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Meat Packaging: Protection,Preservation, and Presentation

R. GRAHAM BELL

MIRINZ Centre AgResearch, Hamilton, New Zealand

I. INTRODUCTION

II. MEAT AS A MICROBIAL SUBSTRATEA. Substrate CompositionB. Substrate pHC. Water ActivityD. Initial Contaminating MicrofloraE. Microbial SpoilageF. TemperatureG. Gaseous Environment

III. FUNCTIONAL REQUIREMENTS OF MEAT PACKAGINGA. ContainmentB. ProtectionC. PreservationD. ApportionmentE. UnitizationF. ConvenienceG. Communication

IV. THE PACKAGING ENVIRONMENTSA. Physical EnvironmentB. Climatic EnvironmentC. Human Environment

V. PRODUCT PACKAGING ENVIRONMENT INTERACTIONS

VI. PACKAGING METHODSA. Nonpreservative PackagingB. Preservative PackagingC. Two-Phase Packaging

Copyright © 2001 by Marcel Dekker, Inc. All Rights Reserved.

VII. PACKAGING FOR CHILLED STORAGEA. Vacuum PackagingB. Saturated Carbon Dioxide Atmosphere Packaging

VIII. PRODUCT-LIFE EXPECTATIONS FOR CHILLED PRESERVATIVELY PACKAGEDFRESH MEATSA. Product Safety

IX. PACKAGING FOR RETAIL DISPLAYA. Overwrapped TraysB. Modified Atmosphere Packaging

X. TWO-PHASE PACKAGINGA. Mother PacksB. Gas-Exchange PacksC. Removable Top Web Packs

XI. CHILLED MEAT PACKAGING REQUIREMENTS

XII. FROZEN MEAT PACKAGINGA. Types of Packaging for Frozen Meat

XIII. PRODUCT DETERIORATION DURING FROZEN STORAGEA. Gross Carton Deformation and BreakdownB. Freezer BurnC. Frost FormationD. RecrystallizationE. Freeze-Thaw CyclingF. RancidityG. Nutrient Loss

XIV. STORAGE LIFE EXPECTATIONS FOR FROZEN MEAT

XV. SUMMARY

REFERENCES

I. INTRODUCTION

Although fresh meat is highly perishable, its packaging has, until relatively recently, beena matter of minor concern to meat traders, health officials, and the public. Unwrapped freshor frozen carcasses have long been the currency of the wholesale meat trade. Small meatspecies such as rabbits and poultry are, in many parts of the world, still traded commerciallyin natural packaging, their own skins. This is also the primary packaging choice of mostcommercial and recreational hunters.

At retail, the grease-proof and wrapping paper package has given way to the over-wrapped polystyrene tray typical of self-service merchandising of meat. The very obviousdifference between full-service and self-service packaging is not simply a matter of moder-nity but is a reflection of different functional requirements of the two retailing systems.This chapter will consider the functional requirements and principles of packaging in rela-tion to its various and changing roles in the protection, preservation, and presentation ofmeat. Those interested in the chemistry and functional characteristics of individual poly-mers and polymer combinations used in proprietary packaging films should consult manu-facturers or specialist texts such as Brown (1992) and Brandrup et al. (1998).

Packaging is not simply the materials immediately surrounding a product but is thesynthesis of product, processing, labor, and machines, for addressing specific functionaland/or marketing requirements. These functions relate to all aspects of distribution, storage,

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and merchandising: containment, protection, preservation, apportionment, unitization, con-venience, and communication.

With meat and meat products, packaging should provide users—whether intermedi-ate traders, processors, or the end consumers—with appropriately portioned product in asafe and wholesome condition. Meat packaging has to function in a physical environmentthat threatens product damage and loss of pack integrity; a climatic environment present-ing temperature, moisture, light, oxygen, and microbiological challenges; and a complexhuman environment that includes functional, psychological, and legislative elements.

The functional requirements of meat packaging systems are dictated by the requiredmarketing performance. Obviously the packaging requirements for international trade inchilled meat differ from those for domestic supply. The overriding performance require-ment is, however, the same in both cases: adequate storage life ensuring product resilienceto meet customer expectations. With chilled products, the onset of microbial spoilage, con-sidered fully in Chapter 10, is the usual determinant of the end of practical storage life.Hence, storage life of chilled meat and microbial growth limitation are inextricably linked.Microbial spoilage can be easily prevented if meat is frozen to temperatures too low for mi-crobial growth to occur. However, even frozen meat is subject to deterioration through des-iccation and chemical changes leading to changes in color and palatability. It is these qual-ity changes that packaging of frozen product must minimize or prevent.

Successful meat packaging and storage denies, or limits, the opportunity for micro-bial growth. Consequently, it is appropriate to preface this discussion of packaging with abrief overview of meat as a substrate for microbial growth.

II. MEAT AS A MICROBIAL SUBSTRATE

Fresh meat is an oxygen-sensitive chemical entity that provides an excellent substrate formicrobial growth, allowing contaminating bacteria to proliferate rapidly while conditions,particularly temperature and the gaseous environment, remain favorable. Such microbialgrowth will eventually cause spoilage and can also pose a hazard to health. In normalhealthy slaughter animals, the tissues destined for the table are generally sterile. However,during slaughter and dressing, microorganisms contaminate the surface of carcasses andmeat cuts. Microbial growth is limited to surface sites until spoilage is well advanced.

