 1981; Olson et al, 1981; Vitkov, 1973, 1974).Edible films and coatings can be used in combination with treatments suchas refrigeration and controlled atmosphere to improve the microbiologicalquality of certain foods. For example, calcium alginate-based films weretested to limit microorganism contamination on the surface of beef pieces(Williams et al., 1978). These films were found to have a significant effect

on microbial growth (natural microflora and coliform inocula). The specificantimicrobial activity of calcium alginate coatings has not yet beenexplained, but could be partially due to the presence of calcium chloride. ElGhaouth et aL (1990, 1991) used chitosan-based coatings to protect freshvegetables (cucumbers, peppers). These coatings improved fruit appearanceand reduced microbial degradation. This effect, which has also beenobtained on strawberries, can be attributed to the specific antifungalproperties of chitosan molecules (Allan and Hadwiger, 1979; Hirano andNagao, 1989; Stossel and Leuba, 1984). Zein-based films have been found tosuccessfully reduce microorganism penetration into chicken egg shells aftersurface inoculation (Tryhnew et aL, 1973).

Development of processes that specifically enhance surface microbialstability is required; food processors have used preservative dips and sprays.Potassium sorbate (or sorbic acid), which has a wide range of bacteriostaticand mycostatic properties, can be used by dipping to reduce the total numberof viable bacteria at both refrigeration and elevated temperatures(Cunningham, 1979; D'Aubert et aL, 1980; Lueck, 1984; Robach, 1979,Robach and Ivey, 1978; Robach and Sofos, 1982; To and Robach, 1980;Torres et aL, 1985b; Zamora and Zaritzky, 1987a, b). However, the shelf-lifeextension achieved by these surface treatments is limited by problemsrelated to potassium sorbate (or sorbic acid) stability and diffusion. Thestability of sorbic acid (in its active non-dissociated form) is dependent onapplication conditions; e.g. a lowered pH improves stability (Eklund, 1983).The diffusion into the core of the food results in a reduction in preservativeconcentration on the surface which allows microorganisms to overcome thesorbate-induced bacteriostasis (Greer, 1981; Torres, 1987).

It is important to be able to predict and control surface preservativemigration between phases during food treatments (e.g. absorption of sorbicacid during the processing of dried prunes, or its loss during cooking offabricated foods), storage of composite foods (e.g. dairy products or cakescontaining pretreated fruits), storage of foods in contact with wrappingmaterials or films containing sorbic acid (absorption by dairy productscovered with paper saturated with sorbic acid) and storage of foods coatedwith an external edible layer highly concentrated in sorbic acid (Guilbert,1988).

Edible films and coatings can be used as food preservative media(particularly as antioxygen and antifungal agents) and as surface retentionagents to limit preservative diffusion in the food core (Guilbert, 1986, 1988;Kester and Fennema, 1986; Torres et aL, 1985b; Vojdani and Torres, 1989a,b, 1990). Maintaining a local high and effective concentration of pre-servative may allow, to a considerable extent, a reduction of its total amountin the food for the same effect (as schematized in Figure 5.1), i.e. at thesurface of the food to reduce aerobic contamination and/or oxygeninfluence.

Time (days)

Figure 5.6 Tocopherol retention at 25°C in gelatin layers and in gelatin layers treated with tannicacid in contact with aqueous model food (aw = 0.95) or in contact with margarine (according toGuilbert, 1988). D, Gelatin layer - aqueous model food; •, gelatin layer treated - aqueous model

food; o, gelatin layer - margarine; • , gelatin layer treated - margarine.

Guilbert (1988) determined the retention rate of a-tocopherol in gelatin-based films formed on intermediate moisture foods and margarine (Figure5.6). The rapid decrease of tocopherol content in the gelatin layer in contactwith margarine is due to the high apparent solubility of tocopherol in fattymaterial. The low solubility of tocopherol in water, and so its low diffusivityvalue, explains the retention effect exerted by the gelatin layer in contactwith aqueous model food. Cross-linking treatment with tannic acid increasestocopherol retention by the gelatin layers in contact with margarine. Theseactive layers enriched with tocopherol could be used to reduce the oxidativedeterioration in some food products.

Guilbert (1988) also investigated the retention of sorbic acid in gelatin andcasein films treated respectively with tannic and lactic acid, and placed overan aqueous model food system (aw = 0.95). After 35 days at 25°C, aretention of 30% was observed with the treated casein film. Swelling andpoor retention were observed with the gelatin film (30% retention after 10days).

Sorbic acid retention has been studied in zein-based (Torres et aL, 1985b)and casein-based films (Guilbert, 1986, 1988), gluten-based and pectin-based films (De Savoye et aL, 1994), and in composite polysaccharide-lipidderivative films (Vojdani and Torres, 1990). The sorbic acid permeabilityvalues determined for these films can also be compared with publishedapparent diffusion values for sorbic acid in food systems. In an intermediate

% Retention

Table 5.5 Apparent coating diffusion coefficients for sorbic acid in inter-mediate moisture model food (agar model or cheese analog) at 24°C and aw =0.88, in agar gel (with sucrose or glycerol) at 25°C and aw = 0.88, and incheese analog coated with edible zein films

Diffusion coefficientExperiment (m2 s~')

Agar gel and sucrose(1) 0.5 X IO"10

Cheese analog(2) 1.0 X IO"10

Agar model(2) 2.0 X 10"10

Agar gel and glycerol(1) 3.5 X 10"10

Cheese analog and zein film(2) 3.3 X 10~13

Cheese analog and two zein films(2) 6.8 X 10~13

(According to Giannakopoulos and Guilbert, 1986a(l); Torres et al.,1985b(2).)

moisture agar model system, Guilbert et al. (1985) reported a value of 2.0 x10~10m2/sec. Torres et al. (1985b) found with an intermediate moisturecheese analog a value of 1.0 x 10"10m2/sec (Table 5.5). The sorbic aciddiffusivity values in edible films were found to be 150- to 300-fold lowerthan those determined for model intermediate moisture foods (Table 5.5).These few examples indicate that edible films could be used for additiveretention on the surface of food products.

Preservative diffusion through edible films is influenced by variousparameters: film characteristics (type, manufacturing procedure), foodcharacteristics (pH, aw), storage conditions (temperature, duration, etc.) andsolute characteristics (hydrophilic properties, molar mass).

The effect of the film-forming material on sorbic acid permeability hasbeen studied for various edible films (Table 5.6). It appears that filmcomposition (type of film forming agent, presence of lipids) affects even thesorbic acid permeability. For example, a 65% reduction of the sorbic acidpermeability of a methylcellulose edible film is observed when palmitic acidis added to the hydrocolloid matrix, there is a 75% reduction for ahydroxypropyl methylcellulose edible film.

Table 5.6 Sorbic acid permeability of various edible films at 24°C and aw =0.77

Sorbic acid permeabilityFilm (x 108 g mm m"2 s'1 (g/1)"1)

Chitosan 0.865MC 0.334HPMC 0.830

MC and palmitic acid (weight ratio 3:1) 0.120HPMC and palmitic acid (weight ratio 3:1) 0.205Zein Similar values

(According to Torres et al, 1985b; Vojdani and Torres, 1989a, b.)(HPMC = hydroxypropylmethylcellulose; MC = methylcellulose.)

According to Vojdani and Torres (1990), sorbic acid permeabilitydecreases in composite films (hydroxypropyl methylcellulose or methylcel-lulose and fatty acids) as the lipid derivative concentration increases; it alsodecreases as the length of the carbon chain in fatty acids increases and withthe presence of double bonds. This is consistent with published data on thesolute permeability of synthetic lecithin liposomes. De Gier, et al. (1968)found that increasing the lecithin fatty acid chain length decreased thepermeability of glycerol and erythritol through these artificial membranes.McElhaney et al. (1970) showed that the permeability of liposomes wasaltered by the geometrical configuration and the number of double bonds inthe fatty acid component.

The composite film-forming technique used to form composite films orcoatings (emulsions or multilayers) also affects the sorbic acid barrierproperties. According to Vojdani and Torres (1989a) the lowest permeabilityvalues are found with bilayer composite films. For example, the sorbic acidpermeability for an emulsion composite film of methylcellulose and palmiticacid was 3 times lower than for bilayer composite film.

As noted for water vapor and gas permeability, temperature and relativehumidity affect the permeability of edible film to sorbic acid (Figure 5.7). Atconstant temperature, the sorbate permeation rate decreased as aw decreased(Vojdani and Torres, 1989a; Rico-Pena and Torres, 1991). This is consistentwith studies in food model systems (Giannakopoulos and Guilbert, 1986a)where sorbic acid diffusivity rises at high aw.

Sorbic Add Permeability Sorbic Add Diffusivity(xlO f g.m.nr2 .*1 .**/!)-1) (xlOlf m ' . s 1 )

Water activity

Figure 5.7 Effect of water activity on sorbic acid permeability of edible multicomponent filmscomposed of methylcellulose and palmitic acid (weight ratio 3:1), at 24°C (•), (according to Rico-Pena and Torres, 1991); and on sorbic acid diffusivity in agar gels with sucrose (•) or glycerol (o)

at 25°C (according to Giannakopoulos and Guilbert, 1986b).

Temperature (0C)

Figure 5.8 Effect of temperature on sorbic acid permeability of edible multicomponent filmscomposed of methylcellulose (MC) or hydroxypropylmethylcellulose (HPMC) and palmitic acid(weight ratio 3:1); of edible chitosan films, at aw = 0.77 (according to Vojdani and Torres, 1989band 1990); and of edible pectin films, at aw = 1.0 (according to De Savoye et al., 1994). o, MC; •,MC and palmitic acid; a, HPMC; •, HPMC and palmitic acid; A, chitosan; A, pectin (SAP X

0.005).

An increase in temperature causes a decrease in film sorbic acid barrierproperties (Figure 5.8). These variations can easily be analysed throughArrhenius-type representations. Vojdani and Torres (1989b) noted thatactivation energy is affected by the solvent embedding the film, whichsuggests that the diffusion process in the film occurs mainly through theaqueous phase. Consequently, the performance of edible coatings controllingsurface preservative concentration will depend strongly on the compositionof the aqueous phase of the coated food.

The sorbic acid permeability of edible films varies according to pH(Figure 5.9). At constant aw and temperature, sorbic acid permeabilitydecreases when pH increases. An increase in pH lowers permeability,possibly due to a change in the sorbic acid formed around the pKa (equal to4.8), thus modifying the diffusion properties. Longer surface retentions ofsorbic acid are possible at higher pH (and also at lower aw and lowertemperature). It should be noted that the higher retention at higher pH wouldhelp balance the lowering of sorbic acid effectiveness as pH is increased(Eklund, 1983).

The microbiological analyses carried out have confirmed the sorbic acidretention efficiency of zein-based edible films, formed on the surface ofintermediate moisture cheese analogs, relative to microbial stability of thefood product (Torres et al., 1985b; Torres and Karel, 1985). These edible

Sorbic Add Permeability (SAP), (x 108 g . m . nr*. s ! . (g /1) ' )

pH

Figure 5.9 Effect of pH on sorbic acid permeability of edible multicomponent films (•) composedof methylcellulose and palmitic acid (weight ratio 3:1), at aw = 0.77 and 24°C (according to Rico-Pena and Torres, 1991) and of edible pectin (Psa X 0.01) films (D) at aw = 1.0 and 200C (according

to De Savoye et ah, 1994).

barrier layers double the shelf-life of the food product before the appearanceof microorganisms. Torres et al. (1985b) have assessed the potential sorbicacid retention efficiencies of edible films according to utilization conditions.For example, a composite film (methylcellulose-palmitic acid) applied to afood item (at pH = 5.0 and aw = 0.8) stored at 24°C reduces the amount ofsorbic acid diffused in the food mass by more than 50%. The effect thatedible active film (according to sorbic acid permeability) has on increasedsurface microbial stability has been estimated by Torres (1987). Forexample, with the use of a methylcellulose and palmitic acid (weight ratio =3:1) edible film on food stored at 24°C, then the permeability = 0.042 g mmm"2 24 h"1 (g/1)"1; the surface protection can be predicted to last 30 and 120days for a 0.1 mm film and 0.2 mm film respectively. If the item is storedunder refrigeration (at 5°C), these values increase to 82 and 328 daysrespectively (Torres, 1987; Vojdani and Torres, 1989a). According to theseauthors, these estimations need to be confirmed using a specific food systemand challenging microorganism.

Guilbert (1988) conducted microbiological tests on intermediate moisturefruit pieces (Table 5.7). Significant improvement of the microbial stabilitywas observed with coatings containing sorbic acid and no spoilage could bedetected after 40 days' storage in the case of papaya cubes coated withcarnauba wax containing sorbic acid. The coating efficiencies were in thefollowing order: carnauba wax + sorbic acid > carnauba wax > casein +sorbic acid > casein > no coating. The fact that casein film with sorbic acidwas less effective than the carnauba wax with sorbic acid may be explained

Sorbic Add Permeability (SAP), (x 10f g. m. m2 . s '. (g / W )

Table 5.7 Stability of coated intermediate moisture papaya cubes (as a function of aw) at 300Cinoculated with Aspergillus niger(a) or Staphylococcus rouxii{h)

Delay for first apparent spoilage aw aw aw aw

(days) 0.71 0.75 0.84 0.90

Control (non coated) (a) >40 >40 13 4(b) >40 >40 3 1

Coated with casein (a) >40 >40 12 4(b) >40 >40 2 2

Coated with casein and sorbic acid (a) > 40 > 40 22 10(b) >40 >40 >40 17

Coated with carnauba wax (a) > 40 > 40 > 28 10(b) >40 >40 >40 14

Coated with carnauba wax and (a) > 40 > 40 32 10sorbic acid (b) > 40 > 40 > 40 > 40

(According to Guilbert, 1988.)

by the poor retention properties and the high initial pH of the casein film(Guilbert, 1988).

The improvement of food microbial stability can also be obtained byreducing surface pH. This can be achieved by using films or coatings thatimmobilize either specific acids or charged macromolecules. Torres andKarel (1985) succeeded in obtaining a temporary pH difference between thesurface and the core of intermediate moisture foods by adding lactic acid tozein-based edible films. The development of edible films that immobilize

Contamination: togN (cells / cm2)

(uncoated control)

(reduced pH surface)

Time (days)

Figure 5.10 Effect of reduced surface pH on the microbiological quality of an intermediate moisturecheese analog coated with a carrageenan and agarose film; challenged with Staphylococcus aureusS-6 (aw = 0.88 and 35°C) (after Torres and Karel, 1985).

charged macromolecules has made it possible to obtain a pH differencebetween the surface and core of enveloped intermediate moisture foodproducts. A Donnan equilibrium model for semipermeable membranescontaining charged macromolecules can be used to assess the possibility ofobtaining a permanent pH difference between two components separated bya membrane. The use of agarose- and carrageenan-based edible films (withcharged macromolecules) decreases the surface pH by about 0.5. In additionto carrageenans, slightly esterified pectins could probably be used for similarapplications (Guilbert and Biquet, 1989). A decrease in surface pH can alsoimprove the stability and efficiency of antimicrobial agents such as sorbicacid.

Microbiological analyses have confirmed the antimicrobial efficiency ofthis type of surface treatment (Torres and Karel, 1985). Studies of bacterialgrowth (Staphylococcus aureus) on intermediate moisture foods revealedimproved microbiological stability with the use of carrageenan-based ediblefilms (Figure 5.10).

Edible films and coatings are a useful mean to reduce the rate of somedeteriorative reactions such as surface microbial development or oxidation.They could also be used to achieve the slow release of flavor compoundsduring food storage or consumption. Initial choices of film-formingmaterials and of fabrication conditions allow modification of solute retentionproperties of these edible active layers, and allow selection of the efficientfunctional properties in relation to each specific application, according tospecific needs.

5.5 Conclusion

Edible films and coatings can be used to control gas exchange (water vapor,oxygen, carbon dioxide, etc.) between the food product and the ambientatmosphere, or between components in a mixed food product, and to modifyand control food surface conditions (pH, level of specific functional agents,etc.). It should be stressed that the characteristics of the film or coating andthe application technique must be adapted to each specific utilization.

Edible superficial layers provide supplementary and sometimes essentialmeans to control physiological, microbiological and physicochemical chan-ges in food products. The active edible layers concept can thus be extendedto new fully adapted superficial or internal applications for food products.Among these new potential applications, layers enriched with susceptors formicrowave treatments or with catalyst for specific reactions on the one hand,or with flavor, preservative, ethanol, etc., for slow-release systems on theother hand, can be mentioned. The development of an active edible layer ismainly limited by formulation constraints, i.e. the layer composition must becompatible with the product characteristics and regulation, and by industrial

production constraints, i.e. coatings or preformed films applications proce-dure must be realizable on the industrial scale and be easily integratable inthe food processing line.

Acknowledgements

Dalle Ore Florence (CIRAD-SAR, Montpellier, France), ThibautRomain (CIRAD-SAR, Montpellier, France), Aymard Christian (CNRS,Montpellier- CIRAD-SAR, Montpellier, France) and Cuq Jean Louis(Universite de Montpellier II, France) are gratefully acknowledged forhelpful discussions and technical assistance. Part of the work presented herewas supported by the EC project: AIRl - CT92 - 0125.

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6 Interactive packaging involving sachettechnology

J.P. SMITH, J. HOSHINO and Y. ABE

6.1 Introduction

Over the past decade, there has been a tremendous growth in interactivepackaging for shelf-life extension of food. Packaging can be defined as'interactive' when it 'performs some role in the preservation of the foodother than providing an inert barrier to outside influences' (Rooney, 1992),There are many examples of interactive packaging technologies includingantimicrobial and antioxidant films, ethylene absorbing sachets and tem-perature control indicators, some of which have been discussed in previouschapters of this text. However, perhaps one of the best examples ofinteractive packaging, and one which fulfils the above definition in allaspects, is modified atmosphere packaging (MAP). MAP has been defined as'the enclosure of food products in a high gas barrier film in which thegaseous environment has been changed or modified to slow respiration rates,reduce microbiological growth and retard enzymatic spoilage with the intentof extending shelf-life' (Young et aL, 1988). The growth in MAPtechnology has resulted from advances in packaging technology, the foodindustry's need for less energy-intensive forms of food preservation thandrying, freezing or thermal processing, and consumer needs for conveniencefoods with extended shelf-life yet retaining their fresh characteristics. Theatmosphere in MAP foods has traditionally been modified by vacuum or gaspackaging, the latter involving various mixtures of CO2, either alone or inconjunction with nitrogen and sometimes oxygen depending on the productgas packaged, e.g., meat or fruits and vegetables (Smith et al., 1990).Although this form of interactive packaging can be used to extend the shelflife and keeping quality of food, aerobic spoilage can still occur in thesepackaged products depending on the level of residual oxygen in the packageheadspace. The level of residual oxygen in vacuum/gas packaged productscould be due to a number of factors such as oxygen permeability of thepackaging material; ability of the food to trap air; leakage of air throughpoor sealing; and inadequate evacuation and/or gas flushing (Smith et ai,1986).

In recent years, novel methods of oxygen control and atmospheremodification have been developed, primarily by the Japanese. These include

oxygen/carbon dioxide absorbents, oxygen absorbents/carbon dioxide gen-erators, and ethanol vapor generators. This technology involves the use ofsachets which can be placed alongside the food and actively modify thepackage headspace thereby extending product shelf-life. This chapter willreview the various types of absorbent/generator sachets available in themarketplace, the methods by which these sachets actively modify the gasatmosphere in the packaged product, and their uses, advantages anddisadvantages for shelf-life extension of food.

6.2 Oxygen absorbents

One novel and innovative method of oxygen control and of atmospheremodification involves the use of oxygen absorbents. Oxygen absorbents canbe defined as 'a range of chemical compounds introduced into the MAPpackage (not the product) to alter the atmosphere within the package' (Agri-Food Canada, personal communication). Developed in Japan in 1976,oxygen absorbents were first marketed by the Mitsubishi Gas Chemical Co.,under the trade name Ageless (Table 6.1). Several other Japanese companiesalso produce oxygen absorbents with the best known being the ToppanPrinting Company which produces a range of oxygen absorbents under thelabel Freshilizer Series (Smith et al., 1990). In 1989, almost 7000 millionsachets were sold in Japan with sales of absorbents growing at a rate of 20%annually. The Mitsubishi Gas Chemical Co. dominates the oxygen absorbentmarket (73%) while the Toppan Printing Co. has about 11% of the marketwith nine other companies sharing the remaining 16% of the market.Oxygen absorbent technology has been successful in Japan for a variety ofreasons, including the hot and humid climate during the summer monthswhich is conducive to mold spoilage of food products. Another importantfactor for their success is that Japanese consumers are willing to pay higherprices for preservative-free products with an increased shelf-life. Severalyears after the Mitsubishi Gas Chemical Co., successfully launched oxygenabsorbent technology onto the marketplace, Multiform Desiccants in theUnited States introduced FreshPax, the first North American oxygenscavenger (Table 6.1). Like its predecessors, FreshPax is extremely effectivein lowering package oxygen from 20% to less then 0.05% in about 36 hours.However, unlike the Japanese market, acceptance of oxygen scavengers in

Table 6.1 Major companies producing oxygen absorbents

Company Product name

Mitsubishi Gas Chemical Co., Japan AgelessToppan Printing Co., Japan FreshilizerMultiform Desiccants, US Freshpax

North America and Europe has been slow, although several majorcompanies in both continents are now using this technology.

Oxygen absorbents comprise of easily oxidizable substances usuallycontained in sachets made of air-permeable material. These sachets come ina variety of sizes capable of absorbing 20-2000ml of headspace oxygen.When placed inside the packaged food, they actively modify the packageheadspace and reduce the oxygen levels to < 0.01% within 1-4 days atroom temperature. However, some are designed to scavenge oxygen atrefrigerated or frozen storage temperatures and are used to further extend theshelf-life and keeping quality of muscle foods. This oxygen-free environ-ment protects the food from microbiological and chemical spoilage and isalso effective in preventing damage by insects. A combination of some or allof these factors helps maintain the quality and freshness of food, whichfacilitates the marketing of oxygen absorbents. The classification and themain types of oxygen absorbents will now be briefly reviewed.

6.2.7 Classification of oxygen absorbents

Oxygen absorbents can be classified into several different categories asshown in Table 6.2. Each category will be briefly reviewed.

6.2.1.1 Classification according to material. In theory, any material thatreacts easily with oxygen can be used as an oxygen scavenger. However,because the technology involved is used mainly for food preservation, thematerial used inside the sachet must meet the following criteria prior toapproval by regulatory agencies.

• It must be safe.• It must be handled easily.• It must not produce toxic substances or offensive odors/gases.• It must be compact in size.• It must absorb a large amount of oxygen.• It must have an appropriate oxygen absorption speed; and• It must be economically priced (Harima, 1990).

Iron powder and ascorbic acid are commonly used in existing oxygenabsorbers with powdered iron being most frequently used either alone or inconjunction with other specific chemical compounds in dual functionabsorbents (Table 6.2).

6.2.1.2 Classification according to reaction style. Water is essential foroxygen absorbents to function. In the self-reaction types, water required forthe chemical reaction is added and the absorbents must be handled carefullyas the oxygen absorbing reaction commences as soon as the self-reactingabsorbent is exposed to air. In moisture dependent types, the oxygen

absorption reaction only takes place after moisture has been absorbed fromthe food; these type of absorbents are easier to handle as they do not reactimmediately upon exposure to air. However, they absorb oxygen quicklyafter sealing and oxygen can be absorbed within 0.5-1 day in certainproducts. Examples of self-reacting and moisture dependent types of oxygenabsorbents are shown in Table 6.2.

62.13 Classification according to reaction speed.. Oxygen absorbentscan be classified as immediate effect, general effect, and slow effect types(Harima, 1990). The average time for oxygen absorption is 0.5-1 day for theimmediate type; 1-4 days for the general type; and 4-6 days for the slow

Table 6.2 Classification of oxygen absorbents

Function

O2I

O2I &CO2I

O2I &CO2T

O2I &EthanolT

Reactant

Iron

Catechol

Iron + Calcium

Ascorbic acid

Ascorbic acid +Iron

Iron +Ethanol/Zeolite

Application

Self-working type

Moisturedependent type

Self-working type

Self-working type

Self-working type



Dryaw < 0.3Tea; nuts

Medium aw

aw < 0.65Dried beef

High aw

aw > 0.65CakesBakeries

Frozen temp+ 3 to - 25°CRaw fish

High aw

aw > 0.85Pastas

Medium aw

aw < 0.65Nuts

High aw

aw > 0.65CakesRoasted coffee

Medium aw

0.3 < aw < 0.5Nuts

High aw

aw > 0.85Cakes

High aw

aw > 0.85Cakes

Absorptionspeed

4 to 7 days

1 to 3 days

0.5 days

3 days at- 25°C

0.5 days

3 to 8 days

1 to 4 days

Product

Ageless Z-PKVitalon T

Ageless ZKeplon TS

Ageless SSequl CA

Ageless SS

Ageless FXVitalon LTM

Tamotsu A

Tamotsu P

Ageless E

Ageless GToppan C

Vitalon GMA

Negamold

Time (day)

Figure 6.1 Effect of storage temperature on oxygen absorbing speed of Ageless S-IOO, a selfworking type (fast working type) of oxygen absorber.

reacting type (Harima, 1990). The reaction time depends on storagetemperature and water activity (aw) of the food. The effect of storagetemperature on oxygen absorbing speed is shown in Figures 6.1 and 6.2.Most oxygen absorbents are used with foods stored at ambient temperature.

O 2 c

once

ntra

tion

(%)

O2 c

once

ntra

tion

(%)

Time (day)

Figure 6.2 Effect of storage temperature on oxygen absorbing speed of Ageless Z-100, a selfworking type (medium working type) of oxygen absorber.

However, if used with refrigerated or frozen products these absorbents reactvery slowly. To overcome this problem, some absorbents can now scavengeoxygen rapidly from the package headspace of food stored under low-temperature conditions. A comparison of the oxygen absorbing speed of onesuch type of oxygen absorbent (Ageless SS) with absorbents which functionmainly at ambient storage temperatures, is shown in Figure 6.3.

6.2.1.4 Classification according to use. Oxygen absorbents can be usedfor a variety of foods with different moisture contents ranging from verymoist to very dry food products. In general, foods with a high moisturecontent are more susceptible to mold spoilage. Therefore, an immediateeffect absorbent would be used with these products to rapidly absorb oxygenand extend the mold-free shelf-life of the product. General type absorbentsare used with intermediate moisture food products where a moderate speedof oxygen absorption is required. For low moisture food products, which arenot susceptible to microbial spoilage but to chemical spoilage, a slow effectoxygen absorbent can be used. Examples of the application of the variouscategories of oxygen absorbents are shown in Table 6.2.

6.2.1.5 Classification according to function. The majority of absorbentshave only one function - absorption of oxygen. However, dual functionalabsorbents have been developed for use in specific products. These includeoxygen-carbon dioxide absorbents for use in coffee, and oxygen absor-

02 c

once

ntra

tion

(%)

AGELESS FX-100

Time (day)

Figure 6.3 Effect of frozen temperature (-250C) on oxygen absorbing speed of three types ofoxygen absorbents.

bents-carbon dioxide generators. The latter type of sachets absorb O2 andgenerate the same amount of CO2 as that of absorbed oxygen. They aremainly used in products where package volume and package appearance iscritical, e.g., packaged peanuts. These sachets contain iron carbonate andascorbic acid as the reactants. Another dual functional absorbent scavengesoxygen and releases alcohol vapor. These absorbents could be used tocontrol the growth of facultative bacteria and yeasts which grow underreduced oxygen tensions. However, this type of dual functional absorber isnot widely used by the Japanese food industry (Harima, 1990). Examplesof single function and dual function oxygen absorbents are shown inTable 6.2.

6.2.2 Main types of oxygen absorbents

6.2.2.1 Ageless. Ageless, made by the Mitsubishi Gas Chemical Co.,consists of a range of gas scavenger products designed to reduce oxygenlevels to less than 100 ppm in the package headspace. While both organictypes (based on ascorbic acid) and inorganic types (based on iron powder)are available, the inorganic types are most commonly used in the Japanesemarket. The basic system is made up of finely divided powdered iron which,under appropriate humidity conditions, uses up residual oxygen to form non-toxic iron oxide, i.e., it rusts. The oxidation mechanism can be expressed asfollows (Harima, 1990; Smith et ai, 1990; Smith, 1992):

Fe -> Fe2+ + 2e

^O2H- H2O + 2e -> 2OH-

Fe2+ + 2(OH)- -> Fe(OH)2

Fe(OH)2 + J O2 +\ H2O -> Fe(OH3)

To prevent the iron powder from imparting color to the food, the iron iscontained in a sachet (like a desiccant). The sachet material is highlypermeable to oxygen and, in some cases, to water vapor.

Since Ageless relies on a chemical reaction and not on the physicaldisplacement of oxygen as in gas packaging, it completely removes all tracesof residual O2 and protects the packaged food from aerobic spoilage andquality changes.

Several types and sizes of Ageless sachets are commercially available andare applicable to many types of foods including those with a high moisturecontent, intermediate moisture products, low moisture product foods andfoods containing or treated with oil. The main types of absorbents are typesZ, S, FX, E and G. Three new types of Ageless are commercially availablein Japan. These are Ageless types SS, FM and SE (Table 6.3).

Table 6.3 Types and properties of Ageless oxygen absorbents

Type

ZSSS

FXFME

G

SE

Function

Decreases O2

Decreases O2

Decreases O2

Decreases O2

Decreases O2

Decreases O2

Decreases CO2

Decreases O2

Increases CO2

Decreases O2

Increases ethanol

Moisture status

Self-reactingSelf-reactingSelf-reacting

Moisture dependentMoisture dependentSelf-reacting

Self-reacting

Moisture dependent

Water activity

<0.65> 0.65> 0.85

>0.85> 0.85< 0.3

0.3-0.5

>0.85

Absorption speed(day)

1-30.5-2.0

2-3 (0-40C)10 ( -25 0 C)

0.5-1.00.5-1.0

3-8

1-4

1-2

Type Z is designed for food products with water activities of less than0.65 and reduces residual headspace oxygen to 100 ppm in 1-3 days. It isavailable in sizes that can scavenge 20-2000 ml of oxygen (an air volume of100-10 000 ml).