A. Substrate Composition

All meat spoilage bacteria utilize low-molecular-weight, soluble components of muscle tis-sue for their growth, particularly glucose and amino acids. Generally, glucose is the pre-ferred substrate and as long as it is available at the meat surface other substrates are not sig-nificantly degraded. When bacterial numbers at the meat surface are high enough that theorganisms consume glucose more rapidly than it can diffuse from within the tissue, the or-ganisms begin to attack amino acids, releasing large amounts of ammonia and smalleramounts of malodorous organic sulfides and amines. Consequently, the glucose content ofmeat is a critical factor determining the relationship between spoilage flora developmentand the time to spoilage onset.

The glucose concentration in normal pH meat varies between 100 and 1000 �g�g�1,being low in beef, intermediate in mutton, and high in pork (Newton and Gill, 1978). Offodors begin to develop when bacterial numbers reach about 108 cm�2 where the initialmuscle glucose concentration is about 100 �g�g�1. When the initial muscle glucose con-

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centration approaches 1000 �g�g�1, odor development and slime formation may occur si-multaneously when bacterial numbers reach 109 cm�2 (Gill, 1976). By contrast, high ulti-mate pH meat, also known as dark firm dry (DFD) meat, is low in glucose, and spoilagemay occur when bacterial numbers approach 106 cm�2 (Newton and Gill, 1978).

B. Substrate pH

Growth of the major aerobic and anaerobic spoilage organisms, Pseudomonas spp. and lac-tic acid bacteria, respectively, is insensitive to pH within the fresh meat range (5.3 to 7.0).Meat pH has little effect on the onset of aerobic spoilage but is important under anaerobicconditions because it affects the composition of the developing microflora.

C. Water Activity

For most meat spoilage microorganisms, water activity (aw) values above 0.98 are optimalfor growth. As fresh meat has an aw above this value, inhibition of microbial growth by aw

reduction needs to be engineered either by drying and/or—in cured products—by the addi-tion of solutes such as salt or sugar. The spoilage bacteria of chilled meats are generallyvery susceptible to lowering of the aw at product surfaces. However, when product is pack-aged to prevent moisture loss, surface drying can play no part in controlling microbialspoilage.

D. Initial Contaminating Microflora

The major sources of microbial contamination are the slaughter animals themselves, theprocess workers, and the processing environment (Empey and Scott, 1939). Most microor-ganisms present in the initial contaminating microflora are unable to grow at chill temper-atures. (These are termed mesophilic; cold-tolerant microorganisms are termed psychro-tolerant or psychrophilic.) However, a few psychrotolerant organisms will also be presentand will grow at chill temperatures, causing spoilage. In temperate regions, the proportionof psychrotolerant bacteria present in the initial contaminating microflora ranges from ap-proximately 1% in summer to 10% in winter (Newton et al., 1978).

E. Microbial Spoilage

It is remarkable that so many societies independently came up with the same solutions tothe problem of microbial spoilage of fresh meat. These classical preservation methods in-clude drying, smoking, salting, curing, pickling, fermenting and, in colder latitudes, icingor freezing. These processes all make meat a less desirable substrate for microbes by eitherreducing its aw, changing its acidity, adding toxic substances, or lowering its temperature.However, these processes, excepting cooling, produce products with sensory characteris-tics that differ markedly from those of fresh meat. The secret to meat preservation withoutsensory compromise lies primarily in the control of storage temperature.

F. Temperature

Microbial spoilage of meat can be prevented if meat is frozen to temperatures too low formicrobial growth. Without resort to freezing, the effective product life of fresh meat can begreatly extended through chilling. Chilled storage of meat at between �2 and 5°C preventsthe growth of mesophilic pathogens and delays the onset of spoilage. Spoilage microflora

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developing on chilled meat are composed of psychrotolerant bacteria carried on the hidesof slaughter animals and contaminating carcasses during dressing.

Microbial spoilage is influenced by the size and composition of the initial contami-nating microflora. The rate at which the psychrotolerant component of that microflora pro-liferates determines the time to spoilage onset.

At chill temperatures, meat spoils most rapidly through microfloras dominated byPseudomonas spp. (Ayres, 1960). These microorganisms are described as having a highspoilage potential because they produce offensive metabolic by-products as they grow onmeat.

G. Gaseous Environment

The gases that have the most marked influence on meat spoilage microflora developmentare oxygen, carbon dioxide, and nitrogen.

Oxygen is the universal electron acceptor necessary for the growth of strict aerobes.Consequently, establishment of an oxygen-free environment will prevent the growth of themeat spoilage pseudomonads.

Facultative anaerobes such as Shewanella putrefaciens also use oxygen as an elec-tron acceptor, but in its absence are capable of fermentative metabolism. When the meat pHis greater than 6.0, anaerobic growth of Shewanella putrefaciens is accompanied by copi-ous H2S production, resulting in a green discoloration due to sulfmyoglobin. Brochothrixthermosphacta will accelerate spoilage of anoxic meat above pH 5.8, as will some psy-chrotolerant enterobacteria (Grau, 1980, 1981).

Carbon dioxide inhibits the growth of many microorganisms, but because all organ-isms are not equally sensitive (Enfors et al., 1979), increased concentrations of this gaswithin a package change the microflora composition. Pseudomonas spp. are particularlysensitive, whereas lactic acid bacteria are largely unaffected (Coyne, 1933). By increasingthe carbon dioxide concentration in a packaging atmosphere, the microflora is shifted fromhigh to low spoilage potential as lactic acid bacteria progressively displace the pseu-domonads. Under aerobic conditions, there is an initial large decrease in pseudomonadgrowth rate as the carbon dioxide concentration is increased to 20% but with little addi-tional inhibition at higher concentrations (Gill and Tan, 1980). In anaerobic systems, bycontrast, the inhibitory effect on facultative anaerobes increases progressively, with themaximum effect being achieved with a pure carbon dioxide atmosphere (Gill and Penney,1988). Perhaps most important of all, from a preservative perspective, is that at any con-centration of carbon dioxide, its inhibitory effect increases as storage temperature de-creases (Adams and Huffman, 1972).

Nitrogen and other inert gases do not in themselves inhibit bacterial growth. How-ever, by displacing the oxygen fraction in air they influence microbial growth by loweringthe in-pack oxygen concentration. Nitrogen is also often included in pack atmospheres toprevent pack collapse as carbon dioxide used in meat packaging is absorbed by the pack-aged product.

III. FUNCTIONAL REQUIREMENTS OF MEAT PACKAGING

A. Containment

This basic function of packaging is so obvious that it is often overlooked. With the possi-ble exception of carcasses, meat and meat products must be contained before they can be



moved from one place to another. The containment function of meat packaging extendsfrom the packing plant to the consumer’s refrigerator. Imagine the response, at retail, ofcustomers offered a pile of pork chops but no containers in which to take their selection tocheckout.

B. Protection

Packaging isolates its contents from environmental effects such as dust, microorganisms,water, chemicals, gases, odors, shocks, and compressive forces. Conversely, packagingalso protects the environment from its contents. Consider the pile of pork chops at the meatcounter. Although paper bags would satisfy the containment function, plastic bags wouldafford both containment and protection.

C. Preservation

For meat and most other perishable products, the major cause of deterioration ismicrobial spoilage (discussed above and in Chapter 10). Other causes of product deteri-oration include moisture loss (desiccation), color change, and oxidative rancidity. Toaccomplish the preservative function, packaging must restrict microbial growth,prevent moisture loss, and control gaseous exchange between the package and ambientatmospheres.

Loss of water from fresh meat reduces the weight of meat available for sale andin extreme cases renders the product unsaleable because of its unsightly appearance.However, enclosing meat within a water-impermeable film, while preventingmoisture loss, will accelerate the onset of microbial growth because water activityremains high.

The color of raw meats is determined by the oxidation state of the muscle pigment,myoglobin (Chapter 5). When no oxygen is present, myoglobin is in its oxygen-free form(deoxymyoglobin), which gives meat in anoxic packs its characteristic purple-red color.Oxygenated myoglobin (oxymyoglobin) is bright red, a color consumers associate withfreshness. The deoxymyoglobin/oxymyoglobin reaction is reversible and occurs in re-sponse to changes in the partial pressure of oxygen surrounding the meat.

Oxidized myoglobin (metmyoglobin) is brown, a color consumers associate withstaleness and the loss of nutritional and sensory quality. Metmyoglobin forms most rapidlyat low oxygen concentrations, around 0.5%. The relationship between oxygen concentra-tion and the chemical state of myoglobin (Forrest et al., 1975) shows that metmyoglobinformation is minimal in atmospheres containing less than 0.1% or more than 15% oxygen.Therefore, either a highly aerobic or a completely anoxic environment is required to pre-vent browning. However, as the consumer associates a bright red meat color with freshness,the packaging requirements for retail display and extended storage are unlikely to be thesame.

The reaction of fats with oxygen is responsible for the development of rancidodors and flavors. Oxidation takes place with atmospheric oxygen and is often acceler-ated by heat, light, high energy radiation, and various oxidation catalysts. In meats,the development of rancidity results primarily from a free radical chain reaction.This means that an anoxic packaging environment may slow but will not prevent the de-velopment of rancidity if product is packaged after the initial oxygen–fat reaction hasoccurred.

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D. Apportionment

This function of packaging is to reduce industrial output (i.e., dressed carcasses) to an ap-propriate size for further processing or consumer use.

E. Unitization

Unitization describes the function by which primary packages are consolidated for ship-ment. Primary packages are unitized into secondary packages, for example by placementinside a cardboard carton. The secondary packages in turn are unitized into a tertiary pack-age; for example, a stretch wrapped pallet that may, in turn, form part of a quaternary pack-age—a shipping container or truck load. Unitization allows optimization of materials han-dling by minimizing the number of discrete packages that need to be handled. On delivery,the process is reversed from distributor to consumer so the latter is presented with a primarypackage and (fortunately) not a container load.

F. Convenience

In modern industrialized societies the time spent by the consumer in obtaining and prepar-ing food continues to fall. Although meats are not in the forefront of the convenience mar-ket, microwaveable packs and meat-based whole meals are appearing on supermarketshelves.

G. Communication

There is an old packaging adage that says “a package must protect what it sells and sellwhat it protects.” In the case of meat, “sell what it protects” requires that product be pre-sented in an attractive manner and also that the product be clearly recognizable. The lattermeans branding or other distinctive labeling that will identify the product to the consumer.Currently this is a grossly underdeveloped aspect of meat retailing.

IV. THE PACKAGING ENVIRONMENTS

A. Physical Environment

The physical environment is the environment in which physical damage can be caused di-rectly to the product or indirectly to the product through damage to the package. Such dam-age includes shocks from drops, falls or dumps, vibration, compression, and crushing. Lossof pack integrity—for example, puncture of a vacuum pack—will compromise the preser-vative function of that package.

B. Climatic Environment

The climatic environment can cause meat deterioration as a result of contact with gases,(especially oxygen), water or water vapor, exposure to light (particularly ultraviolet wave-lengths), and the effects of heat and cold. Packaging acts as a barrier separating the ambi-ent environment from the package’s internal environment.