Two other types of Ageless (FX and S) work best at higher wateractivities and have a faster reaction rate (0.5-2 days). They have the sameoxygen scavenging capacity as above. Type FX is moisture dependent anddoes not absorb oxygen until it is exposed to an aw greater than 0.85. Thus,it can be easily handled if kept dry. Type S, on the other hand, containsmoisture in the sachet and is a self-working type. This type of absorbentrequires careful handling since it begins to react immediately on exposure tooxygen. Absorbent type SS is similar to type S. However, it has the abilityto rapidly scavenge oxygen under refrigerated and frozen storage conditions.These absorbents (Ageless type SS) are widely used to extend therefrigerated shelf-life of muscle foods such as fresh meat, fish and poultry.Yet another new absorbent is type FM which can be used with micro-waveable products (Table 6.3).

A commonly used absorbent is type E which also contains Ca(OH)2 inaddition to iron powder. Type E scavenges CO2 as well as O2. It is used forground coffee, where CO2 removal reduces the chance of the packagebursting. Marketed under the brand name Fresh Lock, it is used in MaxwellHouse ground coffee cans (Table 6.3).

Two other types commonly used in the Japanese market are type G andtype SE. Type G is a self-working type and absorbs oxygen and generates anequal volume of CO2. It is used mainly with snack food products, such asnuts, to maintain the package volume and hence appearance of the product.Another new innovation is Ageless type SE. This absorbent is moisturedependent and absorbs oxygen and generates ethanol vapor. It is used toextend the mold-free shelf-life of bakery products in Japan. The varioustypes of Ageless and their characteristics are summarized in Table 6.3.

Table 6.4 Types and properties of Freshilizer oxygen absorbents

Type

F SeriesFDFHFT

C SeriesC

CW

CV

Function

(ferrous metal)Decreases O2

Decreases O2

Decreases O2

(Non-ferrous metal)Decreases O2

Increases CO2

Decreases O2

Increases CO2

Decreases O2

Decreases CO2

Moisture status

Self-reactingSelf-reactingMoisture dependent

Self-reacting

Self-reacting

Self-reacting

Water activity

<0.80.6-0.9> 0.8

< 0.8

0.8-0.9

< 0.3

Absorption speed(day)

1-30.5-1.00.5-1.0

3-5

2-3

3-8

Courtesy of Toppan Printing Co., Tokyo, Japan.

6.2.2.2 Freshilizer series. The Freshilizer Series of freshness keepingagents made by Toppan Printing Co., Japan, consists of the F series and theC series (Table 6.4). Series F Freshilizers use mainly ferrous metal andabsorb only oxygen. Three types are commercially available - types FD, FHand FT. Type FD is designed for use in food products with aw values lessthan 0.8 (nuts, tea, chocolate) while type FH is suitable for use in productswith a range of aw values ranging from 0.6 to 0.9 and is used mainly withbeef jerky and salami to maintain the color of these products. Type FTworks best in foods with water activities greater than 0.8 such as pizzacrusts. Series F absorbents can absorb 20-300 ml O2 corresponding to apackage volume of 100-1500 ml of air (Toppan Technical Information,1989).

The Freshilizer C series of absorbents comprises types C, CW and CV.These sachets consist of non-ferrous metal particles and can therefore beused in products which must pass through a metal detector. Types C and CWabsorb oxygen and generate an equal volume of CO2 thereby preventingpackage collapse. Type C is used in foods with an aw of 0.8 or less (nuts)while type CW is suitable for foods with higher aw values (i.e. > 0.8). TypeCW is commonly used to prevent mold growth in sponge cakes. Type CVabsorbs both oxygen and carbon dioxide and was developed for use withroasted or ground coffee (Table 6.4; Toppan Technical Information,1989).

6.2.2.3 FreshPax. FreshPax is a patented oxygen absorbing systemdeveloped by Multiform Desiccants, a leading manufacturer of desiccantsand other protection products for more than 30 years. Manufactured in theUnited States, FreshPax, like other oxygen absorbent technology provides analternative to gas/vacuum packaging to extend the shelf-life and keepingquality of products while simultaneously reducing costs and increasing

Table 6.5 Types and properties of Freshpax™ oxygen absorbents

Absorption speedType Function Moisture status Water activity (day)

Type B Decreases O2 Moisture dependent > 0.65 0.5-2.0Type D Decreases O2 Self-reacting < 0.7 0.5-4.0

at 2° to - 200CType R Decreases O2 Self-reacting 0.3-0.95 0.5-1.0

(depending on temp.)Type M Decreases O2 Moisture dependent > 0.65 0.5-2.0

Increases CO2

Courtesy of Multiform Desiccants, Buffalo, NY.

profitability. Produced in sachet form, FreshPax absorbs headspace oxygento < 0.1% using safe, non-toxic ingredients (mainly iron oxide) that rapidlyabsorb oxygen before the food deterioration process begins. Four main typesof FreshPax are commonly available - type B, type D, type R and type M(Table 6.5). Type B is used with moist or semi-moist foods while type D canbe used with dehydrated or dried foods. Type R can be used to scavengeoxygen at refrigerated or frozen storage temperature and is similar toAgeless type SS. It is mainly used to extend the shelf-life and keepingquality of muscle foods. Type M is used with most or semi-moist gasflushed products to maintain package volume and to remove all traces ofresidual oxygen (Table 6.5; FreshPax Technical Pamphlet, 1994).

6.2.3 Factors influencing the choice of oxygen absorbents

Oxygen absorbent sachets come in a range of sizes capable of absorbing 5ml to 2000 ml of oxygen. Several interrelated factors influence the choice ofthe type and size of absorbent selected for shelf-life extension of foodproducts (Harima, 1990; Smith et ai, 1990; Smith, 1992). These are:

• The nature of the food, i.e., size, shape, weight.• The aw of the food.• The amount of dissolved oxygen in the food.• The desired shelf-life of the product.• The initial level of oxygen in the package headspace.• The oxygen permeability of the packaging material.

The latter parameter is critically important for the overall performance of theabsorbent and shelf-life of the product. If films with high O2 permeabilitiesare used, e.g., > 100 cm3 mil m~2 day"1 atm"1, the oxygen concentration inthe container will reach zero within a week but then returns to ambient airlevel after 10 days due to the fact that the absorbent is saturated. However,if films of low O2 permeability are used, e.g., < 10 cm3 mil m 2 day1 atm1

such as PVDC coated nylon/LDPE, the headspace O2 will be reduced to 100ppm within 1-2 days and remain at this level for the duration of the storage

Table 6.6 Examples of films used with oxygen absorbents (FreshpaxTechnical Pamphlet, 1994)

Film laminates OTR MVTRincluding (ml/m2/d) (g/m2/d)

Long-term preservation Aluminium < 0.6 < 0.6EVOH < 3 < 4PVDC < 15 < 8

Short-term preservation Nylon < 16 < 40PET < 15 < 100

Not appropriate Cellophane < 200 <20PP < 2000 < 6PE < 3000 < 5

OTR = oxygen transmission rate; MVTR = moisture vapor transmissionrate; EVOH = ethylene vinyl alcohol; PVDC = polyvinylidene chloride;PET = polyester; PP = polypropylene; PE = polyethylene.

period providing packaging integrity is maintained. Examples of appropriatepackaging materials for use with oxygen absorbents and their permeabilitycharacteristics are shown in Table 6.6.

A rapid and efficient method of monitoring package integrity throughoutthe storage period is through the incorporation of a redox indicator, e.g.,Ageless Eye. When placed inside the package alongside an Ageless sachet,the color of the indicator changes from blue (oxidized state) to pink (reducedstate) when the oxygen content of the container reaches < 0.005%. If theindicator reverts back to its blue color, it is indicative of poor packingintegrity caused by poor sealing of the film or minute pin holes in die film.Similar redox indicators are made by the Toppan Printing Company.

6.2.4 Application of oxygen absorbents for shelf-life extension of food

6.2.4.1 In Japan. Several studies have reported significant extension inthe chemical and microbiological shelf-life of low, intermediate and highmoisture food products using oxygen absorbent technology (Nakamura andHoshino, 1983; Abe and Kondoh, 1989; Harima, 1990; Minakuchi andNakamura, 1990). As a result of these studies, oxygen absorbent technologyis being used extensively in Japan to prevent chemical problems, e.g.,discoloration problems in highly pigmented products, such as cured meatproducts and tea, and rancidity problems in high fat foods, such as peanuts,fried rice cake and chocolate covered almonds (Figures 6.4 and 6.5; Table6.7). Oxygen absorbents are also used effectively to inhibit growth ofaerobic spoilage microorganisms, especially molds, in intermediate moistureand high moisture bakery products. Examples of products found in Japanesesupermarkets with extended chemical and microbiological shelf-life as aresult of oxygen absorbent technology are summarized in Tables 6.8-6.11.More detailed information on the use of both Ageless and Freshilizer oxygen

(MONTHS)

Figure 6.4 Effect of an oxygen absorbent on the vitamin C content of Green tea. Bag made fromoriented nylon/polyethylene/foil/polyethylene. Conditions: Material, 100 g; air, 200 ml; storage

temperature, 25°C; absorber, Ageless Z-50.

Tota

l Vi

tam

in C

(mg/

100g

)PO

V (p

erox

ide

value

: m

eq/k

g)

AGELESS

AIR

AIR N 2 gas

AGELESS

Time (day)

Figure 6.5 Effect of oxygen absorbents on the peroxide value (POV) of packaged fried rice cakesby comparison under photo-irradiation. S, Sunlight; F, fluorescent light.

Table 6.7 Effect of an oxygen absorbent on the shelf life quality ofchocolate-coated almonds.

40 days 3 months 6 months

Ageless O2(%) 0.01 0.01 0.01POV almond 2.3 1.2 1.5POV chocolate 1.1 1.0 1.0Flavor score 5 5 4

Air O2(%) 18.2 13.3 9.5POV almond 7.8 16.4 23.5POV chocolate 0.9 1.2 2.1Flavor score 4 3 2

(Almond without chocolate coating)Air O2(%) 14.3 6.7 1.7

POV almond 14.0 21.1 28.8Flavor score 2 1 1

Material: 20Og in an ON/PVDC/PE bag; Water activity: 0.31;Moisture content: 2.1% (almond), 0.8% (chocolate); Stored: 25°C;Ageless: Z-50PK. Flavor score: 5 = no rancid smell, good aroma andgood flavor; 4 = aroma slightly lost but good flavor; 3 = little rancidsmell but good flavor; 2 = rancid smell and flavor weakened;1 = obvious rancid smell and flavor loss.

absorbents for shelf life extension of food can be found in the excellentarticles referenced at the beginning of this section.

62.42 In the USA. While oxygen absorbents are used extensively in Japan,their use in North America is still in its infancy. Examples of the known UScompanies currently using oxygen absorbents for shelf-life extension ofproducts are shown in Table 6.12. The earliest use of this technology was withMaxwell House coffee which used a dual function absorbent (Ageless E). Thisabsorbent, marketed under the trade name Fresh Lock, contains iron powderfor absorption of oxygen and calcium hydroxide which scavenges carbondioxide. The use of this absorbent in coffee delays oxidative flavor changesand absorbs the occluded carbon dioxide produced in the roasting process and,which if not removed, would cause the packages to burst. More recently, amold-free shelf-life of 1 year has been achieved for a specialty therapeuticgluten-free bread, Ener-Getic. The bread is packaged in a copolymer film ofMylar/EVOH/Surlyn film with 100% carbon dioxide and a Freshilizer oxygenabsorbent/carbon dioxide generator (type CW) supplied by the Toppan PrintingCompany, Japan. The bread remained mold-free for 1 year at room temperatureand physico-chemical changes, i.e., staling, were minimal at the end of storagein the gluten-free bread (Anon., 1988).

Studies by Powers and Berkowitz (1990) at the US Army Natick Research,showed that a FreshPax oxygen scavenger enclosed in a high gas barrier pouchmade of polyester/aluminium foil/HDPE with baked, meal ready-to-eat (MRE)bread prevented mold growth on bread for 13 months at ambient storagetemperature. Based on the results of this study, two companies, Sterling Foods

Table 6.8 Use of Ageless for shelf-life extension of bakery products

Shelf-life @ RT*PreventsType and size ofAgelessPackaging materials

Water activity offoodType of food

1 month1 month1 month1 month3-4 weeks3-4 weeks6 months7-10 days1 month1 month2 months2-3 weeks

Mold growth/oxidationOxidation/mold growthMold growthMold growthMoid growthMold growthOxidationMold growthMold growthMold growthMold growthMold growth

S200SIOOS200SlOOFXlOOSlOOZ200S50S200S10-15ZPTl 00-200S200

KONKONKONKONKONKOPKON/BoxPET/PVDC/PEKONKONKON/PS TrayPP/EVAL/KON/PE

0.88-0.920.9

0.86-0.880.90.90.9

<0.60.92-0.940.88-0.9

0.930.850.9

CheesecakeButtercakePlain sponge cakeEgg breadPumpernickel breadPita breadTortilla chipsCustard cakesBlueberry cheesecakeSoy bean pieBean/jam cakeSteamed bean/jam cake

*RT = Room temperaturePET = polyester; PE = low-density polyethylene; AC = acrylonitrile; KON = PVDC-coated nylon; KOP = PVDC-coated polypropylene; PVDC = polyvinyli-dene chloride; PP = polypropylene; EVAL = ethylene vinyl alcohol; PS = polystyrene.

Table 6.9 Use of Ageless for shelf-life extension of fish products

Shelf-life @ RT*PreventsType and size ofAgelessPackaging materials


1 year3 months6 months1-2 months1 month6 months3 months1 week at - 30C6 months at-200C1 month at 100C1 month3-6 months6 months at - 3°C3 months

2 weeks at - 5 0C

Oxidation/DiscolorationOxidation/DiscolorationOxidationOxidation/Mold growthOxidationOxidationOxidation/Mold growthOxidation/DiscolorationDiscoloration

Mold growth/Discoloration/OxidationOxidation/Mold growthOxidation/DiscolorationDiscolorationOxidation/Mold growth

Discoloration/Bacterial growth

ZlOOZ50Z100-200S200S2000ZlOOZlOOSSlOOFXlOO

ZlOOFXlOOTypeZ200+100%N2

SSlOOZ200

SSlOO

PET/AC/PEKONKONKONKON/PET TrayKOPKONNylon/PVDC/PPKON

KONKONKONKONKOP

Wood Tray/KON

<0.30.7-0.8

< 0.6-0.880.8

0.7-0.8< 0.60.7-0.8

0.990.99

0.8< 0.850.6-0.70.8-0.90.7-0.8

0.98

Dried seaweedDried salmon jerkyDried sardinesDried shark's finDried rose mackerelDried codDried squidFresh yellow taileSliced salmon

Dried/smoked salmonDried octopus legDried bonitoSalmon roeDried squid/vinegar/soybean sauce

Sea urchin

* RT = Room temperature

Table 6.10 Use of Ageless for shelf-life extension of meat and deli products

Shelf-life @ RTPreventsType and size ofAgelessPackaging materials


2 months at 50C2 weeks at 50C3 weeks at 0-40C3 weeks at 0-40C3 weeks at 0-40C1-2 months1 month

Mold growthMold growth/DiscolorationDiscoloration/RancidityDiscoloration/RancidityDiscoloration/RancidityDiscolorationDiscoloration

FX200S200FX50SlOOFXlOOZ50SlOO

KONKONKONPS Tray/KONKONKONKON

~ 0.94-0.950.990.990.990.990.8-0.850.8-0.85

Pizza crustPizzaFresh carved sausagePre-cooked hamburgersPre-cooked chicken nuggetsSalami slicksSliced salami

Table 6.11 Use of Ageless for shelf-life extension of miscellaneous products

Shelf-life @ RTPreventsType and size ofAgelessPackaging materials


1 year6 months9 months9 months1 month at 0-40C1 month at 0-40C3 months6-12 months1 year1 year

Oxidation/RancidityOxidation/RancidityOxidation/RancidityOxidation/RancidityOxidation/RancidityOxidation/Mold growthDiscolorationOxidation/RancidityOxidationOxidation

ZlOOGlOOZ200Z30-100FX50+100%N2

S30-50FX20ZlOOZ50Z50

Tin/Plastic/Aluminium lidKONKON/Paper boxKONKONKONPaper/AC/EVATin canTin canKON

<0.3<0.3

0.5-0.60.5-0.60.9-0.95

0.90.8-0.85

< 0.3<0.3

0.5-0.6

Rice crackers/fried beanPeanutsPeanut butter/chocolate coatedChocolate peanutsCandy-type cheeseSmoked cheeseSoy bean pasteBakery food (milk powder)Green teaFlavored tea

EVA = ethylene vinyl acetate.RT = Room Temperature.

Table 6.12 Use of oxygen absorbent technology in USA_ _ -

Company Product (months @ RT1)

General Foods Maxwell House coffee 12-36Ener-G Foods Ener-Getic gluten-free bread 12Sterling Foods MRE2 bread 36Franz Bakery MRE bread/buns 6The Famous Pacific

Dessert Company Fruit tortes 6-91RT = room temperature2MRE = meals, read-to-eat

uses Ageless oxygen absorbents to prevent rancidity and mold problems inpackaged tortes (Table 6.12). Clearly, there is an increasing interest by manycompanies, particularly baking companies, in the use of oxygen absorbenttechnology as an alternative or adjunct to gas packaging to prevent the perennialproblem of mold growth.

6.2.4.3 Research activities in oxygen absorbent technology. Although theuse of oxygen absorbent technology has not been accepted commerciallywith the same enthusiasm in North America compared to Japan, there is,however, tremendous interest and ongoing research in this innovativemethod of atmosphere modification.

Studies in our laboratory have shown oxygen absorbents to be three timesmore effective than gas packaging for increasing the mold-free shelf-life ofcrusty rolls (Smith et al., 1986). While the mold-free shelf-life of crusty rollscan be extended by packaging in 60% CO2 and 40% N2, mold growth stilloccurred after 19 days due to the oxygen permeability of the packaging film.Studies have shown that Aspergillus and Penicillium species, the major spoilageisolates of crusty rolls, can tolerate and grow in < 1% (v/v) headspace oxygeneven in the presence of elevated levels of CO2. However, when Ageless (TypeS or Type FX) was packaged alongside crusty rolls, either alone or inconjunction with gas packaging, headspace O2 never increased beyond 0.05%and the product remained mold-free for > 60 days at ambient storagetemperature (Smith et al., 1986). While a longer extension in the mold-freeshelf-life was possible using oxygen absorbents, mold problems occasionallyarose in the Ageless packaged product. This was due to absorption of headspacegas and package collapse resulting in some products being tightly wrappedwith the packaging film. This created pockets of localized environmentsbetween the product surface and the film where the oxygen concentration mayhave increased to a level sufficient to permit mold growth. This clearlydemonstrates the need for a free flow of gas around the product if Agelessis to be totally effective as an oxygen scavenger (Smith et al, 1986).

More recently, Ageless oxygen absorbents have been used in ourlaboratory to extend the mold-free shelf-life of fresh bagels and pizza crusts.

with the packaging film. This created pockets of localized environmentsbetween the product surface and the film where the oxygen concentration mayhave increased to a level sufficient to permit mold growth. This clearlydemonstrates the need for a free flow of gas around the product if Agelessis to be totally effective as an oxygen scavenger (Smith et a/., 1986).

More recently, Ageless oxygen absorbents have been used in ourlaboratory to extend the mold-free shelf-life of fresh bagels and pizza crusts.Furthermore, the use of specific types of absorbents appeared to have ananti-staling effect on bagels. Further studies are now underway to comparethe anti-staling effects of oxygen absorbents with other methods ofatmosphere modification for shelf-life extension of bakery products.

The use of absorbent technology for shelf-life extension of food has alsobeen extensively studied by Cryovac, a world leader in vacuum packagingtechnology. Cryovac in the US has been given the distribution rights toAgeless in North America by the Mitsubishi Gas Chemical Company.Researchers at Cryovac have found that oxygen absorbent technology can beused to prevent mold growth, discoloration and flavor changes in cookedcured sausage, cooked poultry and smoked poultry chops. Using acombination of oxygen absorbent technology and Cryovac's high barriershrink film, headspace oxygen (which can reach 4-7% due to entrappedoxygen in the foam tray) is reduced to < 0.3% before products are exposedto retail display lighting. Thus, oxygen absorbent technology can be used toinhibit photodegradation of meat pigments in cured meat products therebyenhancing consumer appeal for such products (Cryovac, personal commu-nication). Oxygen absorbent technology has also been investigated to inhibitmold growth and rancidity problems in fresh pasta and pizza crusts. Otherareas of applied research by Cryovac involving oxygen absorbents includeprevention of rancidity problems in snack food; prevention of flavor changesin wine stored in oxygen permeable PET bottles using a barrier pouchoverwrap with an oxygen absorbent; and master packaging of fresh meat andpoultry. This latter concept is in response to consumer concerns aboutabsorbents being visible inside the packaged food product. In the MasterPak,retail cuts of meat/poultry are packaged in trays with an oxygen permeableoverwrap. These are placed inside a cardboard carton with a high gas barrierfilm and absorbents added. The system is then evacuated, gas flushed andheat sealed and distributed under refrigeration to the retailer where theindividual packs are removed from the MasterPak and displayed on retailshelves. Color or 'bloom' returns to the meat/poultry due to the low barriercharacteristics of the overwrap.

Extensive research on Freshilizer absorbents has been conducted byMultiform Desiccants in conjunction with Dr Joe Hotchkiss at CornellUniversity. Studies by Alarcon and Hotchkiss (1993) have shown thatFreshPax oxygen absorbents can prevent mold growth in preservative-freewhite bread and low moisture, low fat mozzarella cheese for 8 weeks at

ambient/refrigerated temperature. Furthermore, oxygen absorbents were alsoeffective in reducing the oxidative formation of rc-hexanal (an indicator ofoff-flavor development) in both sunflower seeds and cornchips. Sensorypanel analysis also showed that absorbents inhibited the formation ofundesirable rancid odors during accelerated storage tests (Alarcon andHotchkiss, 1993). These authors also reported that FreshPax oxygenabsorbents were also effective in controlling permeated oxygen, and henceiiiold growth ai/id rancidity problems, in bread and cheese packaged injnedium barrieij films with an OTR of about 32 ml/m2/day. Therefore,medium barrier] films could be substituted for a high barrier and moreexpensive film if an oxygen absorber pack was included in the package(Alarcon and Hotchkiss, 1993).

6.2.5 Advantages and disadvantages of oxygen absorbents

Oxygen absorbents have several advantages for the food processor, bothfrom a marketing and food quality viewpoint (Harima, 1990; Smith et ai,1990; Smith, 1992). These are:

• Inexpensive and simple to use.• Approved by the US Food and Drug Administration as non-toxic and

safe to use.• Prevent aerobic microbial growth and extend shelf-life of product.• Arrest the development of rancid off-flavor in fats and oils.• Maintain flavor quality by preventing oxidation of the flavor com-

pounds.• Maintain product quality without additives.• Increase product shelf-life and distribution radius.• Allow a long time between deliveries and allow fewer, longer deliv-

eries.• Increase length of time product can stay in the distribution pipeline.• Reduce distribution losses.• Replace chemical pesticides to prevent insect damage of foods.• Reduce evacuation/gas flushing times in gas packaged products thereby

increasing product throughput.• Reduce costs required for gas flushing equipment.

A few of the disadvantages of oxygen absorbents are (Harima, 1990; Smithet aL, 1990; Smith, 1992):

• There needs to be a free flow of air surrounding the sachet in order toscavenge headspace oxygen if an O2 absorbent is used alone.

• May cause package collapse; this can be overcome by using an O2

absorbent/CO2 generator.

• O2 absorbents/CO2 generators may cause flavor changes in highmoisture/fat foods due to the dissolution of CO2 in the aqueous/fat phaseof product.

• Cost - approximately 2.5-10 cents per sachet depending on size ofsachet and volume ordered. Many processors regard oxygen absorbentsachets too expensive.

• Consumer concerns about sachets inside the packages and possibleconsumer misuse of sachets.

• May promote growth of potentially harmful anaerobic bacteria.

The two latter concerns will be briefly discussed.

6.2.5.1 Consumer resistance to sachets. The two main consumer con-cerns about sachets being placed inside the packaged food product are: fearof ingestion even although the contents are safe and the label clearly says'Do not eat'; and spillage of sachet contents into food and consequentadulteration of the food product. These concerns have been addressed by theMitsubishi Gas Chemical Company by producing the deoxidizer in a tabletform or in a board form and affixing it to the lid or to the bottom ofpackages. This approach has been successfully used for health foods sold inbottles and with individually wrapped teacakes in the Japanese marketplace.An alternative method is to package the primary packages inside a secondarypackaging containing the absorbent. This is the principle behind theMasterPak system, i.e., bag-in-box system, designed to hold 12-48 con-sumer-size primary packages made of material with a higher oxygentransmission rate than for secondary containers, as discussed in the previoussection. The main advantage of this approach is that the consumer never seesthe absorbents, thus eliminating the possibility of consumers misusing thepackets or reacting negatively to the absorbents (Smith, 1992).

Another approach to overcome consumer resistance to sachets is toincorporate the oxygen scavenging capacity in the form of a package label.This approach has been developed by both Mitsubishi Gas ChemicalCompany and Multiform Desiccants. This latter company currently marketsboth Type B and Type M absorbents in the form of labels marketed underthe trade name FreshMax. This label can be backed with various adhesivesfor different product applications. The technology has a printed surface andis contact acceptable (oil/grease resistant). The label can be applied atconventional line speeds and is compatible with both moist and dry foodproducts. The prototype FreshMax is 2.5" x 2.5" and 15 mil thick. It iscapable of absorbing 100 ml of oxygen in 24 h and can be made into variousshapes. The labels can reduce headspace oxygen to less than 0.01% (100ppm) and can be used to retard both chemical and microbiological spoilagein stored products. Marks & Spencer Ltd., London are now using FreshMaxlabels to extend the shelf-life and keeping quality of deli meats. The new

labels cover only one third of the lid surface, providing consumers with aclearer view of the product yet absorb oxygen and protect products from theadverse oxidizing effect of light and oxygen on meat pigments (MultiformDesiccants, personal communication).

6.2.5.2 Public health concerns. Another consumer and regulatory con-cern about oxygen absorbent technology is that the anaerobic environmentcreated inside the package may be conducive to the growth of facultativeanaerobic or strictly anaerobic food pathogens. These concerns are justifiedin view of the ability of many food pathogens to grow at refrigeratortemperatures, e.g. Listeria monocytogenes and Clostridium botulinum typeE; the inhibition of aerobic microorganisms as indicators of spoilage; andthe potential for temperature abuse. Recent studies have shown that averagetemperatures of retail cases in supermarkets were about 12°C whiletemperatures of domestic refrigerators ranged from 2 to 200C (Palumbo,1986). Nakamura and Hoshino (1983) reported that an oxygen-freeenvironment alone was insufficient to inhibit the growth of Staphylococcusaureus, Vibrio species, Escherichia colif Bacillus cereus and Enterococcusfaecalis at ambient storage temperature. For complete inhibition of thesemicroorganisms they recommended combination treatments involving oxy-gen absorbents with thermally processed food or refrigerated storage or aCO2 enriched atmosphere. Using this latter combination, i.e., low O2-highCO2 gas atmosphere, they found that this modified atmosphere inhibited thegrowth of S. aureus and E. coli but promoted the growth of E. faecalis.While low levels of CO2 (20%) failed to inhibit the growth of B. cereus,higher concentrations (100%) completely inhibited the growth of thismicroorganism in an oxygen-free environment. When similar studies weredone with Clostridium sporogenes, some interesting results were obtained,as shown in Table 6.13.

These studies demonstrated that an O2/CO2 absorbent will inhibit thegrowth of C. sporogenes while an O2 absorbent-CO2 generator will enhanceits growth, indicating the importance of selecting the correct absorbent tocontrol the growth of Clostridium species in MAP food. In challenge studieswith C. botulinum type A and B spores, Lambert et al. (1991a) observed that

Table 6.13 Effect of various gas atmospheres on the growth of C. sporogenes(adapted from Nakamura and Hoshino, 1983)

Growth ofPackaging conditions C. sporogenes_ _

Ageless oxygen absorbent generating carbon dioxide 4 +Ageless oxygen absorbent not absorbing carbon dioxide 2 +Ageless oxygen absorbent absorbing carbon dioxide 1 +

- = no growth; 1+ = slight growth; 2+ = medium growth; 3+ = heavygrowth; 4+ = extensive growth.

package headspace was not significant in affecting the time until toxinproduction (Lambert et al, 1991b,c).

In more recent agar plate studies with Listeria monocytogenes, high CO2

levels (> 60%) promoted the growth of this pathogen at 10-150C (Morris etal, 1994). However, when an oxygen-free environment was achieved usingAgeless SS, growth of L. monocytogenes was completely inhibited even atmild temperature abuse storage conditions. Further studies are nowunderway to determine the antimicrobial efficacy of Ageless SS and gaspackaging on L. monocytogenes in packaged pork.

6.2.6 Effect of oxygen absorbents on afiatoxigenic mold species

While the use of oxygen absorbent technology may fail to completely inhibitthe growth of facultative or strictly anaerobic pathogenic bacteria, it is veryeffective in controlling the growth of Aspergillus flavus and Aspergillusparasiticus. In inoculation studies with these aflatoxigenic molds in peanutspacked in air alone and with an oxygen absorbent, mold growth andaflatoxin were completely inhibited in absorbent packaged peanuts while~1000 ppb (1000 ng) of aflatoxin B1 were detected in all air-packagedsamples after only 6 days at room temperature (Ellis et al, 1993). Similarcontrol of A. parasiticus has also been reported in inoculation studies withpeanuts using oxygen absorbent technology (Ellis et al, 1994). However,this control was dependent on both the OTR of the packaging films used andstorage temperature. While packaging in high-medium barrier filmsinhibited aflatoxin B1 production, aflatoxin was detected in all absorbentpackaged peanuts using a low barrier film (OTR -4000 ml/m2/day).However, when absorbent packaged peanuts were packaged in mediumbarrier films (OTR -50 ml/m2/day), aflatoxin production occurred in allpeanuts stored at 300C (Ellis et al, 1994). This was attributed to the greaterpermeance of the film to oxygen at higher storage temperatures, resulting insaturation of the absorbents, a concomitant increase in headspace oxygen,and subsequent mold growth and aflatoxin production. Similar results wereobtained with a CO2 generating oxygen absorbent (Ageless G), indicatingthe importance of packaging film permeability to ensure the efficacy ofoxygen absorbents and the public health safety of absorbent packagedpeanuts.