The water and gas barrier properties of the packaging material define an internal en-vironment that may differ markedly from the ambient external atmosphere. Consequently,it is essential that barrier and other functional properties do not change over the expectedstorage period and range of external and internal environmental conditions.



Temperature is probably the most important climatic factor because of its influenceon the functional properties of packaging materials. Most meat primary packagings haveno effect on the temperature of the packaged product, which tends toward that of the ex-ternal environment. However, secondary and tertiary packaging can have a marked effecton the rate of initial product cooling and on product temperature changes during storage anddistribution. Among other parameters, temperature changes influence oxygen permeabil-ity, microbial growth and color deterioration rates.

The permeability of packaging films to oxygen decreases with temperature. How-ever, permeability is particularly affected around the freezing point of water. At sub-zerostorage temperatures, the difference in the permeability of different films is dramaticallyreduced (see Fig. 1), perhaps arising from the freezing of water associated with the film(Lambden et al., 1985). In practical terms, oxygen transmission rates at subzero tempera-tures cannot be accurately predicted from data obtained at higher temperatures (Table 7).A small decrease in temperature below 0°C can alter both the absolute and relative perme-abilities of packaging films.

At chill temperatures, the rate of growth of psychrotolerant bacteria is inversely re-lated to temperature. Consequently, the optimum storage temperature for chilled meat in re-spect to length of storage life is the lowest temperature that can be maintained withoutfreezing. Within the chill temperature range (�2.0 to �5.0°C), the storage life of packagedmeat reduces by approximately 10% for every 1°C that the product temperature exceeds the�1.5°C optimum.

Deterioration of meat color is extremely temperature sensitive, with the rate doublingfor every 5°C rise in temperature (Greer and Jeremiah, 1981). The effect of temperature onthese oxygen-mediated changes (see Chapter 5) is critically important in successful retail-ing of fresh meat. Exposure of meat, in particular cured meat, to light can increase the rateof color deterioration. Cured meat products tend to fade when exposed to light.

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Figure 1 Influence of temperature on the oxygen transmission rates of a nylon and a vinylidene-based laminated film used for vacuum-packaging of meat. (Data from Lambden et al., 1985.)


C. Human Environment

This is the environment in which the package interacts with people, including regulatorybureaucrats and the consuming public. The most easily understood interactions are thoseassociated with regulatory requirements, such as the use of approved packaging materialsand appropriate labeling. Provided these and functional requirements are adequately satis-fied, the interaction with the consuming public enters the realm of psychology and includesexperience, perception, recognition and preference.

V. PRODUCT PACKAGING ENVIRONMENT INTERACTIONS

In the preceding sections, an attempt has been made to separate product properties, pack-aging functions, and environmental factors. Such a separation is arbitrary as these elementsinteract to produce the conditions experienced by the packaged product. In some casesthose conditions differ little from those found in the external environment (e.g., meatwrapped in greaseproof paper) whereas in others the difference is profound (e.g., meat heldin a modified atmosphere pack). The more important factors contributing to the synthesisof the in-pack environment are presented in Table 1. The term intrinsic refers to variablesassociated with the product at the time of packaging. Extrinsic, on the other hand, describesenvironmental and packaging-associated variables. The interaction of the intrinsic and ex-trinsic factors determines the in-pack conditions presented to the packaged product by anygiven packaging system. The in-pack conditions determine product performance charac-teristics. Selection of a packaging system requires matching of performance capabilitieswith specific functional and/or marketing requirements.

VI. PACKAGING METHODS

Both chilled and frozen raw meats are generally protected by flexible plastic packaging.However, consumer packs can take a variety of forms which for convenience can begrouped into rigid (glass jars, cans, etc.), semi-rigid (plastic trays, boxes, etc.) and flexible


Table 1 Intrinsic and Extrinsic Factors That Determine In-Pack Conditions and ConsequentProduct Performance

Extrinsic

Intrinsic factors Environmental factors Packaging factors In-pack factors

Temperature Temperature Insulation TemperatureAir Gas barrier

Pack atmosphereGaseousenvironment

Water activity Relative humidity (RH) Vapor barrier aw and RH(aw) Light Opacity Light

Composition

SubstrateAcidity (pH)Redox PotentialPreservativesInitial microflora Contaminants Protection Microflora

Product performance


(plastic bags, pouches, etc.) packagings. The characteristics of the most common types ofpackagings used to contain, protect, and preserve raw meats are detailed below.

A. Nonpreservative Packaging

This type of packaging contains and protects the product from contamination and water losswithout creating in-pack conditions very different from ambient. Consequently, unless mi-crobial growth is prevented by freezing or retarded by chilling, product in such packs ishighly perishable—i.e., has a very short product-life.

Wrappers are the simplest type of flexible package, in which a sheet material is usedto enclose a quantity of product. Examples of this type of packaging relevant to meat in-clude greaseproof paper and plastic cling films. Such packages are generally not sealed byplastic welding.

Overwrapped trays are widely used in supermarkets for fresh meat and poultry. Freshmeats are placed on a semi-rigid plastic tray that is overwrapped with a plastic cling filmof high oxygen permeability. As with wrappers, overwrapped trays are not sealed and be-cause of the high oxygen permeability of the films used, provide aerobic conditions aroundthe product.

Loose-fitting plastic bags and pouches are not generally used at retail because ofunattractive product presentation. However, such packagings are widely used as primarypackaging for fresh carcasses and for frozen bulk or individually wrapped cuts and offals.Bags and pouches can be heat sealed, in which case the degree of product protection is de-termined by the permeability of the packaging material to water vapor and gases.