6.3 Ethanol vapor

The use of ethanol (ethyl alcohol) as an antimicrobial agent is welldocumented. Alcohol was used by the Arabs over 1000 years ago to preservefruit from mold spoilage. However, alcohol is most commonly recognized asa surface sterilant or disinfectant. In high concentrations (60-75% v/v),

ethanol acts against vegetative cells of microorganisms by denaturing theprotein of the protoplast (Seiler and Russell, 1993). Lower concentrations ofalcohol (5-20% v/v) have also been shown to have a preserving actionagainst food spoilage and pathogenic microorganisms in agar modelsystems. In tests with surface inoculated agar medium containing concentra-tions of ethanol ranging from 4 to 12% (v/v) ethanol was shown to beeffective in controlling 10 species of mold including Aspergillus andPenicillium species; 15 species of bacteria including S. aureus and E. coli;and 3 species of spoilage yeasts (Freund Technical Information, 1985). Mostmolds were inhibited by 4% ethanol while yeasts were more resistant andrequired 8% ethanol for complete inhibition. However, S. aureus required12% ethanol for complete inhibition (Freund Technical Information, 1985).Shapero et al. (1978) reported that the effectiveness of ethanol against S.aureus was a function of water activity (aw). They reported that in brothmedia adjusted to aw 0.99, 0.95 and 0.9 with glycerol, inhibition of S. aureusoccurred at 9, 7 and 4% of ethanol respectively. A similar synergismbetween aw and the antimicrobial efficacy of ethanol against A. niger and Rnotatum has been observed by Smith et al. (1994). In studies withunadjusted Potato Dextrose Agar (PDA) plates a concentration of 6%ethanol was required for complete inhibition of these common moldcontaminants of bakery products. However, complete inhibition of A. nigerand P. notatum was observed with 4 and 2% of ethanol respectively in platesadjusted to aw 0.95 and 0.9 with glycerol. These results confirm previousobservations that lower concentrations of ethanol can be used to extend themold-free shelf-life of food products with a low aw.

The antimicrobial effect of low concentrations of ethanol for shelf-lifeextension of bakery products has also been demonstrated. Plemons et al.(1976) showed that the mold-free shelf-life of pizza crusts could be extendedfor about 20 weeks at ambient temperature by spraying the crusts with 95%ethanol and overwrapping in a large unsealed polyethylene bag (Seiler,1978; Seiler, 1988). Seiler and Russell (1993) also reported a 50-250%increase in the mold-free shelf-life of cake and bread sprayed with 95%ethanol to give concentrations ranging from 0.5 to 1.5% by weight ofproduct. Both these studies indicate the potential of ethanol as a vapor phaseinhibitor. However, a more practical and safer method of generating ethanolvapor is through the use of ethanol vapor generating sachets, which will nowbe discussed in greater detail.

6.3.1 Ethanol vapor generators

A novel and innovative method of generating ethanol vapor, againdeveloped by the Japanese, is the use of ethanol vapor generating sachets orstrips. These contain absorbed or encapsulated ethanol in a carrier materialand enclosed in packaging films of selective permeabilities which allow the

Table 6.14 Manufacturers of ethyl alcohol generators

Trade name Manufacturer Sachets/yr Entered

Antimold Freund 60m 1978(Ethicap)Negamold Freund - 1982Oitech Nippon Kayaku 9 m 1980ET Pack Ueno Seiyaku 1 m 1988

6J.I Ethanol vapor generators

A novel and innovative method of generating ethanol vapor, againdeveloped by the Japanese, is the use of ethanol vapor generating sachets orstrips. These contain absorbed or encapsulated ethanol in a carrier materialand enclosed in packaging films of selective permeabilities which allow theslow or rapid release of ethanol vapor. Several Japanese companies produceethanol vapor generators, with the most commercially used system beingEthicap or Antimold 102 produced by the Freund Industrial Co., Ltd., Japan(Table 6.14).

Ethicap consists of food grade alcohol and water (55% and 10% byweight respectively) adsorbed on to silicon dioxide powder (35%) andcontained in a sachet made of a laminate of paper/ethyl vinyl acetatecopolymer. To mask the odor of alcohol, some sachets contain traces ofvanilla or other flavors as masking compounds. The sachets are labelled 'Donot eat contents' and include a diagram demonstrating this warning. Anothertype of ethanol vapor generator is Negamold (Table 6.15), which scavengesoxygen as well as generating ethanol vapor. Negamold, like its competitor,Ageless type SE, can be used to extend the shelf-life and keeping quality ofbakery products. However, Negamold is not widely used since it does notgenerate as much ethanol vapor as Ethicap (Freund Technical Information,1985).

Sachet sizes of Ethicap range from 0.6 to 6 G or 0.33 to 3.3 g of ethanol,which can be evaporated. The actual size of the sachet used depends on theweight of food; aw of food; and the desired shelf-life of product. Thisinterrelationship is shown in Figure 6.6. For example, if the aw of the productis 0.95 and a 1-2 week shelf-life is desired, then a 2 G size of Ethicap

Table 6.15 Types of alcohol generators_ _

Type Function Application products

Ethicap (Antimold 102) Generates EtOH vapor Moisture dependent Cakes/ > 0.85Bread

Negamold Absorbs O2 Moisture dependent Cakes/ > 0.85Generates EtOH vapor Bread

(~ 1.1 g ethanol) should be used per 100 g of product. However, if a longershelf-life is desired (2-3 months), a 4 G size of Ethicap should be used forthe same product (Freund Technical Information, 1985).

When food is packed with a sachet of Ethicap, moisture is absorbed fromthe food, and ethanol vapor is released from encapsulation and permeates thepackage headspace. However, both the initial and final level of ethanol vaporin the package headspace is a function of sachet size and water activity, asshown in Figure 6.7. Smith et al. (1987) examined this relationship in PDAplates adjusted to aw 0.95 and 0.85 with glycerol and packaged in a film oflow ethanol vapor permeability with a 1 G and 4 G (E1 and E4) size ofEthicap respectively. After day 1, the level of headspace ethanol generatedfrom a 1 G sachet of Ethicap (E1), ranged from 0.7% v/v at a water activityof 0.99 to 0.5% v/v at a water activity of 0.85 (Smith et al, 1987). However,after 7-10 days' storage, headspace ethanol was approximately 0.4% v/v ata water activity of 0.85 compared to 0.2% v/v at the highest water activityunder investigation and remained at these levels for the duration of thestorage period (Smith et al., 1987). A similar, higher trend was noted forheadspace ethanol generated from 4 G of Ethicap, E4 (Smith et al., 1987).These studies indicated the importance of product aw on the vaporization ofethanol into, and absorption of ethanol from, the package headspace.

Req

uire

d s

ache

t si

ze o

f E

TH

ICA

P®

per

10O

g o

f fo

od

for long term

for short term

Water activity of food

Figure 6.6 Relationship between aw of food and required size of Ethicap sachet. Short term, 1-2weeks; long term, 8-13 weeks. (Reproduced with permission from the Freund Technical Co., Ltd.,

Japan.)

DAYS STORAGE 25 *C

Figure 6.7 Effect of water activity (aw) on generation and absorption of ethanol vapor. •, 0.85 + E4;A 0.95 + E4; • , 0.99 + E4; o, 0.85 + E1;*, 0.95 + E1; o, 0.99 + E1.

Another important factor in using Ethicap as a food preservative is topackage the product in a film with a high or medium barrier to ethanolvapor. The ethanol vapor permeabilities of appropriate packaging materialsare shown in Table 6.16. Generally, a film with an ethanol vaporpermeability of < 2 g/m2/day @ 300C is recommended (Freund TechnicalInformation, 1985).

6.3.2 Uses of Ethicap for shelf-life extension of food

Ethicap is used extensively in Japan to extend the mold-free shelf-life ofhigh ratio cakes and other high moisture bakery products. A 5-20 timesextension in mold-free shelf-life has been observed for high ratio cakesdepending on the size of Ethicap used (Freund Technical Information,1985). Results also showed that products with sachets did not get as hard asthe controls and results were better than those using an oxygen scavenger to

-I*OZ<ZUl

UlO

(0

SUlX

Table 6.16 Ethanol vapor permeability of certain plas-tic films (adapted from Freund Technical Information,1985)

Ethanol vapor permeabilityFilm (g/m2/day) @ 300C_ _ _ _ _ ___

OPP/EVOH/LDPE 0.8PVDC/PET/PP 0.9PVDC/OPP/PP 1.0AL/PET/LDPE 1.2PP/PVDC/PP 1.5PET/PP 1.8OPP/PP 2.0HDPE 4.1OPP 4.7PP 8.0LDPE 19.0EVA/LDPE 56.1

PVDC = polyvinylidene chloride; PA = nylon;OPP = oriented polypropylene; PP = cast polypropyl-ene; PET = polyester; AL = aluminium; HDPE =high-density polyethylene; LDPE = low-density poly-ethylene; EVA = ethylene vinyl acetate.

inhibit mold growth, indicating that the ethanol vapor also exerts an anti-staling effect (Freund Technical Information, 1985). Ethicap is also widelyused in Japan to extend the shelf-life of semi-moist and dry fish products.Examples of bakery and fish products preserved by Ethicap in the Japanesemarket are shown in Table 6.17.

Pafumi and Durham (1987) also found that the mold-free shelf-life of 200g Madeira cake could be extended for about 6 weeks at room temperatureusing a 3 G sachet of Ethicap. However, there was a significant change in

Table 6.17 Use of Ethicap for shelf-life extension of food (adapted from Freund TechnicalInformation. 1985")

Food

Bakery products

BreadCupcakeJam doughnutAmerican cakeRice cakeChocolate sponge cake

Fish products

Smoked squidBoiled squidBoiled & dried squidBoiled & dried small fish

aw of food

0.920.850.830.800.760.72

0.850.680.690.63

Size ofEthicap

IG3G3G2G4G2G

IG0.6G0.6GIG

Packagingmaterial

OPPOPP/PPOPPOPP/PPOPP/PPOPP/PP

OPPOPPOPP/PEOPP

Shelf-life @ RT

1 week2 months20 days6 months2 months6 months

3 months3 months2 months3 months

quality after 3 weeks, characterized by a loss of moistness and firmness ofthe texture and development of soapy and rancid flavors. Therefore, whileEthicap could be used to extend the mold-free shelf-life of the product, thesensory quality of the product was not significantly extended (Pafumi andDurham, 1987).

Black et al (1993) examined the combined effect of gas packaging andEthicap (size unknown) to extend the shelf-life of pita bread. They reporteda 14 day mold-free shelf-life for pita bread packaged in 40% CO2 (balanceN2) or 100% CO2. However, the shelf-life was doubled when an Ethicapsachet was incorporated into the gas packaged products (Black et al, 1993).However, these authors reported that ethanol vapor had little anti-stalingeffect on pita bread. While the effect of increased firmness due to stalingwas reversed by microwave heating, it failed to eliminate the stale flavors inthe pita bread (Black et al, 1993). The authors also observed a slightincrease in headspace oxygen in all gas packaged products stored withEthicap. They hypothesized that the ethanol vapor may have dissolved in thefilm and acted as a plasticizer, thereby affecting the permeability of the filmto both oxygen and carbon dioxide (Black et al, 1993).

Ethicap has also been used as a means of further extending the shelf-lifeof gas packaged apple turnovers (Smith et al., 1987). Apple turnovers, witha water activity of 0.93, had a shelf-life of 14 days when packaged in a CO2:N2 (60:40) gas mixture at ambient temperature due to growth of and CO2

production by Saccharomyces cerevisiae. However, when Ethicap (E4) wasincorporated into the packaged product either alone or in conjunction withgas packaging, yeast growth was completely suppressed and all packagesappeared normal at the end of the 21-day storage period (Smith et al,1987).

Ethicap has also proved effective in controlling secondary spoilageproblems by 5. cerevisiae in gas packaged strawberry and vanilla layer cakesand cherry cream cheese cake. Ooraikul (1993) reported that 5 G sachets ofEthicap inhibited growth of, and CO2 production by, 5. cerevisiae in bothstrawberry and vanilla layer cakes. This size of the Ethicap sachet had noadverse effect on the organoleptic quality of cakes. Indeed, Ooraikul (1993)reported that the aroma of the cake packaged with Ethicap was morepleasant than that of the fresh cake and that the taste also remained excellent.However, a 6 G Sachet of Ethicap failed to inhibit yeast spoilage in cherrycream cheese cake (Ooraikul, 1993). He recommended a 7-8 G sachet ofEthicap in conjunction with a preservative, such as ethyl paraben, to controlsecondary yeast fermentation problems in cherry cream cheese cake(Ooraikul, 1993). These studies show that larger sizes of Ethicap, and hencehigher levels of ethanol vapor, are required to inhibit yeast growth comparedto mold growth. Indeed, agar model studies have shown that yeasts cangrow in media containing 8% (v/v) ethanol while most molds were inhibitedby 4% ethanol (Freund Technical Information, 1985).

633 Effect of ethanol vapor on food spoilage/food poisoning bacteria

While most studies to date have focused on the use of Ethicap as anantimycotic agent, few studies have evaluated its potential to control foodspoilage and food poisoning bacteria. Ethanol, when incorporated into agarmedia, has proved to be effective against several spoilage and pathogenicbacteria (Freund Technical Information, 1985; Shapero et al., 1978). Seilerand Russell (1993) also reported that ethanol at a level of 1% by product wt.,delayed the onset of 'rope' caused by the growth of Bacillus subtilis. Theyalso reported that low concentrations of ethanol (0.5-1% by product wt.)inhibited bacterial growth in both whipping cream and custard, two well-known vectors of food poisoning bacteria in filled bakery products. Thesestudies clearly illustrate the antibacterial properties of ethanol whenincorporated directly into media or a food product. More recently, Morris etal. (1994) examined the effect of Ethicap on the growth of Listeriamonocytogenes, a psychrotrophic food-borne pathogen of public healthsignificance. They observed that a 4 G sachet of Ethicap could control thegrowth of L. monocytogenes (Scott A) on agar media at 5, 10 and 15°C, thelatter storage conditions representing mild temperature abuse. Furtherstudies are now underway to determine the volume of ethanol vaporgenerated at these storage temperatures and the effect of these concentra-tions on the microbiological and chemical shelf-life of fresh packagedpork.

63 A Advantages and disadvantages of ethanol vapor generators

According to Smith et al. (1987) the advantages of Ethicap are:

• Ethanol vapor can be generated from sachets without having to sprayethanol directly onto product surface prior to packaging.

• Sachets can be conveniently removed from packages and discarded at theend of the storage period.

• It eliminates the need for preservatives such as benzoic acid or sorbicacid to control yeast spoilage.

• It can control mold spoilage and delay staling in bakery products.

A disadvantage of using ethanol vapor for shelf-life extension is itsabsorption from the package headspace by product. However, the concentra-tion of ethanol found in apple turnovers (1.45-1.52%) was within themaximum level of 2% by product weight when ethanol is sprayed onto pizzacrusts prior to final baking (Smith et al., 1987). However, the level ofethanol in apple turnovers can be reduced to < 0.1% by heating product at375° F prior to consumption. Therefore, while a longer shelf-life may bepossible by packaging product with Ethicap, its use as a food preservativemay be limited to 'brown & serve' or microwaveable products. Anotherdisadvantage is cost, which limits its use to products with higher profit

margins. Nevertheless, Ethicap is a viable alternative or supplement to gaspackaging to extend the shelf-life of baked bakery products in relation toboth mold and yeast spoilage and to prevent staling.

6.4 Conclusion

In conclusion, the use of gas absorbents and ethanol vapor generators is,without doubt, one of the most exciting interactive packaging technologiesavailable to the food industry. While both oxygen absorbent technology andethanol vapor generators are used extensively in Japan to extend the shelf-life and keeping quality of a variety of products, their use to date in theNorth American market is limited due to the cost of the sachets, consumerresistance to the inclusion of sachets in packaged products and lack ofregulatory approval for Ethicap. Nevertheless, the use of gas absorbents/ethanol vapor generator sachets or labels offers the food industry a moreviable alternative method of interactive packaging than vacuum/gas flushingfor shelf-life extension of its products.

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Harima, Y. (1990) Food Packaging, Academic Press PubL, London, UK, pp. 229-52.Lambert, A.D., Smith, J.P. and Dodds, K.L. (1991a) Combined effect of modified atmosphere

packaging and low-dose irradiation on toxin production by Clostridium botulinum in freshpork. / . Food Protection, 54, 97-104.

Lambert, A.D., Smith, J.P. and Dodds, K.L. (1991b) Effect of headspace CO2 on toxinproduction by Clostridium botulinum in MAP, irradiated fresh pork. / . Food Protection, 54,588-92.

Lambert, A.D., Smith, J.P. and Dodds, K.L. (1991c) Effect of initial O2 and CO2 and low-doseirradiation on toxin production by Clostridium botulinum in MAP fresh pork. / . FoodProtection, 54, 939-44.

Minakuchi, S. and Nakamura, H. (1990) Food Packaging, Academic Press PubL, London, UK,pp. 357-78.

Morris, J., Smith, J.P., Tarte, I. and Farber, J. (1994) Combined effect of chitosan and MAPon the growth of Listeria monocy to genes. Food Microbiology; (Submitted for publica-tion).

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Pafumi, J. and Durham, R. (1987) Cake shelf life extension. Food Technology in Australia, 39,286-7.

Palumbo, S.A. (1986) Is refrigeration enough to restrain food borne pathogens? / . FoodProtection, 49, 1003-9.

Plemons, R.F., Staff, CH. and Cameron, F.R. (1976) Process for retarding mold growth inpartially baked pizza crusts and articles produced thereby. US Patent 3979525.

Powers, E.M. and Berkowitz, D. (1990) Efficacy of an oxygen scavenger to modify theatmosphere and prevent mold growth in meal, ready-to-eat pouched bread. /. FoodProtection, 53, 767-71.

Rooney, M. (1992) Reactive Packaging Materials for Food Preservation. In: Proceedings ofthe First Japan-Australia Workshop on Food Processing, Tsukuba, Japan, pp. 78-82.

Seiler, D.A.L. (1978) The microbiology of cake and its ingredients. Food Trade Review, 48,339-44.

Seiler, D.A.L. (1988) Microbiological problems associated with cereal based foods. FoodScience and Technology Today, 2, 37-41.

Seiler, D.A.L. and Russell, NJ. (1993) Food Preservatives, Blackie Academic & Professional,Glasgow, UK, pp. 153-71.

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Smith, J.P., Ooraikul, B., Koersen, WJ. and Jackson, E.D. (1986) Novel approach to oxygencontrol in modified atmosphere packaging of bakery products. Food Microbiology, 3,315-20.

Smith, J.P., Ooraikul, B., Koersen, WJ., van de Voort, F.R., Jackson, E.D. and Lawrence,R.A. (1987) Shelf life extension of a bakery product using ethanol vapor. FoodMicrobiology, 4, 329-37.

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7 Enzymes as active packaging agents

A.L. BRODY and J.A. BUDNY

This chapter discusses the role of enzymes in active packaging, especially asoxygen scavengers.

Almost all mechanisms in which packaging structures function inresponse to a stimulus involve physical, chemical or physiochemical actions.In a physical action, the active element of the material, which is usuallyexternal, opens, expands, closes, contracts, etc. as one or more variables arechanged. In chemical or physiochemical reactions, a component of the totalpackage reacts with the package structure or component, usually in anirreversible manner, with the result that the active component is effectivelyconsumed or changed as the internal package environment is changed.

However, in catalytic processes physiochemical or chemical reactionsoccur in which the catalyst remains effective and intact. Enzymes, which arebiological catalysts, accelerate chemical reactions but are not consumed as aresult of the reactions. Within limits, for as long as reactants or substrates arepresent, enzymes will function to catalyze chemical, or more specificallybiochemical, reactions. When the proper enzymes are introduced under theproper conditions, they are capable of catalyzing reactions which can eitherprevent the product from being changed or extend the function of packagingbeyond its accepted or previously understood functions by actively servingas a processing unit.

7.1 Enzymes

Enzymes are biological catalysts which are found in all living cells, whetherplant or animal. These macromolecular proteins exhibit two outstandingcharacteristics in addition to the fact that they occur naturally and are foundin living systems.

The first characteristic is their catalytic power. Enzymes acceleratechemical reactions that occur in biological systems by factors that exceed amillion over their uncatalyzed rate. In essence, enzymes allow livingsystems to carry out reactions that would not ordinarily occur or occur soslowly that the rates would not be of any practical significance. A simplereaction of the formation of carbonic acid from carbon dioxide and wateroccurs 107 times faster with the enzyme carbonic anhydrase than the non-enzymatic or chemical reaction.

The second important characteristic of enzymes is their specificity.Enzymatic specificity takes on two distinct forms: the type of chemicalreaction; and for any type of chemical reaction, a specificity for the reactantor substrate. Consequently, for each chemical reaction that occurs in abiological system, there is a unique enzyme required for the optimalproduction of reaction products. With so many different biological reactions,it follows that there are many different enzymes.

As with all catalysts, enzymes do not alter equilibrium conditions. Anenzyme increases a forward reaction in the same way and to the same extentthat it increases the reverse reaction, i.e., enzymes accelerate the rate atwhich a chemical equilibrium is reached but an enzyme does not distort theratio of the equilibrium concentrations of the products to reactants.

The catalytic potential of enzymes and the speed at which they facilitatechemical reactions lies in their ability to reduce the Gibbs Free Energy ofActivation (Ea). Enzymes accomplish their catalytic objectives, not byreducing the Ea of the uncatalyzed reaction but by creating a new anddifferent transition state and hence a different reaction path or mechanism.This new or different transition state is the enzyme-substrate complex (ESC)where the reactant becomes associated or bound to the free enzyme at thereactive center or site, followed by the release of the product whichgenerates the free enzyme again. The now free enzyme is once againavailable to combine with another molecule of reactant to repeat the process.The net effect of this sequence is that reactant or substrate becomes productand the enzyme is unchanged.

The active site is an area or region of an enzyme where the bond-breakingand bond-forming of the reactants and products occur. The participation, andhence the reactivity, of an enzyme for a particular substrate-product pair isdetermined by the amino acid sequence and the geometric or spatialarrangement of the enzyme. Because enzymes are high molecular weightpolymers which are made up of amino acids, it is not surprising that theactive site represents only a small percentage of the total enzyme. It is alsonot surprising that the polymeric catalysts are three dimensional, andconsequently the active site has a size (volume) shape to it. This spatialcharacteristic of the active site defines the size, shape and type of substratesor reactants which can be catalyzed by the enzyme.

The kinetics of enzyme reactions are obviously of great importance inconsidering their potential commercial applications. At the outset, enzymaticreaction rates are linear with time until all of the free enzyme is used to formthe ESC. When all of the enzyme exists as ESC, or as soon as the productis formed, the enzyme reacts with another reactant or substrate molecule,and the rate of conversion of reactant to product plateaus at the maximalreaction rate or velocity. Once the initial velocity has been achieved, all ofthe enzyme exists as the ESC.

Although enzymes may be classified according to the substrates theyaffect, as, for example, proteases for proteins, lipases for lipids, etc., inreality, these are designations for broad families to break proteins of entitiesthat are specific to a single protein or lipid under a particular set ofcircumstances. An enzyme suitable for a single 16 carbon fatty acidoxidation reaction will not catalyze an 18 carbon fatty acid oxidation eventhough the actual reactions at the sites may be identical. This characteristicmay be viewed as beneficial in that only the specific reaction and no otheris catalyzed by the enzyme. On the other hand, this attribute may beregarded as undesirable since a specific enzyme is required for a specificreaction, and no single enzyme can effect a series of related reactions.

Enzymes are proteins whose reactivity is quite sensitive to temperature.At temperatures as low as 1400F (680C), the catalytic reactivity of theenzymes may be temporarily or permanently disrupted, thus renderingenzymes among the most vulnerable of all biological matter. This tem-perature sensitivity is an important consideration in the commercialapplication of enzymes in processing operations.

Among the many enzymes functioning in reactions that have been and arebeing used commercially are rennin (chymosin) to precipitate the casein ofmilk in cheese making; proteases in laundry detergents to assist in proteinstain removal; amylase to convert starch to sugar for brewing; lactase tobreak down lactose in milk; various oxidases to accelerate oxidativereactions; and catalase to remove hydrogen peroxide that might be formedduring prior oxidative reactions. Other, more generic applications ofenzymes include stereospecific amino acid production, high fructose sugarproduction, beer and wine fermentation, tenderizing meats, milling andbaking, juice and wine clarification, juice extraction from fruits andproduction of flavor enhancers, to cite a few.

7.2 Potential roles of enzymes in active packaging

In many commercial situations, enzymes may be viewed as chemicals to beadded to the product to catalyze a reaction as one way to affect batchprocessing. The addition can occur to the in-plant batch or individualpackage. For in-package situations, the enzyme may be added directly to theproduct to effect a reaction or may be incorporated into the packagestructure. To function within a package material, the enzyme must beimmobilized and the substrate, reactant or a constituent circulated past thesite to initiate a reaction. Immobilization of an enzyme, or placing it in astatic position where it may function for an indefinite period, may beaccomplished by making the enzyme an integral part of the packagingmaterial.

Active packaging in general often involves the incorporation of a

chemical into the package material. Active packaging employing one ormore enzymes involves the incorporation into the package material of thespecific enzymes in much the same manner as the incorporation of a moreconventional chemical to create the active package. The key differences arethat the enzyme is not changed by the reaction and can continue to functionindefinitely; the enzyme is vulnerable to variations in temperature, pH, etc.;and the range of environmental conditions for the functioning of the enzymeis a relatively narrow band. These key considerations which affect the abilityof the enzyme to function require special processes and techniques forincorporating enzymes into packaging materials. Often, harsh manufacturingprocesses, geometric configurations, etc. that are adequate and evenappropriate for non-enzyme packaging components render the use ofenzymes inappropriate. Consequently, new and innovative methods arelikely to be required for the incorporation of enzymes into packagingmaterials.

Although a broad range of enzymatic reactions stemming from enzymeincorporation into package materials can be conceived, only a relativelysmall number have actually been attempted on a practical basis. Examples ofthose that have been actively pursued include:

• Oxygen removed by means of glucose oxidase plus catalase.• Removal of products of microbiological degradation by glucose oxidase/

catalase.• Incorporation of lactase to remove lactose from milk.• Incorporation of cholesterol-changing enzymes to remove cholesterol

from liquid egg or milk.• Time-temperature integrator indicators which are triggered enzymat-

ically.

Examples of enzymatic reactions that have not found general use butwhich might have some future potential and require development include:

• Conversion of sugar into alcohol and carbon dioxide in secondaryfermentations of wine to produce champagne-like products.

• A United Kingdom patent application (Thomas and Harrison, 1983)which describes an in-package secondary fermentation system usingimmobilized yeast within a liquid porous container immersed in analcoholic beverage. In one manifestation, the container was a flexiblepouch. Further, the inventors refer to isolated enzyme complexes asbeing useful as yeast substitutes. The objective here was to consume theresidual fermentable sugars, converting them into carbon dioxide andwater.

• In-package production of lactic acid for pickles, sauerkraut or sour dairyproducts.

• Production of 'natural' antimicrobial agents such as benzoic or propionicacids to help preserve the product contents.

• Destruction of natural toxins in foods.• Removal of undesirable respiratory end products such as ethylene that

accelerate the respiratory processes of fresh fruits and vegetables.• Removal of undesirable end products of microbiological or endogenous

enzymatic reactions such as polypeptides, carbonyls, ketones, volatileacids, etc.

• Tenderizing of fresh meat such as beef by proteases such as papain.

7.3 History

Although many enzymes and their roles have been known for severaldecades, the notion of incorporating them into package materials to achievea desirable result dates back only to the 1940s. Almost simultaneously withthe idea of protecting against browning of dry foods such as eggs byremoving residual oxygen, the notion of in-package glucose oxidase/catalasereactions was born. In reality, the initial action of glucose oxidase is withresidual quantities of glucose, a reducing sugar active in the non-enzymatic,non-oxidative Maillard browning reactions. Highly reactive hydrogenperoxide is produced by glucose oxidase, and is removed by catalase whichbreaks it into water and oxygen. This concept was put into practice byemploying porous packets of the enzyme mix in which the enzymes slowlyreacted with minute quantities of residual oxygen, an analogue of thecommercial incorporation of sachets of desiccants to reduce the in-packagerelative humidity. The applications during the 1940s and 1950s appear tohave been largely confined to very long term storage of military foods.

The concern for the adverse effects of temperature abuse on frozen foodsled to numerous ventures into development of time-temperature indicators,among which have been enzymatically actuated versions, beginning in the1970s.

The exponential growth of modified atmosphere packaging in the 1980sled to the notions of oxygen and carbon dioxide and moisture control usingin-package sachets of chemicals. Some enzymatic agents were included inthese chemicals.

Towards the end of the 1980s, interest increased with the formation ofPharmaCal, Ltd. whose objective was to develop the application of enzymesin unit size situations. This company and its principal, the co-author of thisarticle, suggested and, in some instances, physically evaluated three areas inwhich immobilized enzymes within package structures would catalyzereactions of products contained within packages.• Lactase to remove lactose.• Cholesterol reductase to remove cholesterol.• Glucose oxidase/catalase to remove oxygen.

Whether or not the communications emanating from PharmaCal were

directly responsible, several other enzymatically based active packagingoxygen control devices have been proposed since that time.

7.4 Oxygen removal

As is documented elsewhere in this book, oxygen is a highly reactive gaswhich can cause deterioration of almost all food products in terms of flavor,color, nutritional value and safety. Further, the presence of oxygen isessential to the growth and potential deteriorative effects of aerobicmicroorganisms including most bacteria, yeasts and molds. Thus, minimiza-tion or removal of oxygen is an important factor in prolonging the qualityretention of many food products.