B. Preservative Packaging

This group of packagings is characterized by an ability to extend product life by modifyingor restricting microbial growth. This is achieved by creating and maintaining in-pack con-ditions that differ markedly from those of the ambient environment.

In vacuum packaging product is placed into a bag or pouch that is evacuated and heatsealed. The packaging material must have a low permeability to oxygen so that the anoxicin-pack environment is maintained. Vacuum packaging is widely used as a primary trans-port and storage packaging for larger (primal) cuts. To date, its use at retail has been lim-ited because of the unattractive presentation of product: a purple-red color due to de-oxymyoglobin, squashed appearance, and drip accumulation. However, vacuum packagingis widely used for sliced processed meats, for which the color is produced by the nitrite cureand is protected by anoxia. Moreover, the rigidity and superior water binding of processedmeats mitigate deformation and drip problems.

In modified-atmosphere packaging (MAP), the gaseous environment around theproduct is modified before heat sealing, and then gradually changes as a result of the inter-action between product and packaging. With meats, the in-pack gaseous environment isusually altered by evacuation followed by gas flushing with the desired gas mixture. Sub-sequent changes in the composition of the in-pack atmosphere are determined by the gasbarrier properties of the packaging and the metabolic activities of the product and its mi-croflora. Respiratory activity of meat contributes markedly to changing the in-pack atmo-sphere if meat is packed before rigor attainment.

As with MAP, in controlled-atmosphere packaging (CAP) the gaseous environmentaround the product is altered but is then maintained at a specified composition regardlessof product or microbial respiration, temperature, or other environmental changes. The prin-

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cipal difference with fresh meat between MAP and CAP is in the gas permeability of thepackaging. As plastic materials are not absolutely impermeable to gases, the compositionof the in-pack atmosphere will change, although very slowly. With CAP, gas-impermeablepackagings such as plastic aluminum foil laminates or metallized films have to be used.

For most contemporary packaging systems, the term “controlled atmosphere” wouldbe a misnomer because it is not possible to control the atmosphere within a pack once it issealed. However, with the development of active packaging systems, the distinction be-tween MAP and CAP is becoming less clear.

As well as acting as a barrier between the in-pack and external atmospheres, activepackaging modifies the in-pack atmosphere. With meat, active packagings include oxygenscavengers and carbon dioxide generators. Active packaging systems may also includeoxygen indicators and time-temperature indicators.

C. Two-Phase Packaging

Two-phase or double-phase primary packs combine extended product life and a retail dis-play potential. The general principle is to change the gaseous environment surrounding themeat between the storage and display phases. This can be achieved either by gaseous ex-change or by removal of part of the package to allow replacement of the carbon dioxidepreservative atmosphere with air. This process will result in the meat color changing frompurple-red (deoxymyoglobin) to the more desirable bright red (oxymyglobin) color for re-tail display.

VII. PACKAGING FOR CHILLED STORAGE

Packaging systems that allow national and international trade in chilled meat must, whilemaintaining sensory quality, prevent the growth of high spoilage potential strict aerobes,and of high spoilage potential facultative and obligate anaerobes. Simultaneously, the pack-aging must retard the growth of low spoilage potential anaerobes. The growth characteris-tics of the major spoilage microorganisms of chilled meat are presented in Table 2.


Table 2 Growth Characteristics of the Major Groups of Chilled Meat Spoilage Microorganisms

CO2 SpoilageSpoilage organisms Oxygen requirement pH requirement sensitivity potential

Pseudomonas Strict aerobe None Higha HighAcinetobacter/Moraxella Strict aerobe None Higha LowEnterobacteriaceae Facultative anaerobe No anaerobic Moderateb High

growth belowpH 5.8

Brochothrix Facultative anaerobe No anaerobic Moderateb Highthermosphacta growth below

pH 5.8Shewanella putrefaciens Facultative anaerobe No growth below Moderate Very high

pH 6.0Lactic acid bacteria Aerotolerent anaerobe None Lowb LowPsychrotolerant Clostridia Strict anaerobe None Lowb High

a Aerobic environment.b Aerobic or anaerobic environment.


The role of psychrotolerant clostridia as agents of chilled meat spoilage was recog-nized when a causal relationship was established between psychrotolerant clostridia and“blown pack” spoilage (Dainty et al., 1989; Kalchayanand et al, 1989). Blown packspoilage is characterized by gas production in non–temperature-abused vacuum packs,leading to gross pack distension during storage Drip in blown packs usually contains largenumbers of psychrotolerant clostridia (Broda et al., 1996). By contrast, blown packspoilage in temperature-abused vacuum packs is usually associated with the growth of psy-chrotolerant enterobacteria (Hanna et al., 1979).

The extension of the storage life of chilled meat achieved by removing oxygen and/orincreasing carbon dioxide concentration was recognized in the 1930s (Killifer, 1930).However, for almost thirty years there was little commercial interest in modified-atmo-sphere preservation of meat. In the 1960s, following developments in the plastics industry,vacuum packaging became a practical, preservative packaging technology for extendingthe storage life of chilled meat.

A. Vacuum Packaging

In vacuum packs, the preservative effect is achieved by maintaining an oxygen-deficientenvironment around the product. Therefore, when considering the product-life extensionachieved by this well-established packaging technology, parameters that have an impact onthe establishment, and maintenance, of that oxygen-deficient environment are important.