According to Baker (1949, but evidently conceived in 1944) the additionof an oxidase to liquid-containing food products such as beer, peas, corn,milk, apple cider or orange juice, protects them from oxidation. 'In someinstances, it is better to produce in the product a substrate for the oxidasethat is to be introduced rather than to use a substrate already present.' Forexample, glucose originally present or added can be oxidized to gluconicacid. Baker's patent indicates that if the oxidase produces an objectionableend product such as hydrogen peroxide, then an additional enzyme might beintroduced to remove the undesirable end product.

Among the interesting aspects of this early patent is the notion that as theoxygen in the product is removed, free oxygen in the headspace is furtherdissolved by equilibrium dynamics, thus removing oxygen from theheadspace. The reaction, now very well known, is

2G + 2O2 + 2H2O -> GO + 2H2O2

where G is the substrate. Since hydrogen peroxide is a very good oxidizingagent, it is 'just as objectionable, or even more so, than is the originalmolecular oxygen.' Thus, catalase is introduced to break down the hydrogenperoxide

2H2O2 + catalase -» 2H2O2 + catalase

The sum of these two reactions yields half the oxygen originally present andtherefore ultimately the free oxygen approaches zero.

Baker's invention was implemented by introducing one or more pellets ofthe enzyme into the product such as beer or orange juice. The patent alsomentions the incorporation of lactase to hydrolyze lactose into glucose andgalactose which are then oxidized in the presence of oxidases. Perhapswithout realizing the significance of this assertion, the patent suggested theuse of enzymes to reduce the lactose content of milk.

The patent does not explicitly describe precisely how the enzymes areincorporated. The inference is that the enzymes are introduced directly into

the product. Expressed differently, this patent does not indicate that theenzymes are either part of the package structure or in an independent packetwithin the primary package. Thus, although the 1949 patent describedperhaps for the first time the employment of enzymes to eliminate in-package oxygen, it did not indicate that the enzymes were part of thepackage material or structure.

This concept of enzyme incorporation into a package material was firstovertly described in a 1956 patent (Sarett, 1956). (Sarett, incidentally, wasthe assignee for the Baker patent.) In this patent, the same basic enzymaticreactions as in the Baker patent were reiterated as a reference, but theenzymes glucose oxidase and catalase in a solution were impregnated into oron a moistureproof or fabric sheet. The enzyme was bound to the sheet witha water-dispersible adhesive such as polyvinyl alcohol, starch, casein orcarboxymethyl cellulose. The enzyme-coating face must contact the moistproduct to ensure that the requisite oxygen reduction reactions take place.The enzyme system was indicated to serve as a barrier to oxygen whichwould otherwise be transmitted through the sheet. Products described asbeing benefited by this system of oxygen reduction include cheese, butter,frozen foods subject to browning, etc.

Although during the period of the patent a Kraft packaging paper calledmoistureproof (which, as it happens, was not actually moistureproof) wasoften used to package butter and cheese, the patent does not indicate the useof this material. Rather, the package material is described as having \ . . anexposed surface covered with a gas-permeable packaging material andhaving an inter layer between and in contact with packaging material and . . .food . . . inter layer providing an oxygen barrier. . . . ' The specific packagematerials identified were moistureproof cellophane, paper, rubber hydrochlo-ride with impregnation employed for the papers and coating for the plasticand cellulose films. Also cited as being suitable substrates were wax paper,styrene, polyethylene and vinyls.

Experiments discussed in the body of the patent indicated results in whichoxidation of cheese surfaces was retarded by the presence of the enzyme-containing package material.

In 1958, Scott (co-inventor on the 1956 Sarett patent) of FermcoLaboratories, published a paper on Enzymatic Oxygen Removal fromPackaged Foods in which enzymes were incorporated into packagingmaterials or introduced into packets. Fermco Laboratories was a manu-facturer of enzymes, one category of which was labeled Fermcozymeantioxidants, and of the packets which were named Oxyban. This papermarked the first publication to our knowledge on the use of packets ofchemicals in packages.

The glucose oxidase/catalase systems were derived from mold myceliawhich were disrupted, filtered and further purified. To be effective in

reducing oxygen, glucose oxidase/catalase systems must be used in gas-tightpackages. Among the applications indicated were:Aqueous foods

• direct incorporation, in mayonnaise or carbonated beverages;• surface treatment in canned dog food;• in packets in situations in which the enzyme and the product should be

kept separate.

Non-aqueous foods

• direct incorporation;• in packets, as for chow mein noodles.

The mayonnaise and carbonated beverage examples involved incorpora-tion of the enzyme system directly into the products, with oxidative ranciditydelayed in the former class of products and color fading (e.g., grape-flavorcarbonated beverages) as well as flavor oxidations delayed in the latter. Thedog food example was also a direct addition to retard surface discolorationon the top of the dog food in retorted cans.

As a coating, the dried enzyme system was coated on the surfaces ofpackage materials for processed cheese. Deposition of the enzymes was insolution form or via incorporation into a dry starch mixture prior to 'dusting'the package material surface. When the dry and therefore inactive enzymepicked up moisture from the product, it was activated and was a sufficientlygood oxygen interceptor to control the formation of brown ring. Anotherseries of experiments focused on obviating oxidative gray coloration on thesurfaces of luncheon meats.

Fermco's Oxyban product was a dry glucose oxidase/catalase/glucose/buffers blend to be incorporated into products to reduce headspace andoccluded and dissolved oxygen in dry foods such as coffee or soup. Inanother manifestation, the Oxyban was placed in small packets in which itreacted with oxygen in packages of roasted and ground coffee, smoked yeastor egg solids. Exactly how the enzyme was activated without moisture wasnot indicated, but clearly some moisture from the product was required. Theauthor noted that this in-package packet was analogous to the desiccantpacket.

Three years later, Scott, then with Hammer (Scott and Hammer, 1958),elaborated on the oxygen-scavenging packet for in-package deoxygenation.Using the same glucose oxidase/catalase packet system described earlierfrom their laboratory experiments, they proceeded forward to a morecommercially viable mechanism. Among the problems they enumeratedwere:

• Oxygen-scavenger surface area owing to the gas phase reaction.• The need for moisture (cited above).• Necessity to neutralize gluconic acid to avoid enzyme deactivation.

• Package material structure allowing passage of oxygen but not mois-ture.

The gluconic acid problem was obviated using phosphate buffers. As little as15 g of Oxyban enzyme mix in a packet was capable of removing allmeasurable oxygen from a sealed No. 2 size can held at ambienttemperature. Once again, the type of package material used for the packetwas not indicated.

An interesting side note was an exploration of the use of glucose oxidasealone which, of course, led to an increase in the amount of hydrogenperoxide which would, in turn, slow the subsequent rate of oxygenuptake.

The products benefited by the total system were primarily dry milk, potatogranules and ice cream mix.

An international patent application (Lehtonen et ai, 1991) described apackage material containing an enzyme system to remove oxygen from theinterior of the package by enzymatic reaction. By removing the oxygen, thegrowth of aerobic microorganisms was significantly retarded, and so thistechnology was favorable to shelf-life from both microbiological andchemical standpoints. The enzyme, for example, glucose oxidase, wasincorporated into a package material with a gas-impermeable layer on theexterior and a gas permeable layer on the interior, i.e., the layer containingthe oxygen-consuming enzyme was sandwiched between two plastic filmlayers.

The background of this patent cited a 1969 German publication describingthe use of glucose oxidase in package materials for the surface protection ofmeats, fish and cheese products but without elaboration. And, of course, theclassical review paper by Labuza et al. (1989) described a similartechnology of coating plastic film with glucose oxidase catalase, with theenzyme system activated by moisture from the food as Scott had previouslycited.

This patent application from Cultor Ltd. of Helsinki, Finland, details aflexible package structure containing an enzyme system in the liquid phasetrapped between films, the outer of which might be polyamide orpolyvinylidene-coated polyester. The inner film would be polyethylenewhich is generally not a good gas barrier.

The enzymes of choice were oxidases of the oxidoreductase family usingoxygenases and hydroxylases which bind oxygen to oxdizable molecules.The enzyme solution contains a buffer and a stabilizer, and may also bemixed with a filler. The enzyme layer was applied on the film by gravure orscreen technique with the layer thickness being about 12 fim. The enzyme isnot directly in contact with the contents.

The film produced was employed either as the cover film layer or as thethermoformable bottom layer for tray-type packages.

The inventors noted that with increasing temperature, the gas permeabilityof package materials increases and so also does the ability of the enzymesystem to reduce the oxygen content from the 20.9% of air to about 1% atambient temperature within 24 hours.

From technological and potential commercial perspectives, this Finnishwork is so precise as to imply a major advance in the ability to implementthe principles of enzymes as active package components.

Co-author Budny and his company PharmaCal, Ltd. have been activelyresearching enzymes for active packaging since the 1980s. The contributionof PharmaCal, Ltd. to enzymes in active packaging was to expand theconcept of packaging beyond the two long-regarded functions of packaging:containment of the product; and protection of the contents. These require-ments originally were embodied in wine skins that ancient goat- and sheep-herders used for their sustenance beverages. Throughout history, while therehave been advancements in materials and approaches, there have not beenany fundamental changes or additions to the necessary requirements forcontainers or packages. Whether they are animal skins, a lid or a multi-layerstock, they should protect the contents and not leak.

PharmaCal, Ltd. added a third dimension to packaging by allowing anindividual package to become a processing unit or to perform a process stepor function that previously was limited to in-plant operations. With acombination of patent applications and proprietary technology, PharmaCal,Ltd. has been able to expand the concept of packaging to include processingsteps, value-addition to packaged products and increased processing effi-ciencies.

PharmaCal, Ltd. has developed a two-enzyme system involving glucoseoxidase and catalase to intercept oxygen and has applied the technology forenzymes in active packaging to improve the proven concept of oxygenremoval with the dual enzyme system of glucose oxidase and catalase. Theuse of the enzymes to remove oxygen has been acknowledged as not new,but their role in enzyme-based active packaging has been regarded as a moreadvanced application. Figure 7.1 illustrates the mechanism in whichpackaged liquid reacts enzymatically with glucose in the package wall toform gluconate. The resulting hydrogen peroxide is enzymatically reactedwith catalase to produce oxygen and water that re-enter the containedproduct liquid.

A container with an internal reactor, in reality an integral section of thepackage wall through which the liquid contents may flow, permits theenzymes to be retained for a reaction described in a 1989 patent application(Budny, 1989).

A 1991 patent (Ernst, 1991), described a glucose/glucose oxidase enzymemixture in a porous precipitated silica acid carrier. Calcium carbonate,calcium hydrogen phosphate, magnesium carbonate or disodium hydrogen

Figure 7.1 Oxygen removal from liquid products.

carbonate may also be employed as carriers or reaction accelerators. Theoxygen scavenger may be in the interior of in-package sachets.

A 1991 patent (Copeland et al, 1991) describes the incorporation ofoxygen scavenging cell membrane fragments which contain an electrontransfer system in solutions containing alcohol or acids to reduce oxygen towater. Although neither purified nor crude enzymes, the active component ofmembrane fragments in this technology must constitute the enzymesystem.

The inventors note that the major mechanism to effect the reaction isincorporation of the membrane fragments into the product, and that theseactive components may also be made part of the package structure. Sourcesof the membrane fragments were cell membrane of bacteria such asEscherichia coli and/or mitochondrial membranes.

Examples of products from which oxygen might be removed by thesystem include beer, wine, fruits, juices and a variety of non-food products.Both red and white wines were treated with materials supplied by Oxyrase,Inc., which is also the patent assignee. Dissolved oxygen was removedwithin 16 minutes at 37°C. Less than 12 minutes was required to remove100% of the oxygen from beer or tomato juice. A five-fold increase in thetime to the onset of browning of cut surfaces of bananas and apples wasobserved at ambient temperature.

Developers from chewing gum producer, William Wrigley, Jr., havedescribed the use of porous polymeric beads containing glucose oxidase inmultilayer flexible package materials (Courtright et al, 1992). The porousparticles are made from styrene divinyl benzene, with the enzyme incorpo-rated mechanically. The beads are then blended into a thermoplastic coatingin the multilayer film.

Headspace

Glucoseoxidase

enzymeGluconate GlucosePackaged

liquid

Outsideof

container

Catalase

enzyme

Container wall

Inside of container

Labuza and Breen (1989) have analyzed the issues involved in theincorporation of glucose oxidase into package materials.

To counteract the quantity of oxygen passing through an aluminum foillamination an enzyme surface will have to react with oxygen in thefollowing manner:

Rate = permeability X area X oxygen pressure differencebetween the outside and inside

Rate = 0.1 X 1 [0.21-0.01] = 0.2 ml per day per m2

= 20 jxl/day

The calculation above assumes air outside and < 1% oxygen inside. For theworst case and with a pinhole or cracked score, there would be the need toscavenge 1 ml/day. A film could be made equivalent to a barrier by bindingthe oxygen scavenging enzyme to the inside surface of the film to react withthe excess oxygen.

Glucose oxidase transfers two hydrogens from the -CHOH group ofglucose to oxygen with the formation of glucono-delta-lactone and hydrogenperoxide. The lactone then spontaneously reacts with water to form gluconicacid. One mole of glucose will consume one mole of oxygen and so apackage with 500 ml headspace is required, to reach zero oxygen, with only0.0043 mole of glucose needed as a substrate. The major factors are thespeed at which the enzyme works, the amount of glucose available, and therate at which oxygen permeates into the package. In the presence of catalase,a normal contaminant of commercial glucose oxidase, the hydrogenperoxide is broken down, and so with catalase one mole of glucose will reactwith only a half mole of oxygen, decreasing the overall effectiveness of thesystem. Pure glucose oxidase without catalase is reportedly expensive.

If no surface exists for the peroxide for diffusion, the glucose oxidase willbe inactivated, precluding this application. Since many foods may haveminimal contact with the package surface, except on the sides and bottom,this may not be the best approach for oxygen scavenging.

At 30-400C, pure glucose oxidase has a rate of oxygen consumption ofabout 150 000 (il/h/mg. Based on this, and spreading 1 mg per m2 on a film,this would be equivalent to reacting with all the oxygen passing through afilm with an oxygen permeability of about 18 000 ml/day m2 atm.

Thus at room temperature, a i m square surface with 1 mg of enzymespread out on it should be able to handle all the oxygen passing through anypackage film. One advantage is that both polypropylene and polyethyleneare good substrates for immobilizing enzymes. One factor to take intoaccount is the stability of the enzyme when bound to the film. An unknownfactor is how stable the enzyme will be on the film over time. Glucoseoxidase bound to a plastic surface has been shown to undergo a 50% drop inactivity in 2-3 weeks followed by little loss over the next four weeks.

The Japanese have worked on binding of enzymes to chitosan, which is aninsoluble polymeric carbohydrate from shellfish shells, but a 70% loss inactivity for bound glucose oxidase has been reported. Glucose oxidaseimmobilized on polyethylenimine-coated glass beads retained 78-87% of itsactivity and was more stable to heat inactivation. Since the enzyme is aprotein and can serve as a nutrient for microbes along with the glucosesubstrate, a microbial inhibitor may be needed in the film.

Besides glucose oxidase mentioned previously, other enzymes havepotential. One such enzyme is ethanol oxidase which oxidizes ethanol toacetaldehyde. The reaction is extremely rapid. Hopkins et a/. (1991) describea package in which alcohol or oxidase or cellular extracts of Pichia pastoriscells containing alcohol oxidase are the enzymes used for oxygen scaveng-ing in dry foods. An alcohol substrate either from the product or introducedinto the package from the exterior is required to remove the oxygen from thepackage headspace.

7.5 Antimicrobial effects

The use of enzymes in active packaging to control microbial growth andsubsequent packaged-product degradation can be achieved by two independ-ent approaches. By controlling the amount of available oxygen, selectivecontrol of aerobic bacteria can occur. However, this method of bacterialcontrol can, under certain circumstances, allow the overgrowth of patho-genic anaerobic bacteria which, from a human view, may be worse thanaerobic bacterial overgrowth. A second approach that has been implementedby several investigators is non-specific relative to oxygen requirements andis a direct attack on the organisms present, independent of whether theorganisms are aerobic or anaerobic. This second approach can be either bya direct attack on bacteria (both aerobic and anaerobic) or by the productionof broad-spectrum antimicrobial agents.

Neither the literature nor the memories of the authors indicates thecommercial implementation of the Fermco products. Meanwhile, the use ofimmobilized enzymes in commerce has increased significantly. During the1970s, Scott (1975), in his continuing research on the technology of glucoseoxidase, noted that catalase-free glucose oxidase might exert antimicrobialeffects due to the production of hydrogen peroxide. At the University ofRhode Island, Rand and his co-workers conducted research and developmenton catalase-free glucose oxidase as a food preservative, especially withregard to fish (Field et aL, 1986).

The enzymes (not coincidentally, supplied by Fermco) were applied tofresh flounder fillets or whole fish by dips, immersion in ice or by enzyme/algin blankets. In some experiments, the enzyme system included catalaseand/or glucose. The university researchers' experiments (which had begun

during the early 1980s) demonstrated that the enzyme treatments retardedthe onset and magnitude of adverse microbiologically triggered spoilageodors. The researchers explained the result as due to reductions in surfacepH under the refrigerated conditions of the test. These changes influencedthe metabolism of putrefactive microorganisms. They also suggested that thegeneration of hydrogen peroxide might inhibit the growth of psychrotropicmicroorganisms which are reported to be sensitive to the chemicals used.Other possible microbistatic agents include gluconic acid, reportedly a metalcomplexing agent, and gluconolactone, reported to be a binding agent forwater and metal ions. Another factor reported by the group was an alteredgaseous microenvironment in which oxygen in the muscle interstices wasdepleted by the enzymatic action thus retarding the growth of aerobicpsychrophiles. This last, of course, is synergistic with the oxygen removalaspects of the enzyme system.

The authors cited a Japanese patent in which catalase-free glucose oxidasewas demonstrated to be effective in preserving other proteinaceous foodssuch as ground chicken and tofu (Fukazawa, 1980).

Although the Rand et al. work did not specifically state the incorporationof enzymes into package materials, the implications were sufficiently clearin the examples of the enzyme-containing ice and the enzyme-containingalgin blanket. Either of these could have been relatively easily substitutedwith a skin package material which had been surface tested with the enzymesystem. The notion of hydrogen peroxide as an intentional active anti-microbial agent is somewhat of a contradiction since this chemical is quitereactive with many food constituents, especially lipids, and residual freehydrogen peroxide is not readily accepted by regulatory officials. If thehydrogen peroxide is fully reacted with microorganisms as in asepticpackaging, however, perhaps the proposed system may warrant furtherconsideration. Unfortunately, work at the University of Rhode Island on thistopic has been discontinued.

A German patent assigned to Continental Group (Anon. 1977) describesincorporation of biologically active enzymes into polymers on the interiorsof package structures to destroy microorganisms of contained products. Theenzymes were intended to destroy microorganisms by breaking cell wallsand also to consume oxygen, thus increasing shelf-life without heat. Theapplicable products were beer and fruit juices.

Enzymes such as muramidase for cell wall destruction and glucoseoxidase for oxygen interception were attached to the internal polymer bycovalent bonds. 'Non-essential' functional groups such as NH2, COOH, OHphenol, imidazole and sulfhydryl were cited as examples.

The polymer was described as a terpolymer of monomer alkyl acrylateand vinyl aromatic applied to the interior of a glass container from a solventand dried by heat. The enzyme was subsequently applied as a coating froman aqueous dispersion.

Tests indicated highly significant reductions in oxygen concentrationswithin the glass jars due to the conversion of glucose to gluconate in anoxidative enzymatic reaction.

This appears to be the first reference to actually incorporating an enzymeinto an interior package wall to achieve an enzymatic antimicrobialeffect.

7.6 Time-temperature integrator-indicators

For many years, efforts have been underway to develop a practical, accurate,reliable and economic indicator of total temperature-time exposure of foodproducts. Among the routes has been the application of the principles oftemperature sensitivities of enzymes. Although the original objectives wereaimed at frozen food defrosting devices, more recent interest has beenfocused on chilled foods. Among the issues are activation only whenactually at the beginning of shelf-life, accuracy over the entire range, howreflective the integrator-indicator is of the actual temperature-time experi-ence, and another basic question, how well the measurement represents theeffect of the temperature-time integral on the food itself.

Kramer and Farquhar (1976) listed a number of the problems in theirevaluation of five commercial, time-temperature indicating and defrostingdevices. No descriptions were given the mechanisms for sensing, integratingor measuring time-temperature.

On the other hand, Blixt and Tiru (1976) described a commercialenzymatic time-temperature monitor, called I-point® TTM. The authors, ofKockums Chemicals of Malmo, Sweden, stated that their device met all therequirements of reliability, accuracy, size, cost, understandable message andability to integrate ' . . . both length and degree of all temperatureexposures.'

The reaction was based on enzymatic degradation to colored end points.The device was a two-part system, one containing an enzyme and pHindicator since the system was based on pH change caused by enzymaticactivity plus a substrate. Because of the enzymatic core of the pH change,the temperature response was exponential with increasing temperature, andso evidently indicative of actual biochemical changes arising due to thetemperature-time experience. Although the indicators reportedly functionedvery effectively, no reference was made to the type of enzyme used. Onemight speculate on the simple glucose oxidase-catalase system producinggluconic acid as the reaction proceeded.

This product was another manifestation of the application of enzymes inpackage systems to an inactive mode.

A 1989 US patent (Klibanov and Dordich, 1989) claimed a temperature-change indicator composed of an enzyme and substrate, a colorimetric

indicator and a trigger mechanism of a solid organic solvent system thatmelted when a specific temperature range was reached to permit the enzymesystem to respond to temperature stimulus over time. The enzyme andsubstrate cited in the reduction process was peroxidase and peroxide with a/7-anisdine colorimetric indicator. Another enzyme cited as being effectivewas polyphenol oxidase. The organic solvents claimed were basicallyparaffins. Applications were as monitors on the exterior of distributionpackages of pharmaceutical and food products.

No further reference to the use of this enzymatic temperature indicator hasbeen found in the literature.

7.7 Lactose removal

Lactose intolerance is a dietary problem affecting a minor but neverthelesssubstantial fraction of the population. Individuals affected by this problemsuffer from a lack of the enzyme lactase in their intestinal wall. Lactase isnecessary to break the disaccharide lactose, or milk sugar, into itscomponent parts glucose and galactose. Since lactose cannot be absorbedfrom the gastrointestinal tract, its presence can cause discomfort in the formof cramps, bloating, flatulence and diarrhea. Persons with lactose intoleranceeither avoid milk or introduce lactase enzyme into their milk prior toconsumption.

A British patent assigned to Tetra Pak International AB (Anon., 1975)describes incorporation of lactase into pasteurized or sterilized milk prior topackaging to split the lactose after packaging. The lactose must be sterileand is added aseptically. The patent notes that the milk must remain forabout a day at a temperature of at least 80C for the lactase to function.

The Tetra Pak approach differs from the previously discussed examples ofactive packaging because the enzyme has no relationship to the packagingmaterial. Rather, a solution of enzyme is added directly to the individualpackage just prior to sealing. In reality, the Tetra Pak approach is batchprocessing done on a miniature scale, within the individual container.However, this approach does point out that an active enzymatic process canbe carried out in a sealed container.

PharmaCal, Ltd. extended and improved the Tetra Pak approach and madethe process a true enzymatic active packaging process. Budny, at Pharma-Cal, Ltd. (1990) incorporated the lactase, using proprietary technology ofPharmaCal, Ltd. with the result that 30-70% of the lactose was removed in24-36 hours at 3-4°C. PharmaCal, Ltd. has proprietary designs andapproaches for commercializing this active package (Figure 7.2).

7.8 Cholesterol removal

The widespread information on the effects of excess cholesterol in the dietdoes not require discussion here. To demonstrate the awareness in the USAof the cholesterol content of foods, all food packages in the United Statesmust be labeled for cholesterol content.

Co-author Budny (1990) suggests the removal of cholesterol which ispresent in whole milk by incorporating the enzyme, cholesterol reductase, inthe package structure. Using much the same proprietary technology ofPharmaCal, Ltd. as he employed for enzymatic oxygen removal or lactosesplitting, the fluid milk contents are exposed to the enzyme to convert itscholesterol to coprosterol which is not absorbed by the intestine.

This system, illustrated in Figure 7.3, reduces the extensive in-plantprocessing required by supercritical fluid extraction systems to producecholesterol-reduced fluid milk products. Rather, active packaging and thetechnology of PharmaCal, Ltd. allows untreated fluid milk to be packaged,

Milk

Lactaseenzyme Glucose

Galactose

Lactose

Outsideof

container

Containerwall

Inside of container

Figure 7.2 Lactose removal from liquid products.

Figure 7.3 Cholesterol removal from liquid products.

and in the time taken to transport the package to the consumer, itconceivably could become free of cholesterol. While the commercialimplementation has not yet been completed, the component elements of theapplication have been successfully demonstrated.

References

Anon. (1977) Packaged foods and drinks in containers coated internally with polymer carryingenzyme with sterilising action. German Patent DE2817854A.

Anon. (1990) Packaged milk containing lactose enzyme-giving milk with reduced lactosecontent. UK Patent Application.

Baker, D.L. (1949) Deoxygenation Process. 20 September. US Patent 2482724.Best, D. (1990) Fermentation opportunities ripen. Prepared Foods, 159, 5.Blixt, K. and Tiru, M. (1977) An Enzymatic Time/Temperature Device for Monitoring the

Handling of Perishable Commodities. International Symposium on Freeze-Drying Biolog-ical Products, 36, 237.

Budny, J. (1989) A transporting storage or dispensing container with enzymatic reactor.International Patent Application WO89/06273.

Budny, J. (1990) Presentation at Pack Alimentaire, San Francisco, California, May.

Milk

Cholesterolreductaseenzyme

Coprosterol

Cholesterol

Outsideof

Container

containerwail

Inside of container

Copeland, J .C, Adler, H.I. and Crow, W.D. (1991) Method and composition for removingoxygen from solutions containing alcohols and/or acids. XJS Patent 4996073.

Copeland, R.A. (1994) Enzymes, the catalysts of life. Today's Chemist at Work, March.Courtland, S.B., McGrew, G.N. and Richey, L. (1992) Food packaging improvements, 30 June.

US Patent 5126174.Ernst, R. (1991) Oxygen absorbent and use thereof 2 July. US Patent 5028578.Field, C , Pivarnik, L.F., Barnett, S.M. and Rand, A.G. (1986) Utilization of glucose oxidase

for extending the shelf-life of fish. J. Food Science, 51.Fukazawa, R. (1980) Methods of preventing spoilage of foods. Japanese Patent 23071180.Hopkins, T.R., Smith, VJ. and Banasiak, D.S. (1991) Process utilizing alcohol oxidase, 10

December. US Patent 5071660.Klibanov, A.M. and Dordich, J.S. (1989) Enzymatic temperature change indicator, 2 May. US

Patent 4826762.Kramer, A. and Farquhar, J.W. (1976) Testing of time-temperature indicating and defrost

devices. Food Technology, 30, 56.Labuza, T. and Breen, W. (1989) Active Packaging. J. Food Processing and Preservation, 13,

1.Lehtonen, P., Karilainen, U., Jaakkola R. and Kymolainen, S. (1991) A packaging material

which removes oxygen from a package and a method of producing the material. InternationalPatent Application WO 91/13556.

Sarett, B.L. and Scott, D. (1956) Enzyme treated sheet product and article wrapped therewith.US Patent 2765233.

Scott, D. (1958) Enzymatic oxygen removal from packaged foods. Food Technology, 12(7),7.

Scott, Don and Hammer, F. (1961) Oxygen scavenging packet for in-packet deoxygenation.Food Technology, 15(12), 99.

Scott, D. (1965) Oxidoreductase. Enzymes in Food Processing, Academic Press, NY.Thomas, K. and Harrison, RJ. (1985) Method and apparatus for secondary fermentation of

beverages. UK Patent Application 2143544A.Wiseman, A. (1975) Enzyme utilization in industrial processes, Handbook of Enzyme

Biotechnology, Ellis Horwood, UK.

8 The history of oxygen scavenger bottle closures

F.N. TEUMAC

8.1 Background

The early history of the use of scavenger chemicals with beer has played animportant part in the development of oxygen scavenger closures.

Gray, Stone, and Atkin (1948) measured oxygen content of bottled beerand correlated oxygen presence with off-flavor development. The reportmade to the American Society of Brewing Chemists concluded that theaddition of anti-oxidants to beer should be studied. The prime candidateswere sulfites and ascorbic acid.

Thomson (1952) reported extensions of the earlier work in the Brewers'Guild Journal. He found that the use of reductones made from sugar reducesoxygen, but increases the level of calcium to a level that forms hazes. Thereactions with sulfur dioxide, sodium formate, and phosphites were tooslow. He recommended adding ascorbic acid just prior to bottle filling.

Reinke, Hoag, and Kincaid (1963) reported that the inclusion of oxygenscavengers in the lining of cans improves the storage stability of cannedbeer. Glucose oxidase-catalase was preferred to sulfur dioxide and iso-ascorbic acid.

Klimovitz and Kindraka (1989) published in the Master BrewersAssociation of the Americas Technical Quarterly that a combination ofsodium isoascorbate and potassium metasulfite when added to the silicahydrogel mixing tanks significantly improved product flavor stability.

8.2 Oxygen measurements

8.2.1 Techniques for measuring the oxygen content of bottles

Before withdrawing gas samples from a bottle for measurement, the bottleshould be equilibrated by shaking. The foam should be allowed to settle. Fora gas sample, this might require several hours. The sample is withdrawnwith a Zahm-Nagel device. Oxygen and nitrogen can be measured in a gassample directly by gas chromatography or by removing carbon dioxide,separating the other gases and measuring with a mass spectroscopy detector.Using chromatography, assumptions and corrections must be made todetermine oxygen. The advantage of the mass spectroscopy detector is that

argon is detected directly. Because nitrogen and argon do not react with thebottle contents, other data can be gained by comparing the ratios of the threegases. The total oxygen and nitrogen can be calculated from the measuredheadspace, the temperature, and the oxygen and nitrogen concentration inthe head-space.

Liquid samples can be withdrawn and measured with polarographictechniques. Again, the bottle should first be equilibrated. The total oxygen ofthe bottle can be calculated.

Since the Zahm-Nagel device pierces the closure, each bottle can only besampled once. In order to follow the changes in the bottles, it must beassumed that all the bottles were the same at bottling. This requires the mostreproducible conditions possible.