As the vacuum is drawn, flexible packaging collapses around the product, squeezingmost of the air from the pack. Any residual air in the pack is largely trapped in film foldsthat restrict its contact with the meat. The vacuum applied is not critical in successful vac-uum packaging because atmospheric pressure collapses the film tightly to the meat surface.The application of a vacuum to 0.8 to 0.9 atmospheres is generally sufficient to produce sat-isfactory vacuum packs. Close contact between the product and the packaging film is en-hanced by heat shrinking after pack sealing. Packaged product is passed through a shrinktunnel that subjects the films to a temperature of about 90°C for 2 to 3 seconds. However,if flexible packaging cannot conform closely to the product surface, or if the product con-tains voids such as the body cavity of a lamb carcass, residual air remains within the pack,resulting in an “evacuated” rather than a vacuum pack. Evacuated packs are less effectivethan vacuum packs in extending chilled meat storage life.

The development of vacuum skin packaging has improved oxygen removal by ef-fectively eliminating the void volume around, but not within, irregularly shaped products.In this thermoforming variant of chamber vacuum packaging, the “skin” is formed bydrawing a high vacuum on the inner and outer sides of the film, and subsequently vent-ing the upper side to atmosphere, forcing the film tightly over the product. The upper filmmaterial is softened by heating, so when it is applied under vacuum to the lower film (theplastic tray), the soft film molds itself to the shape of the meat to produce a skin-tightpackage.

Unfortunately, all transparent plastic barrier films in commercial use today havelow, but measurable, permeabilities to both oxygen and carbon dioxide. Therefore, theconcentration of oxygen within a vacuum pack is determined by the equilibrium estab-lished between the rate of oxygen diffusion through the film and its utilization by therespiring meat and developing spoilage microflora. Measurements of oxygen in vacuumpacks have usually yielded a figure of about 1%, a value more than adequate to allowrapid growth of aerobic spoilage bacteria. However, such high measured oxygen concen-

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trations are sampling-induced artefacts, because they are inconsistent with observed mi-crobial growth and meat color.

Product life of vacuum-packed meat at a given storage temperature is inversely re-lated to the oxygen permeability of the barrier film used (Table 3). The influence of differ-ences in barrier film permeability on product life is markedly reduced at subzero storagetemperatures (Fig. 1). Therefore, a slight rise in storage temperature from below 0°C toabove 0°C will have an effect on product storage life that is disproportionate to the actualtemperature rise.

Any residual oxygen trapped in vacuum packages at sealing will be converted into car-bon dioxide by the respiratory activity of fresh meat. Within a day or two of packaging, theminuscule residual in-pack atmosphere will be predominantly nitrogen but may contain upto 20% carbon dioxide. This accumulation of carbon dioxide inside a vacuum pack as a re-sult of respiration of product contributes to the preservative effect achieved in that pack(Table 4). The reduced preservative performance of polyvinylidene chloride (PVDC) packscompared to foil laminate packs reflects the poorer gas barrier properties of the former pack-aging film in respect to both oxygen ingress and carbon dioxide egress. Plastic films are ap-proximately four times more permeable to carbon dioxide than oxygen. Comparison of foil


Table 3 Relationship Between Film OxygenPermeability of Vacuum Packs and the Storage Life ofNormal-pH Beef at 0°C

Film permeability Storage life(ml.m�2.atm�1.day�1 at 25°C) (days)

0 105190 105290 77–105532 42–63818 24–42920 14–28

Source: Data from Newton and Rigg (1979).

Table 4 Influence of Packaging Film, Meat pH, and Carbon Dioxide Accumulation on SpoilageFlora Development (Aerobic Plate Count, 25°C) on Vacuum-Packed Beef during Storage at 1°C

Storage timeSpoilage microflora (log10 count.cm�2)

(weeks) PVDC Foil laminate Foil laminate � CO2 absorber

Normal pH (5.5–5.7)7 6.59 6.32 6.45

10 7.46 6.53 6.6612 7.36S 6.96 7.38

High pH � 6.07 7.30P 7.11P 6.74P

10 N.D. 8.30P 8.43P

N.D. � Not determined; S � sour odor; P � putrid odor.Source: Adapted from Gill and Penney, 1988.


laminate with foil laminate plus carbon dioxide absorber shows the inhibitory effect of car-bon dioxide accumulation. Vacuum packaging can be regarded as a form of MAP with ele-vated carbon dioxide. However, that gas, because of its high solubility in meat and fat, is notevident within the packs, unlike the situation with saturated carbon dioxide packaging.

B. Saturated Carbon Dioxide Atmosphere Packaging

Saturated carbon dioxide controlled atmosphere packaging (Gill, 1989) for the prolongedchilled storage of red meats may be more correctly described as a modified-atmospherepackaging system. Although the use of gas-impermeable films will prevent gaseous ex-change between the ambient and pack atmospheres, no control is extended over the latter inrespect to its composition or volume after pack sealing. Microbial growth control is achievedby a combination of an oxygen-deficient environment and the antimicrobial properties of ahigh partial pressure of carbon dioxide. A high (100%) carbon dioxide packaging systemwas once believed to be impractical because in packages where air has been largely replacedwith carbon dioxide, red meats discolored rapidly (Ledward, 1970). This discoloration wasin fact oxygen mediated, the result of metmyoglobin formation (Chapter 5).

The relationship between oxygen concentration and the chemical state of myoblobinindicates that the formation of brown metmyoglobin would be minimal in atmospherescontaining less than 0.1% oxygen. Achieving oxygen concentrations at packaging as lowas 0.1% lay beyond the capability of most of the packaging machines available in the early1980s.

The essential elements of generic saturated carbon dioxide packaging systems are asfollow: a packaging film that is totally impermeable to gases; a gas replacement system pro-ducing a pack atmosphere containing less than 0.1% oxygen; and a pack content of carbondioxide sufficient to saturate the product at the anticipated storage temperature. How theserequirements are met is now discussed.