8.2.2 Results of measurements

Depending upon the equipment capability of the brewer, the oxygen contentof bottled beer can be seen to correspond to three categories of brewer:

• Brewers incapable of performing a final blow down with purified carbondioxide and without new high technology fillers. The initial oxygen inthe package is about 1700 ppb. This decreases by 30% duringpasteurization, 42% the first day, 54% the second day, and 95% in aweek. The reaction of oxygen with the bottle contents is rapid. Thesebrewers usually compensate by adding 10 ppm or more of sulfur dioxideto the beer.

• Brewers capable of good oxygen control up to the last step, but do nothave new high technology fillers. These brewers provide an initialoxygen content of about 900 ppb. The oxygen depletion in the bottleproceeds at the same percentage rate as in the first category.

• Brewers that use the best equipment available. The initial values varybecause maintaining the lower value requires the filler to be in topcondition. An initial value of 400 ppb is common. Brewers using 10 ppmor more of sulfur dioxide obtain values of 200 ppb oxygen; brewers with8 or less ppm sulfur dioxide experience values of 350-800 ppb ofoxygen depending on the maintenance of the equipment. The oxygendepletion in the bottle proceeds at the same percentage rate as for theother two classes.

The effect on flavor deterioration of bottling under different conditions isdifficult to gauge. Each beer is different, so comparisons must be made ondifferent crowns under the exact same bottling conditions on the same batchof beer. The few valid comparisons made prior to the introduction ofscavenging crowns indicated that beer bottled with less oxygen had bettershelf-life.

8.2.3 Oxygen ingress

Closure of the bottle does not mean that the battle with oxygen is over. Foryears a crown or closure was defined as a hermetic seal. Wisk and Siebert(1987) at Stroh and Heyningen et al. (1987) at Heineken separatelychallenged this assumption and came to the same conclusion: crowns allowoxygen ingress. ZapatA Industries studied oxygen ingress in crowns;Teumac, Ross and Rassouli (1990) confirmed the earlier conclusions, andrecommended some improvements to eliminate oxygen ingress. This workwas extended to include both plastic and aluminum closures (1991). Theconcept of oxygen ingress into the bottle gained slow acceptance because itis difficult to envision how oxygen will penetrate a bottle with 3atmospheres of pressure within from an ambient pressure of 1 atmosphere.The phenomenon has, however, been proven using several techniques byseveral workers in the references cited above. It is based on a wellestablished equation that describes permeability through a permeablepolymer (the liner or gasket).

(A X p)PERMEABILITY = P - —

L

Permeability as used here means the flow of any gas per unit of time. For acontainer, it is the flow of a specific gas through the portion of the containerin question. P is the permeability coefficient; this is determined empiricallyfor a specific polymer or polymer compound and is specific to the gas andthe conditions of the test. A is the area of the compound surface involved inthe transfer. As metal has no permeability, A for a crown is the area of theliner compound between the metal and the glass. An aluminum closureprovides a very small area. A plastic closure is totally made up of permeablematerial, so the area is quite large. L is the length of the route followed bythe gas. p is the driving force of each gas. It is the difference in the partialpressure between the respective sides of the liner. It should be emphasizedthat it is not the total pressure; it is the partial pressure of the particular gas.A pressure of three atmospheres in a bottle does not mean that all gases willmove outward from a bottle. If there were a physical leak, that would be thecase. For a polymeric material like PVC, EVA, or polypropylene, gas willflow from the higher partial pressure to the lower.

Oxygen ingress can be measured by placing a closure on a bottlecontaining a known amount of oxygen and periodically measuring theoxygen in the bottle. The bottle must contain nothing that can react withoxygen. A simpler method uses an instrument sold by Modern Controls, Inc.There are several models of an instrument commonly called the Mocon. Theinstrument is used primarily to measure transmission through a permeablemembrane. Because of uncertainties of the dimensions of L and A in a

crowned bottle, the measurements on a closure are best made by modifyingthe Mocon to measure the transmission directly for a closed bottle. Thebottle is closed with the test closure and then cut and sealed to a metal blockcontaining a sealed inlet and outlet. Figure 8.1 is a schematic of theapparatus. Oxygen-free nitrogen is flushed into the bottle carrying anyoxygen in the bottle out to the detector. By keeping the nitrogen flow rateconstant, a steady state is reached where the oxygen in the stream is ameasure of diffusion of oxygen through or around the closure.

8.2A Combining the effect of initial and ingress oxygen

The bottler must consider both the oxygen trapped in the bottle at filling andoxygen ingress. For example, a brewer with good oxygen control techniqueswill fill bottles with beer containing 50 ppb oxygen and entrap another 440ppb. The initial oxygen level would be 490 ppb. A crowned 12 ounce bottlewill allow another 750 ppb to ingress in 3 months or 2000 ppb in 8 months.The amount of oxygen available to react with the product can be calculatedfrom measurements and extrapolated.

The amount of oxygen that has reacted with the product can then becalculated by subtracting the measured oxygen from the total oxygen

Brass mtg. plate

Hot melt

§lueormin. epoxy

Solder

Figure 8.1 Schematic of Mocon apparatus.

exposure (initial + ingress). The instruments cited are capable of makingmeaningful measurements that reveal the oxygen chemistry taking place inthe bottle. All of the oxygen that gets into the bottle is either measurable orhas reacted. In other words:

OXYGEN (Initial+ Ingress) = OXYGEN (Measured) - OXYGEN (Reacted)

8.3 Oxygen scavenger liners

8.3.1 Theoretical

Removal of oxygen from a bottle by a closure requires that the reactionoccurs with gaseous oxygen in the headspace of the bottle. About two-thirdsof the oxygen in a bottle is in the headspace. Scavengers can be incorporatedinto the closure by two different means.

(i) A compartment is placed in the closure that separates the scavenger viaa membrane that allows oxygen and water vapor to permeate the liner, butprevents the scavenger from leaching back into the bottle. This approachlessens the concern of product contamination by the scavenger; thus, itincreases the choices of potential scavengers. There are many patentsdescribing this approach. In order to be practical, the design and placementof the compartment must allow normal closure handling and bottlingprocedures. The fabrication of such a closure would add significant cost andrequire process changes by the brewer. This approach has not beencommercially tested.

(ii) The scavenger is included in the liner compound. The scavenger mustbe effective at levels that do not interfere with compound processing, closurelining, or the closure performance on the bottle. To be effective, thecompound must be permeable to water vapor and oxygen. The rate ofoxygen removal will be determined by the concentration and reactivity ofthe scavenger, the permeability of the compound, and the surface area ofliner exposed.

The scavenger should not become degraded during processing therebylosing activity and should be immune to activity loss during normalhandling. For example, enzymes such as glucose oxidase-catalase are veryreactive, but are destroyed by plastics processing conditions and are muchtoo reactive for normal filling procedures. Because of close contact with theproduct the scavenger should not be noxious from a health or organolepticstandpoint. It is not surprising then that the most successful scavengers arethe materials tested earlier as direct beer additives.

8.3.2 Commercial activity

PVC compounds lend themselves particularly well to use as scavengeradditives. Plasticized PVC can tolerate fillers without significant loss of

properties and is sufficiently permeable to oxygen and water vapor to allowgood reaction rates. Polyolefins have sufficient oxygen permeability, but areless permeable to water vapor.

(i) W.R. Grace, through Tapon France (a crown manufacturer), introduceda scavenger product in a polyolefin liner to Heineken in the Spring of 1989.Heineken dropped scavengers when they became more interested in otheraspects of crown performance.

Grace has published and been granted several patents on their scavengersystem. Essentially, they describe using ascorbates and similar chemicalswith or without sodium sulfite in a thermoplastic matrix. By reading thepatents and analyzing liner materials, it is evident that the Grace scavengerscontain up to 7% sodium sulfite and up to 4% sodium ascorbate. Theascorbates by themselves are very weak scavengers, so the sulfite is requiredfor the rate of activity. For some beers the low activity is not a disadvantageand may even be an advantage.

The exact extent of Grace's commercial success is not known outside ofGrace. It appears that a PVC compound is being used on a low alcohol beer,Foster's Special Bitter. Courage Beer uses Daraform 6490 for the Anheuser-Busch beer produced under license in the UK. Daraform 6490 is a polyolefinliner compound containing an oxygen scavenger. A PVC compound fromGrace is being intensively tested at one major Canadian brewer and severalsmaller US brewers. Trials are being performed with Daraform 6490 atseveral European brewers.

(ii) Aquanautics Corporation, now Advanced Oxygen Technologies, Inc.,had developed some expertise in removing oxygen from sea water andrecognized an opportunity in removing oxygen from bottles of beer. Anelaborate business plan was developed and eventually sold to ZapatAIndustries, Inc.

A joint effort was launched in early 1989. A system was developed thatwas based on the beer chemistry described earlier. Several patents have beengranted and applications are in the process of being approved.

The new technology employs ascorbic acid as the reducing agent.Alternatively, alkali metal ascorbates, alkali metal erythorbates, or ery-thorbic acid can be used. The addition of very small amounts of metalcatalysts to ascorbate or erythorbate-containing liners greatly enhanced therate of reaction with oxygen. The preferred catalysts are copper and ironsalts, but all transition metal salts increase the reaction rate. The amount ofascorbate or erythorbate determines the oxygen reduction capacity; the typeand amount of catalyst determines the reaction rate. Placing the samematerials in a plastic liner earlier placed in beer greatly minimizes the fearof product contamination. The small amount of catalyst enclosed in theplastic liner yields undetectable amounts of leach in beer. Separating thescavenger system from the beer, besides keeping it out of the beer, gives asurprising benefit. Different reactions occur. Earlier workers had observed a

reversal of the benefits of adding ascorbates directly to beer; this is notobserved when the ascorbate is placed in the liner. Another surprisingbenefit is that the reaction rate is significantly increased by placing thescavenger in the liner. The reaction with oxygen is enhanced by the paucityof moisture found in the liner.

The Aquanautics-ZapatA liner, Smartcap®, was introduced in a con-trolled manner in 1991. There was concern that the new conditions createdin the bottle would cause off-flavors to be created. The brewers that startedusing the liners commercially in 1991, and those that have startedsubsequently, have not had a documented incident of off-flavor developmentattributable to the liner. Smartcap and the improved PureSeal® crowns aremarketed by ZapatA Industries and affiliated companies in other countries.In 1993, over 1 billion PureSeal crowns were sold. Other crown manu-facturers have been provided with lining compound for crown trials. ZapatAprovides trial recommendations and an oxygen testing service for PureSealtrials. Trials at 60 brewers have proven that the liners always reduce theoxygen level in the bottle and usually reduce oxygen damage to the beerduring storage. The difference is first noticeable between 1 and 3 months ofstorage and is maintained to between 9 and 12 months.

8.3.3 Health and environmental concerns

This history implies ready acceptance by health authorities; each supplierwill document the acceptability to potential customers. The commerciallyemployed oxygen scavenging chemicals have not been listed as environmen-tally harmful.

8.4 The effect of scavenging closures on beer flavor

There are several large brewers, and they can afford and use excellentoxygen control. With one exception, they have been evaluating PureSealoxygen control crowns for about 2 years. Extensive research and large trialsare resolving their concerns and demonstrating the value of oxygenscavenging crowns. Working with these brewers has resulted in a betterunderstanding of the role of oxygen in beer flavor chemistry. The originalgoal of the project was to remove as much oxygen as quickly as possible.For bottles containing more than 600 ppb oxygen, rapid removal isbeneficial. As the initial oxygen approaches 250-350 ppb, rapid oxygenreduction is not always beneficial. The reason for this is that all beerscontain trace amounts of organic compounds, actually hundreds of them.Some of the sulfide-containing organic compounds included in this numberhave a low flavor threshold. Oxygen participates in the reactions that reducethese flavors. 'Sulfury' beers bottled with low initial oxygen require a lower

rate of oxygen depletion to allow some of the oxygen to react with thesulfury components. PureSeal liner compounds are readily adjusted toachieve both goals. A summary of the results was reported at PackAlimentaire (1993).

8.5 The advantages of oxygen control bottles

The advantages are most obvious to exporters. There is little similaritybetween beer purchased in the area of origin and that purchased elsewhere.Beer carefully brewed to have certain flavor characteristics can now bedelivered to customers all over the world in shipments in the samecondition.

The effect on the bottle of day-to-day variations in oxygen level at thefilling line or between filling lines can be minimized. Most large brewershave rigid standards on initial oxygen levels and dump beer that exceeds thelimit. Oxygen control bottles would allow raising of the limit.

Most beer is sold through distributors. Brewers lack the control theywould like on the distribution of their beer; by extending the expected shelf-life, they can now lessen the concern on how long beer is on the shelves orgive the distributors more leeway.

Production departments can use oxygen control bottles as a tool to solvemanufacturing problems; for example, the limiting factor on how fast afilling machine can operate is often the initial oxygen level.

Limitations of filling lines, the amount of oxygen, and the amount ofheadspace place limitations on package design; with oxygen control bottles,new packages can be designed for the same filling line.

Properly designed oxygen control bottles will provide a fresher tastingbeer compared to a bottle with a standard crown after approximately 30days.

There is some evidence that reduction of oxygen in a package can reducespoilage caused by organisms. Acidic beverages, like beer, can be protectedby less severe heat or additive treatment. In fact, total removal of someorganisms has been achieved by rapid depletion of oxygen in the bottle. Thisprovides more freedom of design in the product, processing, package,packaging materials, and distribution.

8.6 The future of oxygen scavenging closures

The use of oxygen scavenging crowns for beer is increasing rapidly.Brewers will become more comfortable with this trend. At the same time,the cost premium over standard crowns will diminish with increasedvolume. Oxygen control liners should be used in the standard crown

employed by the beer industry. Oxygen control liners have been introducedfor aluminum roll-on closures to complete the closure requirements forbeer.

The use of oxygen control for other beverage products is a new frontier.It is a relatively new industry involved in this field, differing in manyrespects from the beer industry. Until recently, refined constituents such assugar, corn syrup, artificial flavors, and citric acid have been used. Therewere relatively few substances that had the potential of becoming oxidizedto off-flavors. As beverage makers begin to use more natural materials suchas fruit juice, the potential for organoleptic problems increases. Theseproblems can be off-set with additives, but additives must be listed on thelabel.

Wines and coolers also contain hundreds of organic compounds that canreact with oxygen. Wine chemistry has dealt with oxygen for centuries.Wine makers understand the role of oxygen in maturation and/or spoilage inwine; it is a matter of how much oxygen at what stage. Oxygen scavengingclosures can be part of the oxygen control procedure of a winery.

Many food products are damaged by oxygen. Damage might be in theform of discoloration, change in texture, loss of flavor, or the generation ofoff-flavors. The effect is obvious and well understood by food processors.Sacrificial reduction of metal and use of preservatives are becoming lessacceptable. Package oxygen control affords a different means of protectingfood from oxygen damage.

Measuring techniques and equipment are now available for evaluation ofthe control of oxygen in any package. Nonetheless, 'quick and dirty'methods are commonly found. This lack of precision will lead to faultyconclusions or indicate no significant difference. Control of the initialoxygen content and a valid means of measuring a change in properties areessential features. The food scientist should become familiar with the latestdevelopments and only then very carefully plan and execute experiments.

References

Gray, P., Stone, I. and Atkin, L. (1948) Systematic study of the influence of oxidation on beerflavor. ASBC Proc, 101-12.

Heyningen, D. et al. (1987) Permeation of gases through crown cork inlays. EBC Congress,679-86.

Klimovitz, R. and Kindraka, J. (1989) The impact of various antioxidants on flavor stability.MBAA Technical Quarterly, (30), 70-4.

Reinke, H., Hoag, L. and Kincaid, C. (1963) Effect of antioxidants and oxygen scavengers onthe shelf-life of canned beer. ASBC Proc, 175-80.

Teumac, F., Ross, B. and Rassouli, M. (1990) Air ingress through bottle crowns. MBAATechnical Quarterly, (27), 122-6.

Teumac, F., Ross, B. and Rassouli, M. (1991) Oxygen Ingress Into Soft Drink Bottles.Proceedings of the 38th Annual Meeting, Society Of Soft Drink Technologists,pp. 201-10.

Teumac, F. (1993) Case Studies of Oxygen Control in Beer. Proceedings of Pack Alimentaire'93.

Thomson, R. (1952) Practical control of air in beer. Brewers' Guild Journal, 38(451),167-84.

Wisk, T. and Siebert, K. (1987) Air ingress in packages sealed with crowns lined withpoly vinyl chloride. /. Amer. Soc. Brew. Chem., 45, 14-18.

9 Commercial applications in North America

S. SACHAROW

9.1 Packaging overview

Packaging exists because it performs four basic functions which may vary inimportance depending on the nature of the products and their modes ofdistribution.

The classic functions are:

1. Protection2. Containment3. Information4. Utility of use

In recent years, these properties have been expanded to include both theenvironmental disposability of the package material as well as the ability ofthe package to perform far beyond the inherent property of the packagemedia. This may include characteristics such as enhanced shelf-life, theability to 'cook' the product or other changes in the product caused by thepackaging material.

Active packaging is the term used for a package that changes thecharacteristics of the product packaged. Examples of active packagingexisting in the North American marketplace will be discussed in thischapter.

9.2 Marketplace susceptors

In its classical definition, an active package (within the microwave field) isone that changes the electric (or magnetic) field configuration and ultimatelythe heating pattern of the product packaged (Packaging Gp., 1987).

Susceptors (also sometimes called receptors) are materials which convertsufficient microwave energy into heat to result in temperature increases thatexceed those produced by either the direct heating of foods or the boiling ofwater into moisture vapour. Temperatures high enough to produce drying,crisping and ultimately browning result, thereby yielding the desirableeffects associated with conventional infrared oven cooking. Microwavecooking alone produces temperatures limited by the temperatures developedby the food components, especially water, sugar and fats, in response toexcitation.

• Foods containing mostly water, such as vegetables, thus reach boilingtemperature. If heating continues in the absence of sufficient relief ofinternally-generated pressures, bursting can result. Thus potatoes bakedin the microwave are first deeply pierced so moisture has exits.

• Foods containing fat, such as bacon, reach temperatures of frying, whichmay exceed 2000C (392°F).

• Foods containing mixtures of water and sugars or fats achieve tem-peratures determined by the concentrations and distributions of ingre-dients, limited of course by the exposure time. This complicates theheating of meals made up of foods differing in response to microwaves,such as vegetables with gravy-covered meat, mashed potatoes and adessert of cherry cobbler.

Foods that require surface drying include pastries, breads, pizza crusts, andother dough-based compositions. Crisping and sometimes browning isneeded in some of these same foods and additionally in certain meatproducts as well as roasts.

Microwaves are not yet suited for crisping and browning. They mustrather be implemented with some method of raising local temperatures to1500C or higher (3000F or higher) to make them function as do browningdishes and conventional ovens or frying pans. Methods include (1) use of abrowning element, an infrared heating source such as a heating coil, in theoven to provide air at temperatures up to 2500C (4800F); and (2) a surface,a susceptor, which reacts to microwaves by becoming hot enough to createthe desired temperature.

The characteristic temperature-time curves for foods in a microwave ovenvary over a considerable range. Among dry foods, watery and high moisturefoods, foods containing fats and oils, and sugary foods, there can exist ordersof magnitude differences in heating rate, exacerbated by initial temperatureand specific composition. Susceptors help to overcome these differences.Two outcomes are desired of a susceptor:

• rapid rise to the required temperature• constant temperature thereafter

Only the first of these has been achieved in practice, but self-limitingsusceptors that satisfy the second are receiving considerable researchattention and should be on the market within a few years.

9.2.7 Susceptor types

In the parlance of deposited films, 'thick' and 'thin' are differentiated by theform which the deposited material takes during the deposition process.

• thin films are direct condensations of individual atoms, ions or moleculesonto a substrate

• thick films are deposits onto a substrate from dispersions of the materialas, for example, from a paste

Thin films may be only a few angstroms up to a thousand or more angstroms(1 angstrom = 10~8 cm = c. 4 x 10~9 inch) in thickness; as depositioncontinues, thickness increases.

The most interesting and potentially most useful effects pertaining toaluminium in microwave packaging are those which occur at thicknessescorresponding to Macbeth optical densities (OD) between 18 and 28. Thesecoatings are largely transparent to visible light (% transmission c. 50%) andin fact overlap the lower end of the range of thicknesses used in windowfilms. In this range, particles form a discontinuous film of non-uniformthickness - an array of electrical resistances - which responds to micro-waves by becoming increasingly hot, through ohmic heating. At thicknessyielding OD = 35, arcing occurs.

Thickness is a misleading term to apply to these extremely thin coatings becausetheir surfaces are quite irregular; perhaps the root mean square thickness might bemore apropos.

Early susceptors (c. 1986 - 1987) yielded promising heating results whichproduced the desired cripsing and browning, but they also producedproblems in some cases, which since that time have been largely correctedor eliminated entirely. Two examples are:

(i) A strong unpleasant odour, emitting from the oven or on opening thedoor after heating, emanated from the paperboard substrate, theadhesive, the film base or metal or combinations;

(ii) Uneven crisping, or lack of crisping in some areas of the food whenother areas were done, detracted from the favourable impression thisnew technology offered.

In addition, the rapid growth in use of compact ovens, typically less than500 W and 201 capacity (0.7 cf) sharply increased demand for conveniencefoods most likely to require crisping and browning. The accompanying rushto formulate suitable foods and packages led to some sub-optimal results.Most of the recent offerings of susceptor-crisped foods seem to overcomethe early problems, though some remain, especially in the frozen categorywhere uniform temperature attainment is difficult at best. In tests of ovalcross-section frozen dough-encased pasties (meat pies), centre line tem-peratures from middle to ends after the recommended heating time variedfrom 71-27°C (160-800F) and were not improved with additional heating upto the maximum recommended using the sleeve susceptors provided.Moreover, the susceptor efficiency was noticeably better at the base of thepies than at the upper surfaces. Standing time of 5 min narrowed thedifference between highest and lowest temperatures from 44 to 33°C (80 to600F).

Further advances in susceptors technology are anticipated. A key need isa susceptor the temperature of which rises quickly to the desired value andholds it nearly constant for the time required. Such a self-limiting devicemay be found in current early stage research and development activities.

Susceptors are made of either aluminium or stainless steel deposited onsubstrates. Other metals may be used in the future. The practical applicationof microwave-susceptible materials to heating of foods is to producecrisping and browning (see Table 9.1). Two classes of materials areavailable for producing susceptors;

(i) Resistive coatings, i.e. materials whose electrical resistance in the formin which they are deposited is high enough to produce ohmicheating;

(ii) Ferromagnetic/electric materials.

Resistive coatings are currently used exclusively in susceptors, but thesecond class may become important when the expected technology isdeveloped during the next five years. These latter offer the possibility ofsetting specific upper temperature limits on susceptors, thus overcomingsome of the disadvantages of resistive coatings, such as local hot spots, andcharring. In an experiment with a sleeve susceptor around a frozen waffle,the time of microwaving was extended by one-fourth with the result that thesleeve and the waffle began to char.

Susceptor substrates thus far have been limited to polyester films andpaperboard. Other materials may find use for specific reasons, not least ofwhich could be greater resistance to heat as higher temperature performanceis achieved. Engineering thermoplastics including the liquid crystal poly-mers, polysulfones, polyarylenes, nylons, and others listed among hightemperature tray materials for dual oven ware, are potential candidates.

9.2.2 Field intensification devices

Field intensification devices focus microwave energy to increase localintensity above that which would otherwise exist. FIDs therefore function ina manner similar to optical focusing lenses. The extent of intensificationdepends on the geometrical design of the focusing system - a metal antenna- and the distance of the target plane(s) from the antenna.

Alcan, Ltd., calls their MicroMatch™ system for field intensification afield management system which focuses and directs the incoming energy. Indevelopment of the MicroMatch container, Alcan found that efficientdesigns consisted of patches of aluminium arranged on a polymeric,microwave-transparent, snap-fit dome used as a cover for the food tray. Thedome positions the aluminium array with respect to the food and, by virtueof an overlap of the tray, precludes direct contact of the tray with oven walls,thereby reducing the chance for arcing to occur.

Table 9.1 Comparative performance of various susceptor technologies

CommentsCommercialstatus

EvenheatingCrispingBrowningFirm

Technologyname

CumbersomeExpensiveOver-engineeredNo tray use

Gaining onecustomer per month

Rapid heating forFrench fries andpizza

Still somewhat ondrawing board

About to be takenoff R & D program

Limited success.Now being licensedunder technicalagreeemnt. Presentlythere are two paidlicensees in NorthAmerica. Only onecommercial productin North America,'Meals on Wheels'.

'Accu-Crisp is themainstay' as a'patterned susceptor'.Other two are not yetcommercial.

Only use is in pizzaboxes (HealthyChoice brand).

Not yet commercial;however, one producton market has beenwithdrawn.

Only limitedcommercial trials. Nomarket success.

Excellent

1. Excellent2. Excellent3. Good

Good

Good

Excellent

Depends on productpackaged

Good

Excellent rapidheating

Excellent

Good

Good

Good

Good

Good

Good

Alcan, Ltd (MontrealP Q )

Printpak, Inc.Advanced Dielectric,Inc.(Taunton, MA)

Printpak, Inc.Deposition Technologies,Inc. (San Diego, CA)

Printpak, Inc.(licensees)

Lawson MardonMidsomer North (UK)

'Micro-Match'

1. 'Acan-Crisp'2. Accu-Wave'3. 'Barrier-

Wave'

'Susceptor Film''InconaT (Alloymetal)

Dupont, Inc.1. Cello-based

demetallized2. PET stainless

steel metallizedfilm

'Micromet'(patternsusceptor)

No commercial application for MicroMatch is yet in place, but licensingis reportedly underway in the USA and Germany. Alcan has concluded thatacceptance would be enhanced if packaging companies better known in thefood industry were to handle commercialization.

Aspects of the system which appear to make it attractive include:

• the ability to design the antennae to provide optimal focusing ondifferent areas of food, thereby facilitating heating of each food in amulti-component meal to its proper temperature

• faster heating• relatively direct application in manufacturing and ease of changing the

required patterns to fit individual food suppliers' needs• functioning from above and without direct contact with the food makes

possible the browning and crisping of foods having soft and stickysurfaces, which is not feasible with contact susceptors

Use of the MicroMatch container does not always obviate the need to rotatethe food to achieve even heating - many cheaper MW ovens have no modestirrer.

The aluminium tray is coated to lend greater assurance against arcing.Coating increases the electrical potential required for arcing from 30 000 Vto c. 50 000 V, but arcing can occur in either case. Another reason forcoating that is not usually mentioned is that coating improves appearanceand corresponding consumer appeal.

Thus, a FID dome on an aluminium tray directs and focuses energy toprovide both control for uniformity and a means of regaining the speed lostby virtue of the tray's inability to transmit MW energy. A FID dome on analuminium composite tray in which the tray base is plastic or paperboardwould overcome the speed loss and would supposedly heat food faster thanthe same tray without the field intensification.

Cost of the FID dome will no doubt be a major factor in determining itsmarket niche. The extra space required to accommodate the dome shape andits manufacturing cost will likely limit its use to the more expensive mealofferings. Consumers will require convincing evidence of resulting betterquality.

9.2.3 Susceptor applications

There are numerous packages in the supermarket that utilize susceptors -from microwaveable popcorn (reducing the amount of unpopped kernels) tomicrowaveable pizza (offering a crisp crust). In addition, entrees, fruit pies,meat pies and various 'crust' items lend themselves quite well to susceptorutilization. Susceptors have been used in Israel for bourekas, New Zealandfor French Bread, the UK for pappadums, and in Sweden for frozen meatentrees.

9.3 Application of temperature indicator to microwaveablepackaging

An interesting American microwave innovation not utilizing a susceptor, butstill an * active package' form is a microwaveable polypropylene bottle forpancake syrup.

Squat PP jugs of Hungry Jack pancake syrup to be heated in householdmicrowave ovens feature thermographic 'temperature indicator' labels thattell consumers when the syrup is hot.

Developed by Pillsbury Co., Minneapolis, MN, the microwave-readybottles are currently in supermarkets across the USA.

The 24 oz bottles stand 16.8 cm (6 § in) high 13.0 cm (5 g in) wide, and7.0 cm (21 in) deep. Like their counterparts, the MW bottles incorporate frontand back paper spot labels. But, on the MW bottles, the face of the front labelsfeatures an illustration of a microwave oven. On the shelf, the door of the ovenappears black. But, when the bottle is put in an oven and heated to bubblingaccording to directions on the black label, the black oven door on the frontlabel fades to yellow and the word 'HOT appears in the centre.

Extrusion blowmoulded by Continental Can Co., Syosset, NY, the squatPP bottle incorporates a fat, hollow handle that is pinched closed where itjoins the container's body and shoulder. The handle design is meant to warnconsumers and prevent them from burning themselves when handling theheated bottle.

9.4 Active packaging - produce

9.4.1 Oya produce bags

Evert-Fresh, a company in Houston, Texas makes a new kind of bag for storingproduce (Evert-Fresh, 1994). The greenish polyethylene bags are impregnatedwith a finely ground stone of the zeolite family that has high absorptionproperties. (Similar minerals are used to make products like Odor Eaters forshoes.) In the bags, the mineral absorbs ethylene gas, which is given off bymany fruits and vegetables and hastens ripening. By absorbing the gas, thebags slow down the ripening process and keep foods fresh longer.

The bags also have minute pores that allow the gas to escape and preventthe accumulation of moisture that could result in the development ofbacteria. The bags are re-usable if rinsed and turned inside out to dry.

'Evert-Fresh Bags' are reported to be impregnated with processed OyaStone which has its origins in a cave in Japan. The cave has been used forthree centuries to store fresh produce. The success of this cave as an idealstorage space can be attributed to constant levels of high humidity, statictemperature, darkness and, most importantly, the ability to absorb the gasesdischarged by the stored produce. The study of the caves gave scientists the

key to developing the Evert-Fresh film that absorbs ethylene, maintainshumidity, is permeable to other gases, and, when refrigerated, maintainstemperature control. AU of these are major factors in successful long-termstorage of produce.

9.4.2 Oya test results

A substantial portion of the vitamins and minerals in the American diet comefrom fruits and vegetables. Approximately 50% of the vitamin A and over 90%of vitamin C come from this food group. Stability of vitamins in produce isaffected by a number of factors, including heat, light, oxygen and pH.

For example, testing with vegetables, such as whole cabbage and green beans,has demonstrated that the susceptibility to heat destruction of 3-carotene, whichis a pre-cursor for vitamin A, can depend upon the nature of the vegetable.Control of temperature and humidity are necessary since if low humidityconditions prevail, rapid transpiration occurs and vegetables wilt. Under theseconditions, vitamin C and (3-carotene losses in leafy vegetables are well over50%. Also proven is the use of modified atmosphere storage to control thecarbon dioxide and oxygen levels which affect vitamin C retention.

It is reported that this bag does effectively reduce vitamin loss.Specifically, the bag is green in colour to reduce light transmission and madebreathable to enhance the transpiration of gases (O2 and CO2). Combinethese two elements with modern refrigeration (temperature) and three of thefour factors that affect vitamin retention are excluded. Storage evaluations ofleaf spinach, lettuce, broccoli, cabbage and green beans indicated thatvitamin C loss was reduced in excess of 50% using the Evert-Fresh bag forlong term storage.

Tests regarding vitamin C retention using the Ever-Fresh bag and ordinarypolythylene bags were conducted in Japan by the Consumer's ProductsCompany. The Vitamin C contents were determined using the IndophenolMethod.

Broccoli Evert-Fresh PolyethyleneAfter 3 days 95% 90%After 6 days 90% 80%After 12 days 77% 50%

Final results indicated that Evert-Fresh reduced vitamin C loss by 54% overa 12 day storage period during the broccoli test.

Crown Daisy Evert-Fresh PolyethyleneAfter 12 days 60% 40%

Final results indicated that Evert-Fresh reduced vitamin C loss by 50% overa 12 day storage period during the Crown Daisy test.

•CROWN DAISY is an edible flower popular in Japan

9.4.3 Modified atmosphere produce

With the exception of the Oya type consumer produce bags, the trend towardfresh produce has resulted in products such as fresh cut packaged vegetables.These are prepared using ultra-clean processing and packaging. The rate ofpackage material gas permeation is controlled to allow for natural respirationto occur with the product distributed under refrigerated conditions. Whilenot strictly an 'active' package form, the concept does utilize controlledpermeation.

9.5 Oxygen absorber food applications

In 1977, Mitsubishi Gas Chemical introduced 'Ageless' oxygen absorbers inJapan, and now reportedly command over 70% of the 10 billion unit per yearJapanese market; the remaining 30% is shared by approximately 15 otherJapanese producers. In 1988 Multiform Desiccants introduced Fresh Pax™oxygen absorbers (Figure 9.1), as the first US producer of oxygen absorbingpackets. Between 1988 and 1994 Multiform developed a family of productsto meet the specific needs of the North American marketplace (Multiform,1994). Products are designed to be moisture-activated preventing primaryoxidation until time of use, while others are for dry applications, or for

Figure 9.1 Fresh Pax oxygen absorbers introduced by Multiform Desiccants, Inc. for use in a widevariety of food packs.

situations where carbon dioxide is present, or to control oxygen removal rateat a wide range of temperature conditions. In 1992 Multiform introducedFreshMax® oxygen absorbing labels (Figure 9.2) designed to meet a marketdesire to make the absorber an integral part of the package system.Formulations are being adapted from FreshPax development, and a widevariety of substrates, adhesives and custom print are available. Fresh Maxcan be automatically applied within packages using conventional labellingequipment.

Outside the Orient, the most common uses for oxygen absorbers are inprotecting processed and cured meats, peanuts, and other nut varieties, highvalue baked goods, refrigerated pasta, snack foods, and dehydrated foods. Inaddition to human foods, oxygen absorbers can be found in medical devices,artemia, pet foods, treats, vitamins and in protecting valuable collectibles.Some common items containing the absorbers manufactured by MultiformDesiccants Inc., are as follows:

• Hormel Foods corp. - FreshPax 3 oz bottled bacon bits, 8 oz refrigeratedsliced peperoni

• Marks & Spencer - St Michael's sliced meat products (UK)• Kraft - DiGiorno refrigerated pasta• Goodmark - beef jerky• Melody Foods - Pioneer beef jerky

Figure 9.2 Multiform Desiccants, Inc. introduces FreshMax oxygen absorbers for processed,smoked and cured meats.

• US Military - shelf stable bread, cake, hamburger buns, chow meinnoodles, potato sticks

• John B. Sanfillipo - bulk peanuts and almonds• NASA - shelf stable tortillas• Dietary specialties - shelf stable bread

In addition to this list, the institutional and food service markets have usedoxygen absorbers to protect such products as processed meats, nuts, potatochips, and whole fat powdered milk. New applications are evolving almostweekly.

Mitsubishi Gas Chemical Americas' 'Ageless' absorbers (Mitsubishi,1994) are used as follows:

• Hormel Foods Co. - sliced peperoni, bacon bits• Kraft General Foods - fresh pasta (DiGiorgio brand)• Penge Foods - beef jerky• Goodmark Foods - beef jerky• Victor coffee - coffee beans• Advanced Development Corp - powdered drink• Tyson Foods - poultry• Dokosil Foods - sliced ham and poultry

9.5.1 Bottle closures - oxygen scavengers

This subject has been extensively discussed in Chapter 8. Aquanautics Corpis the developer of 'Smartcap'®, marketed by ZapatA Industries, and used invarious beer bottle crown liners. At present, an estimated 20 microbreweriesuse the Pureseal® liner in various applications. No large volume beerapplication yet exists; however, ZapatA is actively pursuing this market(ZapatA, 1994). Microbreweries using ZapatA's PureSeal® include SierraNevada Brewing Co., Cellis Brewing Co., Abita Brewing Co., and Full SailBrewing Co.

9.6 Other applications

International Paper, Purchase, NY, has developed an odour-trapping paper inconjunction with UOP, Des Plaines, IL supplier of Abscents deodorizingpowder.

The powder, which absorbs odours instead of masking them, makes up30-35% of the paper's weight, replacing clay and other fillers typically usedin paper. Paper made with Absents powder has been tested in surgical facemasks, feminine hygiene products, and filters. International Paper is stillinvestigating uses to see if paper with Absents is a viable, cost-effectiveproduct.

International Paper foresees applications in the medical industry, foodpackaging and household deodorization. The company is also testing usesfor automotive products.

References

Evert-Fresh (1994) pers. commun. with Evert-Fresh (Houston, TX), August, 1994.Mitsubishi (1994) pers. commun. with Mitsubishi Chemical (New York City., NY) September,

1994.Multiform (1994) pers. commun. with Multiform Dessicants (Buffalo, NY), September,

1994.Packaging Gp. (1987) Microwave Packaging. A multi-client study published by the Packaging

Group, Inc. (Milltown, NJ).ZapatA (1994) pers. commun. with ZapatA Industries, October, 1994.

10 Time-temperature indicatorsJ.D. SELMAN

Time-temperature indicators are part of the developing interest in intelligentpackaging, and there has been considerable interest in small temperatureindicators (TIs) and time-temperature indicators (TTIs) for monitoring theuseful life of packaged perishable products. There are over 100 patentsextant for such indicators based on a variety of physico-chemical principles;however, widespread commercial use has been very limited for a number ofreasons. For example, TTIs must be easily activated and then exhibit areproducible time-temperature dependent change which is easily measured.This change must be irreversible and ideally mimic or be easily correlated tothe food's extent of deterioration and residual shelf-life.

TTIs may be classified as either partial history or full history indicators,depending on their response mechanism. Partial history indicators will notrespond unless some temperature threshold has been exceeded, while fullhistory indicators respond independent of a temperature threshold. Thischapter reviews some of the physico-chemical principles utilised by differenttypes of indicator, and discusses the various issues concerning theirapplication, including consumer interests. Similar principles are being usedin indicator systems for validating heat processes, and some of the latestresearch directions are highlighted.

10.1 Introduction

Time-temperature indicators are one example of intelligent packaging, andinterest in this is growing because of the need to provide food manu-facturers, retailers and consumers alike with assurances of integrity, qualityand authenticity. Other intelligent product quality indicators might includemicrowave doneness indicators, microbial growth indicators, and physicalshock indicators. No microbial growth indicators are commercially availableyet, but they are likely to be based on the detection of volatile microbialmetabolites such as CO2, alcohols, acetaldehyde, ammonia and fatty acids.Tamper evidence and pack integrity indicators are perhaps the most welldeveloped category. The most familiar types include the physical barrierssuch as plastic heat shrink sleeves and neck bands; tape and label seals; andpaper/plastic/foil inner seals across the mouth of a container. Moresophisticated systems include Vapor-Loc introduced by Protective Packag-ing Ltd. (Sale, UK) which provides a tamper evident recloseable pouch that

combines the security of a barrier pouch with the ease of a recloseable zipperseal. Secondary tamper evident features rely on subtle devices based onchemical reactions, biological markers, and concealing techniques. Somethat are now commercially available utilise pattern adhesive labels and tapes,solvent soluble dyes and encapsulated dyes, optically variable films andholographic tear tapes.

A number of other developments are on the horizon, including theapplication of smart cards within caps, magnetically coded closures andelectrochemical devices. However, gas sensing dyes are the most advanced,especially for modified atmosphere packs. For example, a CO2 sensing dyecould be incorporated into the laminated top web film of a modifiedatmosphere pack, and this could be designed to change colour when the CO2

level falls below a set concentration. In the area of product authenticity andcounterfeiting, there is a large range of intelligent package devices which arebeing developed for use in various industrial sectors. Some of these will beapplicable to the food industry and include the use of holograms,thermochromic and photochromic inks, IR and UV bar codes, biotags,optically variable films, computer scrambled imaging, electromagnetic inkscattering, and so on.

There is continuing interest in the monitoring of temperature in the fooddistribution chain from factory to the consumer, and temperature monitoringand measurement, particularly of chilled foods, have been discussed byothers (Woolfe, 1992). As part of the approach to assuring product qualitythrough temperature monitoring and control, attention has focused on thepotential use of indicators. Temperature indicators may either display thecurrent temperature or respond to some predefined threshold temperaturesuch as a freezing point or a chill temperature such as 80C. TTIs usuallyutilise a physico-chemical mechanism that responds to the integration of thetemperature history to which the device has been exposed. Many differenttypes of indicator have been devised over the years and general reviews havebeen presented by several authors, including Schoen and Byrne (1972)covering patent literature from 1933 to 1971, Cook and Goodenough (1975),Kramer and Farquhar (1976), Olley (1976, 1978), Farquhar (1977), Schoen(1983), Ulrich (1984), Selman and Ballantyne (1988), Bhattacharjee (1988),and Selman (1990).

In general terms, indicators must be able to function in order to monitorone or more of the following.

• Chill temperatures (go/no go basis).• Frozen temperatures (go/no go basis).• Temperature abuses.• Partial history (response over threshold).• Full history (continuous response).

In order to achieve the monitoring objectives, there are several importantrequirements for indicators, including:

• Ease of activation and use.- Indicator may need to be stored and stabilised below thresholdtemperature for several hours before use

• Response to temperature or to cumulative effect of time and tem-perature.

• Response accuracy, time and irreversibility.• Correlation with food deterioration.• Correlation with distribution chain temperature/time.

The sensory quality of food deteriorates more rapidly at higher temperaturesdue to increasing biochemical reaction rates. Such increasing reaction ratesare often measured in terms of Q10 (the ratio of the rate at one temperatureto that at a temperature 100C lower). For many chemical reactions Q10 hasa value around 2, i.e., the reaction rate approximately doubles for each 100Ctemperature rise. As different foods lose quality at different rates, it maytherefore be important that the indicator reaction has an activation energythat is similar to that of the food deterioration (Taoukis and Labuza, 1989a;1989b). This is important for two reasons: firstly, the deterioration rates ofstored foods follow similar patterns, although Q10 values may be higher, sayfrom 3 to 20; and secondly, chemical reactions can be used in indicatorsystems so that by design the reaction rate can be made similar to that of therate of deterioration of the food. Tables of product activation energies or Q10

values have been given by Hu (1972) for ambient shelf-stable foods, bySchubert (1977) and Olley (1978) for frozen products, and by Labuza(1982), and Hayakawa and Wong (1974) for the scientific evaluation ofshelf-life.

10.2 Indicator systems

There are a variety of physico-chemical principles that may be used forindicators, including melting point temperature, enzyme reaction, polymer-isation, corrosion, and liquid crystals. Using these systems, many indicatorsgive one of three responses: colour change, movement, or both colourchange and movement. A variety of patents have been recorded and some ofthese are summarised in Table 10.1; a number of types of labels arediscussed below.

Liquid crystal graduated thermometers may be familiar to some (e.g.those manufactured by Liquid Crystal Devices Ltd., Ruislip, UK), and theycan be engineered in different ways, e.g. as a sticky-backed paper label(Avery Label Systems Ltd., Maidenhead, UK) or designed to show selectedtemperatures as with the Hemotemp II (Camlab, Cambridge, UK). The

Table 10.1 Some recent patents - Cold chain monitoring systems

This device reveals an indicator when the frozen liquid thaws

This is a defrost indicator which consists of blotting paper thatbecomes coloured by a frozen aqueous dye when it thaws

A defrost indicator for frozen foods; it uses a windowed packagingsystem to observe change of shape due to thawing

This device makes use of an ice tablet and an empty chamber whichwill fill up with water if the temperature rises

This device consists in developing frozen hemispheres of ice on thesurface. When these thaw they lose their shape

This device consists of an evaluation indicator which is stable whenfrozen but separates on thawing

This indicator uses an irreversible change of state system: once atemperature change occurs it is recorded

This consists of a microporous sheet which becomes wetted when theliquid thaws. The process is irreversible and operates quickly

Use of vegetable leaves to indicate thawing - green colour turns toblack; irreversible on thawing

This device is a sealed unit containing ice which changes shape onthawing

Sphere of ice suspended in the centre of a capsule

This device has a geometrically shaped column of ice coloured withphosphorescent material at the centre. Loss of geometry indicatesthawing

Solvent/membrane indicator; when solvent melts colour is developed

Bi-metal strip flexes to display colour to indicate critical temperaturereached

French Patent 2626-668A 29.01.88


French Patent 2641-61IA 09.01.89

W. German Patent 3716-972A20.05.87


Japanese Patent 0031-809 21.07.82

British Patent 2209-396A 04.09.87

European Patent 310-428A02.10.87

Japanese Patent 2021-229A08.07.88

European Patent 2002-585A10.03.87




W. German Patent 2824-903C13.10.88

Thaw Indicators - Based on Ice Melting

Bigand, F.M.

Fauvart, J.

Gradient, F.

Holzer, W.

Holzer, W.

KAO Corp.

Levin, D.

Minnesota Mining MFG

Mitsubishi Heavy Ind. KK

Perez Martinez, F.

Perinetti, B.

Toporenko, Y.

Uberai, B.S.

Wanfield-Druck KaId

Table 10.1 Continued

Electrochemical Time-Temperature Devices

Temperature history indicating label; the electrodes of a galvanic circuitform a temperature-responsive device

Tungsten trioxide electrode/weak acid

World Patent 9004-765A 24.10.88Also US Patent 4929-020A

US Patent 4804275 14.02.89

Grahm, I.

Johnson Matthey

This is a time indicator to show the expiry of foods started at ambienttemperature. The device consists of a dye diffusing into a gel; the rateis determined by time and temperature

Twin lapse display. Dye diffusion in agar. With retarder, e.g. albumin



Dry Diffusion in Gels

Toppan Printing KK

Toppan Printing KK

Time-temperature indicator based on colour development with timewhen two chemicals are brought into contact, e.g. amino compounds,hydroquinones, quinones and nitro compounds

This sytem comprises liposomes containing a quenched fluorescent dye.The fluorescence is released by lysis when the product temperaturefluctuates. It measures positive and negative temperature deviations

Diacetyiene monomer which polymerises to a dark compound, theintensity of which depends on time-temperature exposure

A thermal inertia temperature indicator which reacts at a certain presetthreshold temperature. It is enclosed in a transparent case. It does notreact to short temperature changes

This device consists of a microcapsule layer containing an achromaticlactone compound pigment precursor and solvent. The sheet indicatesthe time elapsed at 50C temperature intervals


US Patent 4825-447A 21.09.87

US Patent 4892-677 19.12.84



Chemical Reactions

Badische Tabakmanuf

Bramhall, J.S.

Lifelines Tech. Inc.

Rame, P.

Three S Tech BV

Freezewatch indicator (PyMaH Corp., Flemington, NJ, USA) is, by contrast,a simple irreversible indicator based on some threshold temperature,compared to the reversible technology exhibited by liquid crystals. Whenfrozen, the liquid inside the ampoule freezes, causing it to break. If thetemperature rises to -4°C, the liquid thaws and flows out, staining thebacking paper.

Chillchecker operates by means of a meltable, dyed compound containedin a porous reservoir (Thermographic Measurements Ltd., Burton, UK). Inthe inactivated form, a domed indicator paper is separated from a reservoirby a small distance. When the dome is pressed, the two materials come intocontact, allowing wicking to occur when the melt temperature is reached.The Chillchecker can be designed for different threshold temperatures, e.g.+ 9 or + 200C. Thermographics (see above) have now launched theThawalert, a self-adhesive label (18 mm in diameter) which utilisestemperature sensitive paints chosen to respond at a variety of thresholdfreezing and chilling temperatures. The above types are based on simplecolour development; others quantify the change.

Ambitemp (Andover Monitoring Systems Corp., Andover, USA) was atime-temperature integrator which functioned with a fluid that has a specificmelting point related to the product to be monitored. Under abuse conditionsthe melted liquid moves along the capillary tube. Tempchron (AndoverLaboratories Inc., South Weymouth, USA) was a more recent version ofAmbitemp which gave a read-out in degree minutes that could be interpretedfrom a chart. Although these two did semi-quantify the changes, their sizeand cost did not meet the further important requirements for the indicators tobe simple, small and inexpensive.

3M Monitormark indicators consist of a paper blotter pack and trackseparated by a polyester film layer (3M Packaging Systems, Bracknell, UK).Incorporated into the paper blotter pad are chemicals of very specific meltingpoints and a blue dye. The indicator is designed as an abuse indicator whichyields no response unless a predetermined temperature is exceeded. Theresponse temperature of the indicator is therefore the melt point of thechemical used. To activate this partial history indicator, the polyester filmlayer is removed, allowing the melted chemical and dye to diffuseirreversibly along the track. The higher the temperature above the responselevel, the faster the diffusion occurs along the track. If the temperature fallsbelow the response level of the tag, then the reaction stops. Each indicatorhas five distinct windows which allow an estimate of exposure time abovepresent values to be made. Before use the indicator has to be preconditionedby storing at a temperature several degrees below the response temperatureof the indicator, so that at the start of the reaction the chemical/dye mix issolid. Response of the indicator is measured by the progression of the bluedye along the track, and this is complete when all five windows are blue. Anindicator tag labelled 51, for example, would indicate a response temperature

(melt temperature) of 5°C with a response time of 2 days. This responserefers to the time taken to complete blue colour for all five windows at aconstant 2°C above the response temperature of the tag. Similarly, responsetimes of 7 days and 14 days are available on tags, with responsetemperatures varying from -170C to + 48°C (Byrne, 1976; Manske, 1983,1985; Taoukis and Labuza, 1989a, 1989b; Morris, 1988; Ballantyne,1988).

I Point labels are 'full history' indicators showing a response independ-ently of temperature threshold (I Point A/B, Malmo, Sweden). The deviceconsists of a two-part material, one part containing an enzyme solution, theother a lipid substrate and pH indicator. To activate, the seal between thetwo parts of the indicator is broken and the contents become mixed. As thereaction proceeds, the lipid substrate is hydrolysed and a pH change resultsin colour change through four colour increments (0-3, green to red). Thisreaction is irreversible and will proceed faster as temperature is increasedand slower as temperature is reduced. Each label has a colour scale to beused as a matching reference, which can also be expressed as a percentageof set time-temperature tolerance (TTT) elapsed (colour 1: 80% TTT; colour2: 100% TTT; colour 3: 130% TTT). These labels have been the subject ofseveral studies (Byrne, 1976; Blixt and Tiru, 1977; Blixt, 1984; Singh andWells, 1987; Grisius et ai, 1987; Ballantyne, 1988; Taoukis and Labuza,1989).

An alternative I Point indicator (type B) is also available. Each indicatormodel is provided with the same time-temperature characteristics as type A,but the difference occurs in the colour change interval. In model B only twovisible colours are seen: green and yellow. Only in the final 5% of presetTTT (95-100%, time to colour in type A) does the indicator change fromgreen to yellow. So, whilst responding to the temperature history, theindicators actually remain green for most of the storage life. The develop-ment of a yellow colour then indicates product approaching the end of itsshelf-life. This single colour change was designed to reduce variability incolour determination by different personnel, which was a common com-plaint with type A models. A range of indicators (A and B with varyingTTT) are available, lasting from 2 years at -18°C to 2 days at + 300C.Activation energies of the models 2140, 2180 and 2220 range from 14.0 to14.3 kcal/g mole (Wells and Singh, 1988c). The biochemical solutions mustbe accurate; results may tend to become less reproducible at longer intervals.Using the same technology, I Point have made a freezer indicator. Anotherenzyme based time-temperature indicator has been experimentally devel-oped by Boeriu et ah (1986). This is based on enzymic reactions takingplace many orders of magnitude faster in liquid paraffins than in solid ones.The device works as a thaw indicator by triggering off an enzymic colourreaction when the solid paraffin melts.

Lifelines' Fresh-Scan labels provide a full-history TTI, again showing aresponse independently of a temperature threshold. The Lifelines systemconsists of three distinct parts: a printed indicator label incorporatingpolymer compounds that change colour as a result of accumulatedtemperature exposure; a microcomputer with an optical wand for reading theindicator; and software for data analysis (Lifelines Technology Inc., MorrisPlains, USA).

The indicator label consists of two distinct types of bar code. The first isthe standard bar code, providing information on product and indicator type,and the second is the indicator code containing polymer compound thatirreversibly changes colour with accumulated temperature exposure. Thecolour change is based on polymerisation of diacetylenic monomers, whichproceeds faster at higher temperatures, leading to more rapid darkening ofthe indicator bar (Fields and Prusik, 1983,1986; Byrne, 1990). Initially,reflectance of the indicator code is high (approximately 100%), subsequentlyfalling during storage as the reaction proceeds and the colour darkens. Oncemanufactured, Lifelines' labels immediately start reacting to environmentaltemperature. Therefore, to maintain high initial reflectance values, indicatorsmust be stored at temperatures of - 200C and below. Studies have found thatthe colour changes correlate well with quality loss in tomatoes and UHTmilk, with activation energies for the indicators ranging from 17.8 to 21.3kcal/g mole (Wells and Singh, 1988a, 1988b). The portable hand-heldcomputer reads both the bar codes and the indicator codes. The softwarepackage has been designed to correlate reflectance measurements topredetermined time-temperature characteristics. Data from the hand-heldcomputer are transferred to a host computer, product freshness measure-ments are entered into the system, and a comparison is made between theproduct freshness curve and the response kinetics of the Lifelines labels(ZaIl et al., 1986; Krai et ai, 1988). A mathematical model can then beprepared to compensate for the differences in reaction rates of indicators andproduct degradation and allow prediction of product quality from oneindicator reading. Trials at Campden and Chorleywood Food ResearchAssociation found these labels to be more reliable than I Point indicatorlabels (Ballantyne, 1988).

The Lifelines Fresh-Check indicator has been developed for the consumerin a simple visual form (Anon., 1989). A small circle of polymer issurrounded by a printed reference ring. The polymer, which starts out lightlycoloured, gradually deepens in colour to reflect cumulative temperatureexposure. Again, the higher the temperature, the more rapidly the polymerchanges. Consumers may then be advised on the pack not to consume theproduct if the polymer centre is darker than the reference ring, regardless ofthe use-by date (Fields, 1989). Once again the required polymer responsecan be engineered. During the last two years several American companieshave been using these labels on a trial basis, and the system has been found

useful for determining shelf-life expiry when products are held under properrefrigerated conditions. However, use is still limited by the lack of responseto short periods of temperature abuse, and the polymerisation reaction isinfluenced to some extent by light. The latest types are light-protected by ared filter. There is at present considerable interest in these indicators, forexample for fresh eggs where short time-temperature rises may not directlyaffect quality. Lifelines Inc. also claim good correlation with the quality lifeof cooked ready meals, fresh chicken and yoghurt. During 1991, Lifelinescontinued to evaluate their polymer-based indicators used in both the foodand pharmaceutical industries, and their Fresh-Check label has been trialledin some of the department stores of the French company Monoprix, wherethey have been applied to over a dozen types of chilled retail products(Monoprix, 1990). The most prominent of the indicators to date have beenthe three referred to above, i.e., 3M Monitormark, the I Point type, and theLifelines Fresh-Scan and Fresh-Check. These have been the subject of anumber of independent validation tests, and the test systems and referencesare given in Table 10.2.

Marupfroid (Paris, France) has developed a partial history freezer labelbased on the melting point of ice. The part of the tag containing the red-coloured ice is located inside the pack next to the frozen food, with a hazardwarning area visible externally. If thawing has occurred, the red dye movesalong the label and exposes a warning printed in hydrophobic white ink. Onevery important point must be highlighted here, and that is that all otherindicators are placed on the outside of a pack and therefore respond to theenvironmental temperature. The packaging itself may provide the food withsome insulation from the environment and the food temperature willtherefore lag behind any changes in outside temperature. In the case of thislabel, the indicator system is placed inside the pack but with its responsechange visible externally.

Johnson Matthey has patented a system based on the corrosion of anindicator strip (US Patent, 1989). It consists of a film of electrochromicmaterial (in this case tungsten trioxide), with a metal overprint at one end,printed onto a card. The dissolution of the metal anode in acid is temperaturesensitive and results in a colour boundary which moves down the strip at arate governed by the temperature. The indicator can be engineered torespond to short total times and shows some promise in this respect, and thepotential exists for miniaturisation of such indicators.

Oscar Mayer Foods Corp. (Madison, USA) have developed a qualityfreshness indicator. This is based on pH-sensitive dyes in contact with a dualreaction system which simultaneously produces acid and alkali to maintaina constant pH. When one of the substrates becomes depleted, a rapid pHchange occurs, resulting in a sharp visual colour change (green to pink). Arise in temperature causes a shift in the equilibrium and the colourchanges.

Table 10.2 Validation tests on time-temperature indicators

ReferenceSystem testModel

Wells and Singh (1988a)Grisius et al. (1987)Wells and Singh (1988b)Wells and Singh (1988b)Wells and Singh (1988b)Wells and Singh (1988b)Malcata (1990)Chen and ZaIl (1987a)Chen and ZaIl (1987b)ZaHetal. (1986)Krall et al. (1988)Krall et al (1988)Singh and Wells (1986)Taoukis and Labuza (1989a)Taoukis and Labuza (1989b)WeUs and Singh (1988c)Fields (1985)Fields (1985)Ballantyne (1988)

Tomato firmness (10-200C)Microbial growth in pasteurised milk (0-50C)Green tomato maturity (10-200C)UHT sterilised milk (5-37°C)Fruit cakeLettucePasteurised milk (pallet)Milk, cream and cottage cheeseOrange juiceUHT milk freshnessOrange juice concentrate (frozen)Fresh produce (chilled)Hamburger pattiesUHT milk freshness (21-45°C)Orange juice (7.2°C)Response to isothermal conditions (4-300C)Response to non-isothermal conditions (4-300C)Response to temperature (0-370C)Response to temperatures (5°C and 100C)

LifelinesFresh-ScanFresh-Check

Wells and Singh (1988b)Wells and Singh (1988b)Wells and Singh (1988b)WeUs and Singh (1988b)

Green tomato maturity (10-200C)UHT sterilised milk (5-37°C)Fruit cakeLettuce

I Point

Table 10.2 Continued

ReferenceSystem testModel

Grisius et al. (1987)Wells et al. (1987)Singh and Wells (1985a)Singh and Wells (1987)Singh and Wells (1985b)Olsson (1984)Olsson (1984)Kramer and Farquhar (1977)Mistry and Kosikowski (1983)Taoukis and Labuza (1989a)Taoukis and Labuza (1989b)Wells and Singh (1988c)Wells and Singh (1985)Ballantyne (1988)

Pasteurised whole milk (00C, 5°C and 100C)Hamburger rancidity (frozen)Hamburger rancidityStrawberries (- 12 to + 350C)Seafood salad (pallets) (- 20 to - 100C)Cod fish (frozen) (pallets)Steak, beef patties, macaroni cheese (pallets) (- 20 to + 300C)Pizza (- 20 to + 300C)Milk (4.4-100C)Response to isothermal conditions (4-300C)Response to non-isothermal conditions (4-300C)Response to isothermal conditionsResponse to isothermal conditions (- 18 to + 5°C)Response to isothermal conditions (+ 2C, + 100C, - 12°C, -100C)

Wells et al. (1987)Singh and Wells (1986)Wells and Singh (1985)Kramer and Farquhar (1977)Mistry and Kosikowski (1983)Taoukis and Labuza (1989a)Taoukis and Labuza (1989b)Ballantyne (1988)

Hamburger rancidity (>- 17°C)

Steak, beef patties and macaroni cheese (pallet loads) (- 23.4 to - 15°C)Milk (4.4-100C)Response to isothermal conditions (4-300C)Response to non-isothermal conditions (4-300C)Response to isothermal conditions (4 - 1O0C)

3M Monitormark

Arnold and Cook (1977)Response to isothermal conditionsUnspecified (two models)

Imago Industries (La Ciotat, France) have launched their re-usablethermomarker. This is solid and relatively large (88 x 53 mm), and theprincipal element in its makeup is a shape memory alloy. The alloyeffectively 'memorises' two distinct shapes associated with predefinedtemperatures. In the device itself, a spring made of shape memory alloychanges size according to predetermined temperatures within a programmedrange. This in turn activates a system which ejects different coloured ballsthat signal the reaching of the various temperature thresholds.

A patent from Microtechnic (Germany) apparently uses the alignment oftwo magnets as an indication of the thawing of a frozen food. At the pointof freezing, two magnets are held unaligned in a small liquid container.However, if the liquid thaws, then the attraction by the opposite poles of themagnets will promote movement and the two magnets come together,indicating that thawing has occurred.