At present the requirement for a total gas barrier can be fulfilled only by plastic lam-inates containing a layer of aluminum foil (foil laminates) or by double metallized films.Both these barrier films have the silver appearance of aluminum metal, making observationof product within a sealed pack impossible. In the future, new “glass” films that incorpo-rate a layer of silica may allow product visibility without compromising gas barrierrequirements.

Currently several atmosphere-replacement packaging machines capable of achievingpack atmosphere containing less than 0.1% oxygen are available commercially. The suck-and-blow action of so-called snorkel-type atmosphere replacement machines imposes flex-ing stresses that can compromise the gas impermeability properties of foil laminate andmetallized films. Also, consistent results have been found to be difficult to achieve usingsnorkel-type machines. These performance problems are particularly severe when packag-ing irregular shaped product (e.g., a lamb carcass) or when multiple units are aggregated ina single pack. Chamber machines are preferable not only because they do not stress thepackaging film but also because they produce consistent pack atmospheres. The residualoxygen concentration at packaging achievable with state-of-the-art chamber packagingmachines approaches that of the claimed oxygen-free carbon dioxide supply. As the oxy-gen concentration cannot be less than that in the oxygen-free carbon dioxide, the quality ofthe gas is critical in saturated carbon dioxide packaging.

Oxygen absorbers can be used to compensate for poor oxygen removal or oxygen en-try occurring when packaging films with inferior barrier properties are used. Oxygen ab-

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sorbers active in high carbon dioxide atmospheres are now available. Their use with clearplastic high barrier films (i.e., an active packaging system) is proving comparable to the useof gas-impermeable aluminum foil laminates or double metallized films. It must be em-phasized that the practical acceptability of oxygen absorber systems and the use of modernclear plastic ultra high barrier films in place of metallized films will be determined by theirability to satisfy marketing requirements.

Carbon dioxide is very soluble in both muscle and fat tissue (Gill, 1988). The in-hibitory effects of carbon dioxide on bacterial growth increase with the equilibrium partialpressure of that gas (Gill and Penney, 1988). If the amount of carbon dioxide added to apack is insufficient to saturate the meat, then absorption will continue until essentially allis absorbed. In this case, the equilibrium partial pressure of carbon dioxide within the packwill be less than atmospheric. Approximately 1.3 to 1.5 liters of gas per kilogram of meatare required for saturation. Failure to introduce sufficient carbon dioxide will result in re-duced inhibition of bacterial growth (Fig. 2). This figure also shows that for a given carbondioxide volume to meat weight ratio, growth inhibition is less effective in high-pH than in


Figure 2 Effect of initial carbon dioxide volume to meat weight ratio, meat pH, and storage time(weeks) on spoilage microflora development (aerobic plate count, 25°C) in beef stored at 1°C in foillaminate packs. (Data from Gill and Penney, 1988.)


normal-pH meat. The pH dependency of inhibition efficacy by carbon dioxide occurs withmeat-borne pathogens as well as spoilage organisms.

The technical requirement that the meat be saturated with carbon dioxide can creatematerial handling problems, as the packs are initially distended by the carbon dioxide theycontain. Subsequent absorption of the gas by the meat can produce change in a pack vol-ume approaching 50%. At chiller temperatures, gas absorption is sufficiently complete af-ter 24 hours to allow packs to be cartoned.

As with vacuum packaging, the product storage life achieved by saturated carbondioxide packaging is similarly inversely related to the storage temperature (Adams andHuffman, 1972). Further, the storage-life differential achieved by saturated carbon dioxideover vacuum packaging decreases to become insignificant at temperature above 12°C to15°C (Gill and De Lacy, 1991). With any chill packaging, the importance of storage tem-perature as the principal determinant of effective product life cannot be overemphasized.

VIII. PRODUCT-LIFE EXPECTATIONS FOR CHILLEDPRESERVATIVELY PACKAGED FRESH MEATS

The chilled product life achieved using preservative packaging, be it vacuum, modified, orcontrolled atmosphere, is determined by the interaction of product, packaging, and storageparameters. Moreover, a product-life estimate varies with the criteria used for assessment.In the early development of saturated carbon dioxide packaging, the principal criterion forproduct-life assessment was product appearance and the absence of off-odor (microbialspoilage) at pack opening. However, meat that is visually and microbiologically accept-able, but with a retail display life of less than a day and a liver flavor, is not marketable.Consequently, extreme caution must be exercised when using research results to set com-mercial product-life specifications.

Consumers demand product that consistently meets high standards of eating qualityand appearance during retail display. If a displayed product does not look good, the cus-tomer will look elsewhere. Current specifications often demand 7 days product life for re-tail distribution, display, and home storage from preservative pack opening. Consequently,buyers are now imposing retail specifications that limit the acceptability of chilled productto well below the longevity claimed for state-of-the-art packaging systems. In the marketplace, preservative packaging systems are being judged not on their ultimate microbiolog-ical product life, but on their capability to preserve quality attributes pertinent to specificclients, and thereby satisfy marketing requirements.

The extension of chilled storage life afforded by the use of saturated carbon dioxidepackaging over that achieved by vacuum packaging is highly temperature dependent. Thestorage life of meat at �1.5°C in a saturated carbon dioxide atmosphere is doubled com-pared with equivalent meat in vacuum packaging, but above 10°C there is little improve-ment in storage life.

In commercial practice with a cold chain nominally operating between �1.0°C to0°C, the increase in storage life over simple vacuum pack is around 50%. Comparable stor-age-life expectations for beef, lamb, pork, and chicken are given in Table 5. Also given inTable 5 are the retail display-life expectations for the four meat species.