Albert Browne (Leicester, UK) make cold chain indicators which canproduce either an abrupt change of colour (yellow to blue) at its end point,or a more gradual change depending on its application. They havespecialised in thermal indicators for many years and are now promoting theirtime-temperature cold chain indicators in both the food and pharmaceuticalindustries. Food Guardian (Blandford, UK) have begun to promote theirlabel which has a thermometer profile. The label indicates the time on thescale for which the temperature has been above the designated temperature.Senders (London) have developed a threshold label for application to largeboxes and pallets, and this consists of both a warning indicator that thetemperature is getting too high, and a second indicator showing the need forrejection. Courtaulds Research (Coventry, UK) have considered developinga temperature-sensitive colour in acetate film. This could be used to detectwhen a product is fully defrosted and ready for cooking, assuming nostorage abuse. Bowater Labels (Altrincham, UK) have recently launchedtheir Reactt TTI self-adhesive label for monitoring freezing and chillingdistribution temperatures (Pidgeon, 1994). The labels remain inert untilactivated, then change from blue to red to reveal underlying graphics whenpreset time/temperature limits are exceeded. Trigon Industries Ltd. (Telford,UK) has also just launched its Smartpak label, which is self-activatingbefore use and shows an irreversible colour change to reveal an underlyingsymbol warning. For example, the Smartpak 1812 label self-activates whenit is frozen below -18°C, and subsequently indicates the temperature risingabove -12°C.

In the case of microwaveable products, research has shown that formicrobiological and other quality criteria, all points within the food shouldbe reheated to an equivalent of 700C for 2 min. To date only two donenessindicators are available. That from 3M (Bracknell, UK) uses a thermo-chromic ink which undergoes an irreversible colour change (Summers,1992). The Reactt doneness indicator from Bowater Labels is a modification

of the TTI self-adhesive label and works on the same colour-changeprinciple described earlier. Other devices are being developed at this time,although the challenge of measuring and correlating cold point temperatureswith overall pack temperatures remains considerable. Risman (1993) refersto the gel indicator technique developed at the Swedish Food ResearchInstitute for assessing the reheating performance of domestic microwaveovens for ready meals.

10.3 Indicator application issues and consumer interests

It is generally agreed that there are a number of potential applications forwhich the above-mentioned indicators could be used regarding the monitor-ing of various aspects and parts of the chilled and frozen distribution chains(Singh and Wells, 1990). However, the industry has been expressing concernregarding several issues about all types of indicator. TIs and TTIs representnew applications of technology, with little or no history of successful andreliable application, and until recently there has been no standard againstwhich their performance could be assessed. Also, the proliferation of TIs andTTIs now being offered, involving many different forms of indication, is ofconcern as this is likely to confuse the consumer. Provided these concernsare addressed by a given indicator for a specified product (or range), thepotential exists for indicators to be used in several ways, including on palletsor consumer packs, for stock rotation, parts or all of the distribution chain,retail shelf-life, and as a simple consumer guide.

Ideally, chilled and frozen foods should be stored at the appropriatetemperature, which should remain constant. However, there may be severalpoints in the distribution chain where the environmental temperature israised. Such periods may be short, from a few minutes to several hours. Todate, most indicators will not react rapidly enough to respond to suchregimes. For example, a Lifelines indicator subject to 24 hours at 5°C, sixhours at 100C, and two hours at 200C did not show a response that wassignificantly different to the control at 5°C (Ballantyne, 1988). Lifelineshave done work over the last two years and now claim that a dual chemistrysystem can be engineered to specifications required. Therefore, there may besome important limitations of some indicators that must be recognised, inparticular relating to reliability and reproducibility, sensitivity to short-time-temperature abuse, response to environment temperature but not necessarilyfood temperature, and cost benefits. For example, in 1988 Lifelines bar codelabels cost 30-70p each (scanning system US$20 000), I Point labels 15-2Opeach, and the 3M Monitormark about £1.50, for small trial quantities. In1991, Lifelines' prices in the USA ranged from 7.5 to 3.50 for bar codelabels and 3.5 to 1.250 for Fresh-Checks. The latter lower cost related toproduction runs in excess of 10 million units.

To be effective and of value to manufacturer and consumer, TIs and TTIsmust provide an indication of the end-life of the product. This should be noless clear and unambiguous to the great majority of the population than thecurrent minimum durability instruction. In particular, some consumers mayhave difficulty in detecting the difference between two colours, or shades ofone colour, where this forms the end point. Related to this, the point atwhich product life starts can be clearly defined for the purposes of declaringa 'best before' or 'use by' date. It is essential that the start point of the lifeof the TTI, i.e. when it is activated, can also be known for certain, with self-indication that this has occurred, and no reasonable possibility of pre-activation, partial activation, or especially post-activation. The legalrequirement for a best before and use by date on the pack will continue forthe foreseeable future. Therefore, consumer instructions on the pack willneed to clearly indicate the action to be taken when there is conflict betweenend of product life indication as given by the best before and use by date andthe TTI. There is also concern that where TIs and TTIs may have a role toplay with regard to product quality over life, unsubstantiated claims shouldnot be made regarding any role in relation to safety.

TTIs in general do not measure product temperature. Only one commer-cially available type is known, which is claimed to measure food surfacetemperature. None is known to measure food centre temperature. Almost allrespond to temperatures on the outside of the pack, where there may besome thermal insulation between product and indicator (Malcata, 1990).Measurement at this point may be of value, but the limitations in terms ofusefulness and relevance of such measurement need to be made clear to theuser and the consumer. A TI or TTI which reflects product temperaturewould be of far greater value and relevance than one which responds to thetemperature on the outer surface of the pack. A TI or TTI also needs to beable to cope with fluctuating temperatures (including elevated temperaturesfor a short time) and to respond accurately and reproducibly at the extremesof temperature likely to be experienced by the product. A TTI may need tomimic the growth of food spoilage microorganisms, or whatever other time-temperature related factor is liable to affect the quality of the foodstuff, overthe full range of temperatures likely to be experienced and when thetemperature fluctuates.

The quality management of the manufacture, distribution and storage ofthe TTI and the reproducibility of its performance must be of at least as highan order as the food product it seeks to monitor. In addition, there is concernthat the wrong TI or TTI may be applied to a given product. An incorrectlyapplied date mark is self-evident, at least to the manufacturer at the point ofapplication. As manufacturers may be producing simultaneously a range ofproducts with different predicted lives, they will require a range of TIs orTTIs designed with related performance characteristics. Hence, everyindicator should be supplied with a clear indication to the manufacturer,

distributor, retailer, and the enforcement authorities of the precise tem-perature threshold or time-temperature integration to which the indicatorwill respond. The TI or TTI needs to be no less resistant to malpractice andtampering than is the printed date on the pack. The indicator or the packageshould self-indicate if removed from the product; at the same time, ifremoved it should damage the packaging in such a way that a fresh indicatorcannot be applied without detection. Finally, TIs and TTIs in themselvesmust not represent a hazard to the consumer, e.g. if swallowed. In particular,care needs to be taken to make the indicator 'child-proof.

In order to address these issues of concern, the industry concludedrecently that a specification was required which could be common to alltypes of TIs and TTIs, and which could be used by manufacturers of suchindicators in order to meet the requirements of the industry and of theconsumer. Such a specification would address the basic technical require-ments for the performance of such indicators, although it is accepted thatcommercial reasons may influence the decision to use indicators for aparticular application. A joint Ministry of Agriculture, Fisheries and Food(MAFF)/industry working party met during 1991 at the Campden andChorleywood Food Research Association, and has completed a foodindustry specification (George and Shaw, 1992). It is hoped that this willprovide a basis for indicator manufacturers to design the performance oftheir indicators to meet the needs of the food industry, and at the same timeprovide a basis for the users of such indicators to check the indicatorperformance against their requirements. This specification defines the testingscope for indicator type and application. It refers to the quality managementof the indicator manufacture, the indicator compatibility with food, the needfor evidence of tamper abuse, and indicator labelling. It then outlines testprotocols for indicator response to temperature, including temperaturecycling and abuse, and the evaluation of the kinetic constants of theindicator. It covers evaluation of the accuracy of indicator activation point,and the clarity and accuracy of end point determination, and finallysimulated field testing.

A survey of 511 UK consumers, carried out by the National ConsumerCouncil (MAFF, 1991), indicated that almost all respondents (95%) thoughtthat TTIs were a good idea, but only grasped their concept after someexplanation, indicating that substantial publicity or an education campaignwould be required. Use of TTIs would have to be in conjunction with thedurability date, with clear instructions about what to do when the indicatorchanged colour. The relationship and possible conflict between the indica-tion of the TTI and the durability date on the food was considered a problem.In the retail situation, nearly half those questioned would trust the TTIresponse if it had not changed but the product was beyond its durability date.If the TTI changed before the end of the durability date when stored athome, the majority of respondents (57%) would use their own judgement in

deciding whether a food was safe to eat, with at least 25% putting some ofthe blame on the food suppliers. However, the value of TTIs was recognisedfor raising confidence in retail handling, and improving hygiene practiceswhen food is taken home and stored in refrigerators. It is clear that there isa future for TTIs in monitoring the chill chain. Development of differentindicators is still in progress and technical difficulties have to be overcomeby carrying out the appropriate tests (George and Shaw, 1992). However, theconsumer can appreciate the concept, and the advantages and benefits ofincreased food safety for the higher-risk foods that would result.

10.4 Chemical indicators for thermal process validation

Similar approaches to temperature indication have been taken for assessingpasteurisation and sterilisation processes, and some examples of commer-cially available indicator systems are summarised in Table 10.3. Most ofthese tend to give qualitative indications. Current research is directedtowards evaluating new systems which may give precise quantitativeindication. Hendrickx et al (1993) have conducted an extensive review andhave classified time-temperature indicators, as shown in Figure 10.1, interms of working principle, type of response, origin, application in the foodmaterial, and location in the food.

For biological TTIs, the change in biological activity such as ofmicroorganisms, their spores (viability) or enzymes (activity) upon heatingis the basic working principle. The use of inoculated alginate particles is anexample of the use of spores (Gaze et al, 1990). Recent studies on enzymeactivity have shown potential for the use of a-amylase, using differentialscanning calorimetry to measure changes in protein conformation (De Cordtet al, 1994). Brown (1991) studied the denaturation of several enzymes andsuggested that an approach which measures the status of a number ofenzymes in terms of pattern recognition would be better than using a singleenzyme to indicate retrospectively the heat process that had been applied.Brown (1991) also determined the feasibility and potential for ELISAtechniques for retrospective assessment of the heat treatment given to beefand chicken. Marin et al (1992) studied the effects of graded heat treatmentsof 30 min from 40 to 1000C on meat protein denaturation. They measuredthe remaining antigenic activity of the meat proteins and found this wassignificantly correlated with the heating temperature. Varshney and Paraf(1990) used specific polyclonal antibodies to detect heat treatment ofovalbumin in mushrooms, and could identify whether the ovalbumin hadbeen heated to lower than 65°C or higher than 85°C.

In terms of chemical systems, potential has been shown for correlating theloss of food pigments such as chlorophyll, and changes in anthocyanins,with heat treatment (El Gindy et al, 1972). Other food compounds may

Table 10.3 Commercially available time-temperature thermal process indicator/integrators

Change characteristicsColourTrade nameManufacturer

121°C for 10-15 min and 134°C for 3-4 min forfully developed colour change

Immediately temperature reached

Set to 121°C for 15 min or 134°C for 5.3 min

Steam autoclaves - colour change over 100-1800Cfor a range of exposure times

Dry heat = 1600C for 120 min to 1800C for 12min

Self-adhesive segmented labels giving colourchange when temperature exceeds set point by1°C

Set at 2400C for 20 min, ketone based

Immediately temperature is reached

Irreversible indicator, eight ranges selectable, semi-integrators using chromium chloride complex fordifferent temperatures (110-126.70C) and times(0-150 min) calibrated against spore destruction

The presence of saturated steam lowers the meltingpoint of a chemical tablet

Diffusion of the blue colour front has beencalibrated against spore destruction (B.stearothermophilus) over a range of time-temperature combinations

Immediately temperature is reached: 54.4-1040C

White to black (stripes)

Silver to black

Yellow to mauve

Red to green

Silver to black

Black to red

White to black or red

Purple to green

A blue colour front diffuses alonga transparent window of anaccept/reject band

White to black

Autoclave Tape

Thermometer Strips

TST

Steriliser ControlTube

ATP IrreversibleTemperatureIndicators

Easterday

Colour-Therm

Cook-Chex

SteriGageThermalog S

Reatec

3M Industrial Tapes and Adhesives(Manchester, UK)

3M Industrial Tapes and Adhesives(Manchester, UK)

Albert Browne Ltd. (Leicester, UK)

Albert Browne Ltd. (Leicester, UK)

Ashby Technical Products Ltd. (Ashbyde Ia Zouch, UK)

Cardinal Group (Tiburon, CA, USA)

Colour Therm (Surrey, UK)

PyMaH Corp. (Flemington, NJ, USA)(Temperature Indicators Ltd., Wigan,UK Agent)

PyMaH Corp. (Flemington, NJ, USA)(Temperature Indicators Ltd., Wigan,UK Agent)

Reatec AG (Switzerland) (BarbieEngineering, Twickenham, UK Agent)

Table 103 Continued

Change characteristicsColourTrade nameManufacturer

Liquid crystal colour change immediatelytemperature is reached

Irreversible colour labels, 40-2600C; lacquers,

40-lOloC; reversible strips, 40-700C

Immediately temperature is reached: 40-2600C

Three-stage semi-integrator using chromiumchloride

Selected precise time and temperature, 121 to134°C

Adhesive strips 40-2600C

Reversible and irreversible inorganic pigmentcolour change either immediately temperature isreached or after a few min exposure, 50-10100C

Immediately temperature reached: 71, 77, 82°C and88°C ratings ± 1°C. Other temperature ratings onrequest

Adhesive strips, irreversible colour change paints,37-2600C

Autoclave ink. Change set for 30 min at 116°C or15 min at 127°C

Organic thermo-chromic ink; colour changesimmediately temperature is reached

From light blue to a colour in thespectrum donating maximumtemperature

White to black

Mauve to green

Brown to black

White to black

For crayons and paints, a range ofcolours dependent on temperaturereached

White to black or white to red

Silver grey to black

Red to green

Red to black

Spectratherm

Temperature Tabs

CelsistripCelsidotCelsipointCelsiclock

Integraph

Cross-checks

Thermindex

PasteurisationCheck

Thermax

Autoclave Indicator

TLC 8

Redpoint (Swindon, UK)

S.D. Special Coatings (Barking, UK)

Spirig Earnest (Germany) (CobonicLtd., Surrey, UK Agent)

SteriTec (Colorado, USA)(Temperature Indicators Ltd., Wigan,UK Agent)

SteriTec (Colorado, USA)(Temperature Indicators Ltd., Wigan,UK Agent)

Thermindex Chemicals & CoatingsLtd. (Deeside, Clwyd, UK)

Thermographic Measurements Ltd.,Burton, S. Wirral, UK (TemperatureIndicators Ltd., Wigan, UK, EuropeanAgent)

Thermographic Measurements Ltd.(Burton, S. Wirral, UK)

Thermographic Measurements Ltd.(Burton, S. Wirral, UK)

TLC Ltd. (Deeside, UK)

exhibit heat-induced changes. For example, Kim and Taub (1993) have beenstudying the thermally produced marker compounds 2,3-dihydro-3,5-dihy-droxy-6-methyl-(4H)-pyran-4-one and 5-hydroxymethylfurfural. Both thesecompounds are produced when D-fructose is heated, and glucose yields onlythe latter compound. Hence, where a food contains either of these sugars,there is some basis for assessing heat treatment received as the kineticcharacteristics make them suitable as markers for bacterial destruction. Asbefore, the kinetic response requirement which a TTI should fulfil can bederived theoretically and should match the response of the target index, suchas a spore or a nutrient, when subjected to the same thermal process.Potential exists for multicomponent TTIs in the evaluation of thermalprocesses (Maesmans et al, 1994).

Regarding the origin of the TTI, an extrinsic TTI is a system added to thefood, while intrinsic TTIs are intrinsically present in the food. In terms of the

Physical

Response Single Multi

Origin

Application

Location

Intrinsic Extrinsic

Dispersed Permeable Isolated

Volume average Single point

Figure 10.1 General classification of time-temperature indicators (after Hendrickx et al, 1993).

Working principle Biological Chemical

application of the TTI in the food product, dispersed systems allow theevaluation of the volume average impact, whilst all three approaches (seeFigure 10.1) can be used as the basis for single point evaluations. Whenusing intrinsic components as the TTI, the TTI will be more or less evenlydistributed throughout the food, and this also eliminates heat transferlimitations. This whole field is currently the subject of a major Europeancollaborative research study co-ordinated by the Centre for Food Scienceand Technology at the University of Leuven in Belgium.

10.5 Conclusions

The interest in this subject has generated numerous research studies andpractical evaluations of indicator systems. It is clear that the food industry,and indeed other sectors such as the medical and pharmaceutical industries,as well as the consumer, recognise a variety of benefits that can stem fromthe application of indicators in aiding the monitoring and assurance ofdistribution chains. This, in turn, is leading to the development of newindicators that are much more precisely designed to meet the needs of thefood industry. In the broader context of time-temperature integration,applications for thermal process assessment are receiving further attentionand novel approaches are actively being researched. Such developments willassist in the assurance in and broader introduction of new heat processessuch as microwave sterilisation. Overall, it is likely that there will continueto be exciting developments during the next five years.

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11 Safety considerations in active packaging

J.H. HOTCHKISS

11.1 Introduction

Ever since Appert discovered that heating food in sealed glass jars produceda stable product, a major goal of packaging has been to safely preserve foodsfor extended periods. A scientific understanding of the relationship betweenshelf-life, safety, processing/storage conditions, and packaging began toevolve in the late 180Os as the theoretical basis for the thermal inactivationof pathogenic spores was developed (Goldblith, 1989). This understanding isstill evolving.

The use of packaging to safely protect and preserve foods has remained acentral focus of packaging development. The primary roles of packaging infood safety have traditionally been to withstand thermal processingconditions and to act as a barrier to contamination. It would be of littlebenefit to process food if there was no way to prevent recontamination. Thesuccess of the metal can over the last 150 years is due to its ability towithstand thermal processing and provide a barrier against chemical andbiological contamination. Modern food packaging can also influence thenutritional and quality attributes of foods and ensure the year-roundavailability of many foods. These factors are important in the health andnutritional aspects of foods.

The major advances in food packaging over the last two decades havebeen the development of new materials, combinations of materials, andcontainers with specific technical and economic benefits (Downes, 1989).Most of these new materials and containers are inactive technologies in thatthey act primarily as passive barriers which separate the product from itsenvironment. However, current research is shifting to the development ofpackaging which actively contributes to the preservation and safety of foods(Labuza and Breene, 1989). Such packaging interacts directly with the foodand the environment to extend shelf-life and/or improve quality.

11.2 Packaging and food safety

Packaging has often been thought of as a source of risk for foods and seldomas a technology which could be used to enhance food safety (Wolf, 1992).

Certainly, when packaging fails to preform its protective functions the resultis an unsafe product (Downes, 1993). For example, safety may becompromised when package components migrate to a food or when there isa loss of integrity resulting in contamination by pathogenic microorganisms.Table 11.1 lists several general ways in which packaging can detract fromsafety.

However, active packaging can directly enhance food safety. Activepackaging can not only prevent contamination but it can also improve foodsafety in several other ways. Examples of 'active' packaging whichimproves food safety include antimicrobial polymers and films which inhibitthe growth of pathogenic and spoilage microorganisms, packages whichreact with toxins and indicate their presence, packaging materials whichprevent the migration of contaminant, and packages which indicate ifpackages are leaking. These and other types of active packaging whichimprove safety and quality are areas of current research and commercialinterest (Ishitani, 1994).

Table 11.1 Types of food safety problems associated with packaging

Examples

Microbial contaminationLoss of integrity

Anaerobiosis

Chemical contaminationMigrationEnvironmental contamination

Recycled packaging

Insect contaminationPost packaging

Foreign objects

InjuryExploding pressurized containersBroken containers

Environmental impact

Loss of nutritional and sensoryquality

Tamper evidency

Inadequate processingConventionalAseptic

Consequences

Seal rupture, leaking cans, incomplete glass fin-ishes allow contamination by pathogenic m.o.Low oxygen environment resulting from productor microbial respiration. Can lead to toxin forma-tion by anaerobic pathogenic microorganisms

Transfer of package components to foodsEnvironmental toxicants can permeate filmsExamples include preservatives used in woodenpallets, diesel exhaustContamination of post-consumer packaging istransferred to foods after recycling

Some insects can bore through many commonpackaging materials

Glass shards, metal pieces

Soft drinks, beer in glass, etc.Cuts, lacerations

Disposal, recycling, CFCs

Aroma and nutrient sorption by polymers

Malicious and innocuous

Underprocessing can lead to food poisoningLoss of integrity or insufficient sterilization ofpackaging can lead to food poisoning

11.3 Passive safety interactions

11.3.1 Barriers to contamination

The major safety and quality function of packaging is to act as a barrierbetween food and the environment. The purpose is to prevent contamination(or re-contamination after processing) of the food from both environmentalchemicals and pathogenic microorganisms. With glass and metal foodpackaging, which are, for practical purposes, absolute barriers, preventingcontamination is usually a function of closure integrity. Considerableexperience with such closures has resulted in a remarkably low riskpackaging system.

The economic and functional disadvantages of metal and glass have led tothe development of polymeric packaging materials. The barrier properties ofthese polymeric materials has been the central focus of packaging develop-ment in recent years. Polymers which are high barriers to both oxygen andwater vapor are now available. Very recent efforts have focused onimproved aroma/flavor barriers.

Post-packaging microbial contamination of foods is now not only afunction of closure integrity and material integrity (Downes et a/., 1985).There are two aspects of package integrity: strength and completeness.Strength implies that the closure or seal is sufficiently strong to withstandthe rigors of distribution without failure. Completeness means that there areno gaps, holes, tears, etc. in the material or the seal. A seal or material canbe strong yet incomplete or can be complete yet have insufficient strengthfor distribution. In some cases, flexible materials can contain minute pin-holes which allow entry to microorganisms yet still not show signs ofleakage (Chen et aL, 1991). Flexible packaging can also develop pin holesduring shipping.

Strength and integrity have become important issues because foods aretransported further and stored for extended periods. The canning industryhas had considerable experience with double seam closure for metal cans,making a container sealed in this way one of the safest available. The lackof such long-term experience with the heat seal as a means of closure hasraised safety concerns. Flexible materials are also more prone to failureduring transport and storage. As the change to polymeric packaging hasoccurred, concern for the integrity of the container has increased. Researchhas been undertaken in an attempt to improve the testing of integrity ofpolymer-based packages. Gnanasekharan and Floros (1994) have reviewedmethodology for detecting leaks in flexible food packaging. No currentlyavailable method is entirely satisfactory for all situations.

Increased potential for chemical contamination has become a concernbecause polymers are permeable to organic vapors, and foods which arehermetically sealed in polymer-based containers can absorb environmental

contaminants. The transfer to foods of potentially toxic compounds used topreserve wooden shipping pallets and wooden container floors has beenreported and is exacerbated by the increase in long-distance shipment offoods (Whitfield et al, 1994). In addition to toxicological concerns, manyenvironmental organic compounds which permeate films impart undesirableodors to foods.

As the pace in the use of recycled materials has gained momentum,concern over microbiological contamination of fiber based packagingmaterials has also increased (Klungness et al., 1990). The paper makingprocess inactivates most vegetative cells but does not inactivate microbialspores. Foods packaged in recycled materials have the potential to haveacquired high spore loads from the packaging (Vaisanen et al, 1994). It islikely that in some cases potentially pathogenic spores could be transferredto foods from recycled materials.

Packaging also protects the nutritional and organoleptic quality of foods.While not directly safety issues, foods which have lost their nutritionalattributes or are not consumed because of poor taste or appearance, becomea health issue. Nutrients and organoleptic properties can be adverselyaffected in several ways. For example, nutrients can be destroyed whenoxygen or light enters a package or when the product is exposed to excessiveheat. The loss 'of vitamin C in orange juice when stored in low barrierpackaging is a prominent example. Sorption of nutrients by the packagingmaterial is also a mechanism of loss.

11.3.2 Prevention of migration

The second major safety function of packaging is to limit the transfer (i.e.,migration) of packaging components to foods. Considerable research hasbeen conducted into the migration of packaging materials to foods (Crosby,1981). Migrants include inorganic toxicants, primarily lead from solderedcans, as well as organic toxicants such as vinyl chloride monomer which isa known human carcinogen. Both the theoretical and empirical aspects ofmigration have been studied in detail and in most cases the process isscientifically understood.

Concern over migration has been recently heightened because of the useof recycled materials or refillable containers for food and beveragepackaging (Begley and Hollifield, 1993). At least two potential problemsexist. One is that non-food grade plastics which may contain additives ormonomers that are not intended for human food use will enter the recyclestream. These additives or monomers could then migrate to foods. Thesecond problem is the potential contamination of food grade polymericpackages by consumers (gasoline and pesticides are commonly mentioned aspotential contaminants). These contaminants could then migrate to foodspackaged in containers made from recycled materials.

Several solutions to the post-consumer contamination of recyclableplastics have been proposed. First is the use of equipment to detectcontaminated containers prior to refilling. These are commonly referred to as'sniffers' and are designed to sample the air inside the container anddetermine if volatile organic compounds such as might be found in gasolineare present. Commercial sniffers are available and in use. The secondsolution is to chemically break down the polymeric structure and subse-quently reform the basic polymer. Any contaminants would be removedduring this processing. The third solution is to construct containers in whichthe recycled polymer is separated from the product by a functional barrier.Such functional barriers are intended to prevent the migration of con-taminants from recycled polymers to products. Combining virgin andrecycled PET by co-extrusion into a PET bottle has been commerciallyundertaken. The virgin PET is expected to be a functional barrier to potentialcontaminants in the recycled layer. The major question is, How effective isthis virgin layer at preventing migration?

Other treatments of polymers such as cross-linking can retard migration.Migration is also affected by the chemical and physical nature of themigrant. Active packaging materials which can minimize or eliminatemigration would be of substantial interest as the concern over theenvironmental cost of packaging increases.

A second recent packaging migration safety concern has resulted from theuse of polymer-based packaging as containers for food during heating suchas in processing low acid foods in a retort or heating foods in microwaveovens. Initially, food polymer-based containers were designed to storeproducts at ambient or sub-ambient temperatures. Thus, most laboratorywork on migration was undertaken at room temperature or lower overextended periods. The advent of the microwave oven has meant that foodsare heated in plastic vessels and on plastic surfaces. Heating plastics has twoeffects on migration. First, migration in general follows Arrhenius-typekinetics and increasing temperature increases migration rates in an exponen-tial fashion. The second effect is that elevated temperature can causedegradation of the polymer and additives which can result in migration ofthe breakdown products. Each of these issues has been addressed by severalregulatory agencies in the USA and Europe.

11.4 Active safety interactions

Active packaging systems face similar barrier and migration safety issues asconventional packaging, as well as some additional issues. While there isconcern that some active packaging systems will detract from safety therealso is the possibility that new active systems can enhance safety. Materialsand containers are being developed specifically to reduce food safetyrisks.

11.4.1 Emitters and sorbers

One the earliest and most successful active packaging concepts was toincorporate a material which either absorbed or emitted vapors or gasesinside a package after closure. This might be as simple as water vaporabsorbers which are designed to control relative humidity, or more complexsubstances which absorb ethylene from produce, absorb undesirable odorsfrom foods, or emit ethanol to control molds in bakery products. Particularlydesirable types of sorbers are those that remove both residual and ingressoxygen after the package has been sealed (Rooney, 1994). Oxygen absorberswhich remove oxygen from the headspace of bottled beer, for example, havebeen successfully tested commercially. Initially, absorbers/emitters werecontained inside packets which were added to the package along with theproduct. More recent technologies have incorporated the sorber/emitter intothe film or container wall. This reduces the likelihood of accidentalingestion. These absorbers are a form of modified atmosphere packaging andcan change the microbiology of foods. The safety implications of suchchanges are the same as those for conventional MAP, as discussed below.There is also the concern that the components making up the absorber/emitter will migrate to the food.

11.4.2 Active packaging and migration

Many active packaging systems incorporate functional additives in foodcontact materials. These may be as simple as iron oxides which absorb O2 oras complex as systems which react with singlet oxygen (Rooney, 1994). Ineach case, the potential for and consequences of migration need to beassessed. For example, there has been some reluctance in some parts of theworld to allow the use of ethanol emitters in foods which will be consumedwithout further cooking or processing. The residual ethanol might beconsidered a food additive and thus be required to undergo the rigors ofcomplete toxicological testing. For those active packaging systems whichindirectly add components to foods, the governmental regulatory and healthissues will be similar to those related to migration of residual monomers orother polymer components (Crosby, 1981). Laboratory investigations will berequired to determine the potential for migration and to quantify the amountof migration. If the amount of additive migrating is considered of potentialsignificance, toxicological testing may be required.

In some cases, active packaging systems may involve migrants for whichthere is little concern in food systems. For example, approved antimycoticagents such as sorbic, benzoic, or propionic acids, would likely be of littleregulatory or safety concern if incorporated into antimicrobial films (Giese,1994). However, in most cases active components and additives will not becommon food additives and potential toxicological concerns will need to beaddressed. The addition of antimicrobial metal ions to food contact surfaces

is likely to result in the migration of small amounts of the metals to foods(Ishitani, 1994). While these metal ions may be of low toxicity, the metalsmay be classified as food additives and require rigorous toxicologicaltesting. The regulatory consequences of intentional addition of even lowamounts of metals will need addressing.

In some cases, the use of functional barriers to prevent migration of theactive components will be required. Incorporating absorbers or scavengersinto the adhesives used to bind layers of inert film as a means of 'burying'the additive is an example.

11.4.3 Barrier to contamination

In addition to migration, active packaging systems must fulfill the safetyrequirement of acting as a barrier to microbial and chemical contamination.The addition of active ingredients to films could decrease their mechanicalproperties resulting in a higher failure rate during transport. Such failuresbecome safety concerns if they allow for contamination by pathogenicmicroorganisms or toxic chemicals. For example, the addition of inorganiccompounds such as metal-coated zeolites, desiccants, or oxygen scavengerswill likely reduce the mechanical properties of films raising the possibilitythat the contamination barrier will be reduced.

The addition of packets or sachets to packages of food raises concern thatthey will be inadvertently ingested. While sachets and packets have been inuse for several years without apparent problems, caution about adding non-edible items to packages should be taken. Incorporation of active ingredientsdirectly into the packaging rather than as sachets seems prudent.

11.4.4 Indirect effects on safety

Active packaging often has indirect as well as direct effects on food safety.For example, packaging which absorbs oxygen from inside a package withthe goal of reducing deteriorative affects will affect both the types andgrowth rate of the microorganisms in products. The inclusion of anti-microbial agents in the contact layer of a packaging material may havesimilar effects. This will result in a change in the microbial ecology of thefood. The type of microorganisms present on a product will thus be differentfrom the same product packaged in a conventional manner. This change inmicrobiology will indirectly influence safety. In some cases, safety may beenhanced such as when carbon dioxide is added to high pH cheeses such ascottage cheese (Chen and Hotchkiss, 1993). In other cases, safety may becompromised as when the growth of Clostridium botulinum is favored.

The effect of such changes in the microbial ecology of foods has not beeninvestigated in detail and only a few reports on changes in microbial ecologyhave been published (Reddy et al.9 1992). Smith and co-workers have

investigated the effects of MAP on the microbiology and toxin productionby Clostridium botulinum in meats (Lambert et al9 1991). Somewhatsurprisingly, toxin was produced most rapidly in samples packaged in air(i.e., 20% O2). This confirms an earlier observation we had made in cookedbeef inoculated with both Pseudomonas and Clostridia spp. It is likely thatthis occurred because the aerobic Pseudomonas grew rapidly and consumedthe oxygen rapidly leaving a highly anaerobic environment for theClostridia. MAP in high carbon dioxide atmospheres inhibited the Pseudo-monas inoculum and left traces of unconsumed O2 which inhibited theClostridia. These results point out that large changes in microbial popula-tions can result indirectly from altering the gases inside a package. Thesechanges can both detract from safety but can also improve safety.

11.4.5 Indicators of safety/spoilage

In addition to decreasing the safety of food, active packaging holds thepromise of reducing risks from certain foods compared to conventionalpackaging. One example is the use of packaging which shows or in someway indicates the condition or history of a product. One currently availabletechnology is time-temperature indicators. These devices integrate the timeand temperature history of a product and give a visual indication if thecombination has exceeded some standard or desirable amount (Taoukis etal9 1991). Shelf-life is related not only to how long a product is stored, butjust as importantly, to the conditions, such as temperature, under which theproduct is stored. For example, pasteurized milk will last weeks at O0C butonly a few hours at 35°C.

Time-temperature indicators can be used on individual packages to warnconsumers that a product has been exposed to a combination of time andtemperature which may compromise safety or they may be used on shippingcartons to alert store personnel of potential quality/safety problems or allowstock rotations based on both time and temperature. Such devices would beespecially useful when combined with other shelf-life technologies such asMAP or sub-sterilization radiation.

The next generation of safety/quality indicators may be more specific thanintegrating time and temperature. In the future it may be possible to directlydetect the presence of specific toxins in packaged foods using biosensors.Immunologically based sensors coupled to packaging could find applicationsin food safety, food processing, and detection of adulteration (Deshpande,1994). Such sensors could, for example, detect the presence of bacterialtoxins in packaged foods. They could also be used to determine if a food hadbeen properly pasteurized or contained enzyme activity. Biosensors whichcombine electronics with biological specificity and sensitivity may find usein packaging as monitors of safety and quality (Deshpande and Rocco,1994). In time, it may be possible to incorporate these or similar biosensors

into food packaging systems for which the risk of toxin formation exists.Reportedly, methods to quantify the presence of microorganisms on freshmeats are near commercialization (Bsat et #/., 1994). Such systems couldeventually be incorporated directly into food packaging.

It may likewise be possible to detect the presence of toxic chemicals usingsimilar technologies. The presence of specific pesticides or other environ-mental contaminants could be detected with immunological-based systems(Deshpande, 1994). Lastly, packaging should provide a margin of safetyagainst tampering. Tamper-indicating packaging has been discussed in detailsince several malicious incidents of tampering with drugs and foods haveoccurred (Hotchkiss, 1983). Several simple and complex tamper-evidentpackaging systems have been developed and a few implemented forfoods.

11.4.6 Direct inhibition of microbial growth

Microbial contamination and growth are the major factors in food spoilageand responsible for food-borne disease outbreaks. Two general approaches,heat sterilization and direct addition of antimicrobial additives, have beenused to eliminate or minimize microbial growth. In conventional thermalprocessing, foods are sealed in a package and the combined product-package thermally processed. This is the basis of the canning industry. Morerecently, the process of the package and the product being sterilizedseparately then filled and sealed aseptically has been used. This is known asaseptic packaging. Foods can also be dried to reduce microbial growth.

Another method to reduce microbial growth is to add antimicrobialadditives directly to foods. This approach usually does not inhibit all growthbut is selective for certain types of microbial growth, molds for example.The use of these additives is regulated and their use, in most cases, must bestated on the label.

11.4.7 Modified atmosphere packaging

Recently, the development of alternative methods of inhibiting microbialgrowth has resulted from a consumer desire for fresher and more naturalfoods. The most successful alternative to canning or the direct addition ofantimicrobial agents has been modified atmosphere packaging (MAP). Thenumber and type of microorganisms present on a food is governed by fivegeneral variables: time, temperature, substrate (food) composition, microbialload (type and number), and gas atmosphere. For a given food productwhich must be held above freezing, alteration of the gas atmospheresurrounding the product is the most accessible method of inhibitingmicrobial growth. However, inhibition is not uniform for all types ofmicrobes. In general (although there are exceptions) Gram-negative rods are

inhibited by a modified atmosphere containing more than 10% CO2 whileGram-positive organisms are not inhibited and their growth can bepromoted.

The major goal of MAP is to reduce the growth rate of microorganismswhich cause the product to become organoleptically unacceptable. However,organisms which cause disease (i.e., pathogens) do not, in many cases, causeorganoleptic changes in foods. Increasing the shelf-life by suppressingspoilage organisms might allow for the growth of pathogenic organismswithout development of the normal organoleptic cues of spoilage that warnconsumers that a product may not be wholesome (Farber et ai, 1990). Thus,the major safety concern with MAP and controlled atmosphere packaging orother technologies which selectively change the microbiology of a food isthat suppression of organoleptic spoilage (i.e., extension of shelf-life) willdecrease competitive growth pressure and provide sufficient time for slowgrowing pathogenic organisms to become toxic or reach infectious numbers(Hotchkiss and Banco, 1992). The knowledge gained over the last decadeabout pathogenic microorganisms which are capable of surviving andgrowing at common refrigeration temperatures increases concern about thesafety of refrigerated extended shelf-life foods (Gormley and Zeuthen, 1990;Farber, 1991). The effect that this change in microbiology might have on therisk of food-borne disease has been debated (Gormley and Zeuthen,1990).

There are several methods of creating a modified atmosphere inside apackage. One is the use of selective or engineered barriers which are usedfor respiring products such as fruits and vegetables. The combination ofproduct respiration rate (i.e., rate OfCO2 formation and O2 consumption) andCO2 egress and O2 ingress results in the formation of an equilibriumconcentration of gases which, if properly designed, will reduce senescenceand extend shelf-life. Alternatively, a specific gas mixture can be directlyintroduced into the package after removal of the air and before sealing. Athird method is to use an additional material contained in a sachet orincorporated into the film which will alter the gas composition after sealing.In each case, the change in atmosphere will affect both the growth rate andtype of microorganisms present. However, temperature will affect therespiration rate to a much greater extent than the permeability. If the productis stored at an elevated temperature, respiration rates will increase and the O2

content of the package may approach zero. At the same time the growth rateof pathogenic microorganisms substantially increases with the increase intemperature. This could allow for the growth of anaerobic pathogens such asClostridium botulinum.

For example, Lambert et al (1991) have shown that toxigenesis occursmore rapidly in aerobically packaged pork samples compared to anaerob-ically packaged samples when Pseudomonas spp. were present along withthe Clostridium botulinum inoculum. It was presumed that the Pseudomonas

rapidly consumed the oxygen allowing the C. botulinum to become toxic.These results agreed with earlier results of Hintlian and Hotchkiss (1987)who made a similar observation.

11.4.8 Antimicrobial films

Packaging may directly affect the microbiology of foods in ways other thanchanging atmosphere. In solid or semi-solid foods, microbial growth occursprimarily at the surface. Surface treatment by spraying or dusting withantimicrobial agents for products such as cheeses, fruits, and vegetables iswidely practised. Antimycotic agents are commonly incorporated into waxesand other edible coatings used for produce items (Peleg, 1985). Morerecently, the idea of incorporating antimicrobial agents directly intopackaging films which would come into contact with the surface of the foodhas been developed.

Antimicrobial films can be divided into two types: those containing anantimicrobial agent which migrates to the surface of the food and those thatare effective against surface growth without migration of the active agent(s)to the food. Several commercial antimicrobial films have been introduced,primarily in Japan. One widely discussed product is a synthetic zeolitewhich has had a portion of its sodium ions replaced with silver ions. Silvercan be antimicrobial under certain situations. This zeolite is incorporateddirectly into a food-contact film. The purpose of the zeolite apparently isallow for the slow release of silver ions to the food. Only a few scientificdescriptions of the effectiveness of this material have appeared and theregulatory status of the deliberate addition of silver to foods has not beenclarified in the US or in Europe.

Several other synthetic and naturally occurring compounds have beenproposed and/or tested for antimicrobial activity in packaging (Table 11.2).For example, the antimycotic (i.e., antifungal) agent, imazalil, is effectivewhen incorporated into LDPE for wrapping fruits and vegetables (Miller et

Table 11.2 Some antimicrobial agents of potential use in food packaging

Class Examples

Organic acids Propionic, benzoic, sorbicBacteriocins NisinSpice extracts Thymol, p-cymeneThiosulfinates AllicinEnzymes Peroxidase, lysozymeProteins ConalbuminIsothiocyanates AllylisothiocyanateAntibiotics ImazalilFungicides BenomylChelating agents EDTAMetals SilverParabens Heptylparaben

al, 1984; Hale et al.> 1986). We have demonstrated that the same compoundis effective at preventing mold growth on cheese surfaces when incorporatedinto LDPE films (Weng and Hotchkiss, 1992). Although imazalil is notapproved for cheese, this work established that antimycotic films could beeffective for control of surface molds in foods. Halek and Garg (1989)chemically coupled the antifungal agent benomyl, which is commonly usedas a fungicide, to ionomer film and demonstrated inhibition of microbialgrowth in defined media. While not directly addressed by the authors, themethod used to determine inhibition of growth indicated that the benomylmigrated from the film to the growth media. It is unlikely that benomylwould be approved for food use for toxicological reasons.

Reports have appeared which demonstrate the effectiveness of addingcommon food-grade antimycotic agents to cellulose-based edible films(Vojdani and Torres, 1990). Films were constructed of cellose derivativesand fatty acids in order to control the release of sorbic acid and potassiumsorbate. These films would seem to have the greatest application as fruit andvegetable coatings. Cellulose films are not heat sealable are not goodbarriers in high humidity situations.

We have spectroscopically demonstrated that propionic acid, which is acommon approved food antimycotic agent, could be coupled to ionomericfilms but that antimycotic activity could not be demonstrated on rigoroustesting (Weng, 1992). Direct addition of simple antimycotic acids such aspropionic, benzoic, and sorbic acids to polymers such as LDPE wasunsuccessful because of lack of compatibility between the acid and the non-polar film. This incompatibility is likely to be due to differences in polarity.We have solved this problem by first forming the anhydride of the acidwhich removes the ionized acid function and decreases polarity (Weng andHotchkiss, 1993). Anhydrides are stable when dry and relatively thermallystable yet become hydrolysed in aqueous environments such as foods.Hydrolysis leads to formation of the free acid which in turn leads tomigration from the surface of the polymer to the food where the free acidscan be effective antimycotics. This is an example of 'switched on'packaging; the active ingredient remains in the film until the film comes intocontact with a food. The activity is initiated by the moisture in the food.

Future work in antimicrobial films may focus on the use of biologicallyderived antimicrobial materials that are bound or incorporated into films anddo not need to migrate to the food to be effective. For example, a group ofsubstances known as bacteriocins, which are proteins derived from micro-organisms in much the same way as penicillin is derived from mold, havebeen described in the literature (Hoover and Steenson, 1993). Bacteriocinsare effective against organisms such as Clostridium botulinum and one suchcompound, nisin, has been approved for food use. These peptides could,theoretically, be attached to the surface of food-contact films. Whether or notsuch bound bacteriocins would be effective remains unclear.

Antimicrobial enzymes might also be bound to the inner surface of food-contact films. These enzymes would produce microbial toxins. Several suchenzymes exist, such as glucose oxidase which forms hydrogen peroxide.

A third possibility for antmicrobial films is to incorporate radiation-emitting materials into films. Reportedly, the Japanese have developed amaterial which emits long-wavelengt IR. This is thought to be effectiveagainst microorganisms without the risks associated with higher energyradiation. However, little direct evidence for the efficacy of this technologyhas been published in the scientific literature.

In general, several questions, including those dealing with safety, shouldbe considered in developing antmicrobial films:

• What is the spectrum of organisms against which the film will beeffective? Films which may inhibit spoilage without affecting the growthof pathogens will raise safety questions similar to those of technologiessuch as a MAP.

• What is the effect of the antimicrobial additives on the mechanical andphysical properties of the film? It is likely, for example, that effectivelevels of antimicrobial agents will reduce seal strength. This mayadversely affect safety.

• Is the antimicrobial activity a reduction in growth rate (yet still a positiveincrease in cell numbers) or does it cause cell death (decline in cellnumbers)?

• To what extent does the antimicrobial agent migrate to the food andwhat, if any, are the toxicological and regulatory concerns?

• What is the effect of food product composition? Some antimicrobialagents, for example, are effective only at acid pH while others mightrequire certain product compositions (e.g. aw9 protein, glucose, etc.) to beeffective.

Each of these questions need addressing before the safety consequences ofantimicrobial packaging can be understood.

11.4.9 Rational functional barriers

As pointed out, one major safety function of packaging is to act as achemical and biological barrier. Films are frequently selected for food usebased on the highest degree of oxygen and/or water vapor barrier at thelowest cost. More recently, the concept of rational design or engineering offilm permeability has evolved. These so-called 'smart' films have barrierproperties which are designed to adapt or change permeabilities according toconditions such as a change in gas composition or temperature.

These engineered barriers have at least two important safety-relatedapplications. The first is to act as a barrier to permeation of contaminants.Packaged foods can be exposed to contaminants from environmental sources

or from the use of recycled plastics in food packaging. Common environ-mental sources of toxic permeants include chemicals used to treat shippingcontainers and pollutants. Chlorinated wood preservatives readily permeatethrough common films and cause taint in foods (Whitfield et al, 1991).Common environmental pollutants such as diesel exhaust and industrialsolvents used in printing also permeate many common food-packagingfilms.

The second use of engineered barriers is in passive MAP systems inwhich an equilibrium in the gas mixture is achieved through the combinationof product respiration and package permeability. This equilibrium resultsfrom the consumption of O2 and evolution of CO2 by the food product at thesame time that O2 is permeating into the package and CO2 is permeating outat a given temperature. At some point these respiration and permeation rateswill reach an equilibrium concentration. Selection of a film with the properpermeability will result in the desired gas mixture. Several mathematicalmodels have been developed which predict the proper permeation ratesgiven by a specific product respiration rate (Mannapperuma et ai9 1991).

There are two difficulties with this concept. The first is that respirationrates of most produce items vary widely, even within the same type of item.Thus, permeation rates will have to be tailored for each individual productitem. The second problem is that CO2 permeation rates for commonpackaging films is 2-A fold or more higher than for O2. This means that CO2

may egress at a faster rate than O2 will enter, making the atmosphereanaerobic. Engineered films which can independently select CO2 and O2

permeation rates as well as films that change permeation at the same rate thatfruits and vegetables change respiration rate with temperature, would bedesirable. As pointed out above, the rate of gas change will determine thetype of microorganisms on the products and, probably, the safety of suchfoods.

We have recently devised equations for achieving optimum atmosphereconcentrations for extending the shelf-life of fresh corn on the cob and headlettuce, each of which illustrates some of the problems with engineeredbarriers. Head lettuce respires relatively slowly and films which will allow apassive modified atmosphere to be established are commercially available.However, about 90 hours are required for establishment of a suitableatmosphere (Morales-Castro et al, 1994a). During this time considerabledeterioration can occur. Sweet corn, on the other hand, respires rapidly andthe establishment of a desirable steady-state atmosphere is not possible withnormal films because the permeabilities are too low even for very lowbarrier films (Morales-Castro et aL, 1994b).

MAP products such as lettuce and corn or other vegetables could becomesafety concerns if the atmosphere were to become anaerobic. This mightoccur if products were stored at a higher than expected temperature. Thiswould cause an increase in respiration beyond that expected and the oxygen

might be substantially depleted. This would leave open the possibility ofgrowth of anaerobic pathogens such as Clostridium botulinum.

11.4.10 Combined systems

The most successful active packaging materials are likely to combinedifferent technologies. A few examples of such combinations have appearedin the literature. For example, Fu and Labuza (1992) have suggested thatMAP might be combined with time-temperature indicators as a means ofextending the shelf-life of perishable foods while at the same timeminimizing food-borne disease risks. MAP would reduce the deterioration offood while the time-temperature indicator would insure that the product wasstored and handled within the time and temperature window for which theproduct was designed, to insure safety. Labuza et al (1992) have suggestedthat predictive microbiology should be used to evaluate the safety of MAPfoods.

Low dose irradiation and MAP have been combined to extend shelf-life(Thayer, 1993). Irradiation reduces the numbers of spoilage and pathogenicvegetative organisms while a modified atmosphere reduces the likelihoodthat those not destroyed will grow significantly in number. Lambert et al(1992) have demonstrated a substantial increase in the shelf-life of freshpork treated with both irradiation and MAP.

Other combinations such as antimicrobial films combined with MAP oroxygen absorbers combined with antimicrobial films may find commercialuses. Zeitoun and Debevere (1991) have suggested that combining a simplelactic acid dip combined with MAP would enhance the shelf-life of freshpoultry.

11.5 Conclusions

It can be expected that safety will continue to be an important attribute forfoods. New packaging technologies which improve quality, usefulness, orreduce environmental impact will also be required to maintain a high levelof safety. Active packaging systems will not be an exception.

Those active packaging systems which reduce the risks associated withfoods may find niche markets for products at the highest risk ofdeterioration. MAP of non-sterile foods is one example where additionalsafety measures such as use of microbial inhibitors or indicators oftemperature abuse would be useful. Recycled materials for food packaging isanother.

Active packaging systems which provide benefits for foods will have toadhere to governmental regulatory standards in most of the world. This willinhibit the introduction of some active systems. Antimicrobial films are a

prime example. Developers of such materials should understand the safetyand regulatory implications of their work early in the process if they expectto be successful.

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256 This page has been reformatted by Knovel to provide easier navigation.

Index

Index terms Links

A absorbers

food constituents 99 100 free oxygen see oxygen absorbents odours and taints 100 213

activation energy permeation 68 respiration model 65

active packaging chemical 21 24 composite 8 definition 1 2 143 203 do-it-yourself 17 economic benefit 76 future potential 32 history 4 horticultural 9 limitations 31 literature 10 multiple effects 9 252 origins 3 physical 20 reasons for 3 regulatory considerations 33 252 reviews 10

257 Index terms Links


active packaging (Continued) scope 12 terminology 1 10 whole packages 29

active packaging plastics and safety 243 combined effect 82 106 commercial use 106 effects on foods 74 environmental considerations 107 migration from 243 regulations 106

Ageless and aw 150 capacity 150 chemistry 149 types 149

aldehydes 100

almonds rancidity and oxygen absorbent 155

amines 100

anaerobic respiration 68

antimicrobials 243 244 248 enzymic 250 silver zeolite 248

antimycotics 249

ascorbic acid and oxygen absorbent 154

Aspergillus spp 159



B bacteriocins 249

bakery products and oxygen absorbent 156 ethanol releasing sachets 166 169 mould growth 159 160

barriers functional 250

beer commercial oxygen scavenger closures 198 oxygen concentration in 194 196 oxygen ingress 195

blueberry 65

broccoli 65 69

C calcium carbonate

as filler 69

cans tinplate 238

carbon dioxide permeability 123

carnauba wax 132

carrots packaging condensation control 98

cauliflower 65

casein films sorbic acid in 128



cheese antimycotic films for 249

Clostridium botulinum 244

Clostridium sporogenes effect of Ageless 163

closure beer bottles 193 benefits 199 beverage bottles 193 oxygen scavenging

with ascorbic acid 198 with sodium sulfite 168

composite films sorbic acid barrier 130

consumer resistance to sachets 162

contamination barriers to 240 244

controlled atmosphere (CA) 55

corn gas atmospheres for 251

crusty rolls and oxygen absorbents 159

D deoxidisers see oxygen scavengers

desiccant retorting 75 79



E edible coatings see edible films

edible films as active packaging 113 as food 112 bilayer 117 composite 111 117 food surface modification 126 formation 112 gas exchange with 121 moisture barrier 114 multilayer 117 permeability 114 plasticisation 120

energy transfer 90

environment 107

enzymes active packaging 176 antimicrobial 186 binding 105 177 function 174 history 178 oxygen scavenging 174 release 105

equilibrium modified atmosphere (EMA) 247 251 see also passive modified atmosphere packaging Clostridium botulinum in 247 generation 56 packaging 2 9 55 66 Pseudomonas spp in 247



ethanol oxidase 186

ethanol vapour absorption 167 antimicrobial effects 165 171 and aw 165 in bakery products 165 and food spoilage 171 and food poisoning 171 generators

advantages 171 aw effects 166 disadvantages 171 required sizes 166 types 166 uses 168

ethylene adsorption 40 chemistry 38 degradation 39 effects 41 interaction with other gases 44 sources 45 46 synthesis 38 39

ethylene scavengers activated carbon 47 activated earth 48 novel approaches 50 potassium permanganate 46



F films

antimicrobial 248 ceramic filled 70 condensation control 98 controlled diameter holes to humidity buffering 96 for MAP 67 microporous 68 moisture control 94 perforated 68 temperature-compensating 70 with oxygen absorbents 92 153

fish products and oxygen absorbents 157

flavour scalping 99

foods oxygen absorbents for 155

Freshilizer and aw 151 reaction speed 151 types 151

FreshMax 212

Fresh Pax and aw 151 reaction speed 152 types 152 uses 211

fruits edible coatings 126 gas atmosphere for 57



functional barriers 250

G gelatin films

sorbic acid retention 128 tannic acid crosslinked 128 tocopherol retention 128

glucose oxidase 181

gluten films with beeswax 124

green pepper 65

H hydrogen oxidation 84

hydrogen peroxide 86

hydroxypropyl methylcellulose 130

I IMF

and Staphylococcus aureus 134 casein-coated 132 microbiological stability 132 papaya cubes 132 with sorbic acid 132

indicator bacterial toxins 245 oxygen concentration 2 safety 245 spoilage 245 temperature 209



indicator (Continued) time-temperature see Time-Temperature Indicators

interactive packaging see active packaging

intermediate moisture food see IMF

L lettuce

gas atmosphere for 251

limonin 99

Listeria monocytogenes 163 ethanol vapour effect 171

M mathematical modelling 251

limitations 64 70 parameters 71 steady state 66 unsteady state 65 variables 66

meat colour and oxygen absorbents 160

meat packaging oxygen scavenging 158

methylcellulose 130 with palmitic acid 132

microbiological stability carrageenan effect 134 lactic acid effect 133 surface pH effect 133

microencapsulation 85



microwave susceptors 203 field intensifiers 206 reflectors 206

microwaveable bottle 209

migration from active packaging 243 from heated plastics 242 from recycled plastics 242 of preservatives 127 prevention of 241

modified atmosphere packaging (MAP) active 55 and Clostridium botulinum 245 247 definition 143 and ethanol vapour 170 experimentation for 60 film selection 66 67 gas concentration boundaries 58 gas tolerance limits 61 feasibility study 59 films for 67 flow chart 56 and irradiation 252 literature review 57 optimisation 60 passive 55 pathogens in 246 and Pseudomonas 247 quality criteria 59 regulation of 252



modified atmosphere packaging (MAP) (Continued) and Time-Temperature Indicators 252

moisture transfer ice cream-wafer 121

mould growth on bakery products 75 on cheese 75

mozarella cheese and oxygen absorbents 160

mushrooms 69

O odours

from oxygen scavengers 82 91

organic acids 243

Ox-Bar 82

oxygen concentration measurement 193 194 permeability of closure liners 195

oxygen absorbents see also oxygen scavengers advantages 161 and aflatoxins 164 and mould growth 159 applications 148 choice of 152 classification 145 CO2 producing 146 definition 144 disadvantages 161 dual effect 146 150 151



oxygen absorbents (Continued) function 148 history 4 in Japan 144 153 in USA 155 159 market statistics 144 reaction speed 146 reactions in 145 requirements for 145 research with 159

oxygen absorbers see oxygen absorbents; oxygen scavengers

oxygen permeation aw effect 124 chemical barrier 92 185 edible films 123

oxygen scavengers 1 see also oxygen absorbents Advanced Oxygen Technologies, Inc. 198 Ageless 211 Aquanautics Corp. 198 ascorbic acid 85 198 ascorbic acid plus sodium sulfite 86 beer bottle closures future 200 beer flavour and 199 bottle closures 213 chemistry of 83 closure liner theory 197 composite systems 8 erythorbic acid 198



oxygen scavengers (Continued) fatty acids 82 91 food applications 212 FreshMax 212 Fresh Pax 211 history 4 hydrogen/catalyst 84 iron see oxygen absorbents Mitsubishi Gas and Chemical Co 211 Multiform Desiccants Inc. 211 novel designs 7 Ox-Bar 83 OxyBan 181 patent applications 5 plastics vs. sachets 81 PureSeal 199 release of carbon dioxide rubbers 89 90

and odours 82 91 SmartCap 199 sulfite oxidation 85 86 198

oxygen scavenging activation see triggering and aflatoxin production 164 and microbial growth 163 by autoxidation 91 enzymic 179 peroxide formation 86 purposes 75

oxygen scavenging packaging forms of 76 81



oxygen scavenging plastics 77 forms of 76 history 80 light-energised 80 81 light triggered 91 metal-catalysed photosensitised 91 MXD-6 nylon 83 permeability effects 99 photoreduction-reoxidation 92 photosensitised 87 potential applications 75 singlet oxygen mechanism 87

oxygen scavenging sachets see oxygen absorbents

Oya Stone 209 broccoli packaging and 210 Evert-Fresh Bag 209

P package integrity 240

packaging active constituent impact on 27 antimicrobial 243 244 246 intelligent 2 215 216 interactive 2 modelling 64 modified atmosphere 2 246 safety problems 239 tamper evident 246 with oxygen scavengers 92 153



patents oxygen scavenging 5 81

pectin films 128

Penicillium spp 159

perforated films computer simulation 69

permeability activation energy 68 composite films 118 edible films 114 effect on oxygen scavenging 78 89 humidity effect 79 modification 105 ratio 67 71 125 smoke 105 table 68 78 116 117

123 124 169 temperature effect 120 130 to ethanol 169 to water vapour 115

peroxide value and oxygen absorbent 154

photosensitisation 87

Pichit 97

pizzas and oxygen absorbents 160

plastics definition 74 for oxygen scavenging 77 functional barrier 242



plastics (Continued) iron in 78 recycled 242

poly(1,2-butadiene) 77 80 91

poly(1,3-butadiene) 89

polydimethylbutadiene 89

polyethylene antimycotics in 248

polyisoprene 89

potassium sorbate 127

processed foods and oxygen absorbents 158

produce packaging 9 46 55 209

propionic acid 249

propionic anhydride 249

Q Q10

62

R radiation

far infra-red 70 250

regulation 106 242 243 252

release antimicrobial agents 102 antioxidants 103 butylated hydroxytoluene 104 enzymes 105 179 181 ethanol vapour 165



release (Continued) flavours 103 food ingredients 102 fumigants 104 hinokitiol 102 Maillard reaction products 104 permethrin 104 silver ions 248 sorbic acid 126

removal aldehydes 100 amines 100 cholesterol 190 lactose 189 styrene 101

respiration rate circulation 62 63 closed system 63 flow-through 61 measurement 62 temperature effect 61

respiratory quotient 61

retortable packaging 3 79

S Saccharomyces cerevisiae

ethanol vapour effect 170

sachets combined effect 146 150 151 152 consumer resistance 162



sachets (Continued) ethanol releasing 165 ethylene-removing 46 oxygen scavenging see oxygen absorbents and public health 163 safety with 244

safety active interactions 242 and packaging 238 indirect effects on 244 passive interactions 240

scalping 99

sheets drip-absorbent 95

shelf life with oxygen absorbents 153

silica as filler 69

silicone film 69

smart films 1 250

sodium chloride 98

sorbic acid 126 diffusivity 129 permeability 129

Staphylococcus aureus 163

strawberries 69

sulfites 79

superabsorbent polymers 96



T taints 99 251

see also odours

Time-Temperature Indicator (TTI) activation 228 bar code 222 classification of 233 consumer attitudes 229 definitions 215 enzymic 188 operating principles 217 reasons for 216 245 requirements of 217 specifications 229 thermal process validation with 230 validation tests 224

toxicology 243

triggering chain reaction 91 of propionic acid 249 oxygen scavenging plastics 83 photoreduction 92

V vegetables

gas atmospheres for 59

vitamin C see ascorbic acid



W water activity (aw)

and ethanol generators 68

Y yeast growth

ethanol effect 4 170 oxygen absorber 4

Z zein films 128

lactic acid retention 134

Active Food Packaging

Documents

active packaging materials

use of active packaging

field of active packaging

benefits of active packaging

forms of active packaging

types of active packaging

packaging requirements

active food packagingedited