A. Product Safety

As product safety is the subject of Chapter 17, discussion will be limited to changes in prod-uct safety effected by meat storage and packaging regimens. The majority of meat-borne

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pathogens are mesophilic (e.g., Salmonella, E. coli O157:H7) and require temperaturesabove about 7°C for growth. Therefore, the health hazard they pose is not increased duringchilled or frozen storage. Psychrotolerant pathogens, on the other hand, may be capable ofgrowth during chilled storage. Health concerns have been raised, particularly for preserva-tive packagings that suppress the growth of competing spoilage microorganisms. In thiscase, it is feared that psychrotolerant pathogens could reach high numbers in product thatneither looks, smells, nor tastes spoiled.

The four psychrotolerant pathogens of concern are Aeromonas hydrophila, non-pro-teolytic strains of Clostridium botulinum, Listeria monocytogenes, and Yersinia enteroco-litica, whose growth characteristics are summarized in Table 6. Adequate refrigeration willcontrol the growth of all four pathogens; however, other variables such as substrate pH andgaseous environment markedly influence the minimum temperature at which they cangrow. Generally, the minimum temperature for growth increases as carbon dioxide con-centration and substrate acidity increases (pH falls).


Table 5 Expected Storage Life for Hygienically Produced Beef, Lamb, Pork and Chicken Heldat �1°C � 0.5°C and Displayed at 4°C; Storage Life is Defined by the Product Attribute That FirstFails to Satisfy Market Needs (Microbiological, Eating Quality, or Retail Display Requirements)

Assured product-life (days)

Packaging system Beef Lamb Pork Chicken

Transport /storage packagingVacuum 84 60 28 25Carbon dioxide 126 90 63 70

Display packagingCling film overwrap (fresh meat) 3–5 3–5 3–5 2–4

(stored meat) 1–3 1–3 1–3 1–2High oxygen MAP (fresh meat) 7–10 7–10 7–10 4–8

(stored meat) 2–6 2–6 2–6 2–4

Table 6 Growth Characteristics on Chilled Meat of Psychrotolerant Meatborne Pathogens

MinimumPsychrotolerant CO2 growthpathogen Oxygen requirement pH requirement sensitivity temperature

Aeromonas hydrophila Facultative anaerobe Sensitive to pHbelow 6.0 High 4–0°Ca

Clostridium botulinum Obligate anaerobe No growth belowpH 4.8 Low 3–3°Ca,b

Listeria monocytogenes Facultative anaerobe Sensitive to pHbelow 5.0 Moderate 4–0°Ca

Yersinia enterocolitica Facultative anaerobe No anaerobicgrowth below High 4–0°Ca

pH 5.8

a Strain and substrate dependent.b Toxin production reported at 2°C (Moorhead and Bell, 1999).


IX. PACKAGING FOR RETAIL DISPLAY

The trend to self-service merchandising of fresh meat requires a high standard of productpresentation. As consumers relate the bright red color of oxymyoglobin with freshness, re-tail packaging of meat normally provides an aerobic environment. Under aerobic condi-tions the meat surface is fully oxygenated, with the concentration of that gas decreasingwith depth below the surface (Chapter 5). About 8 mm below the surface, the oxygen con-centration will favor the development of the brown pigment metmyoglobin. This layer ad-vances with time toward the meat surface, bringing about the meat color deterioration se-quence from bright-red through tired/dull red to brown or even green.

A. Overwrapped Trays

An aerobic display requirement is satisfied by placing meat on a semi-rigid plastic tray andoverwrapping with highly oxygen permeable cling film. This system does not restrict thegrowth of aerobic spoilage microorganisms, in particular the high spoilage potential pseu-domonads. Consequently, the effective display life may be determined either by the onsetof microbial spoilage or by color deterioration. Which occurs first depends on the interac-tion of various parameters, including microbial load, time in storage, and display tempera-ture.

Typical display times for overwrapped trays are given in Table 5. The retail perfor-mance of meat after prolonged chilled storage is shorter than that of fresh meat. There aretwo reasons for this: during storage, metmyoglobin reductase activity eventually stops, anda greater proportion of the myoglobin in the oxidized state (metmyoglobin).

Activity of metmyoglobin reductase requires a biochemical reductant. During pro-longed chilled storage, reductant reserves become exhausted and the enzyme is no longerable to reverse the accumulation of metmyoglobin. The result is more rapid browningduring retail display. During prolonged chilled storage, meat in vacuum packs will reactwith the traces of oxygen that permeate the barrier. Progressively during storage, this re-action converts an increasing proportion of the deoxymyoglobin into brown metmyo-globin. On exposure to the air the meat will bloom (i.e., become bright red) but that meatalready contains a significant proportion of metmyoglobin. This being the case, brown-ing will occur more rapidly in stored meat, as less oxymyoglobin has to be transformedto metmyoglobin. Saturated carbon dioxide packaging with its associated oxygen-imper-meable films affords a display life advantage over vacuum packaging in respect tobrowning.

Greening of the original outside surfaces of preservatively packaged primals storedfor long periods, or retail cuts derived from them, may occur during aerobic retail display.This type of discoloration results from the reaction of myoglobin with hydrogen peroxide,the latter generated by high surface lactic acid bacteria populations on exposure to oxygen.This problem, green choleglobin formation, is not usually seen on fresh cut surfaces but isevident on the edges of cut steaks. It can be particular severe with the first and last steakscut from a strip loin, as in both cases one side is an original outside surface carrying a highlactic population.

Another problem is the greening or graying of fat (Bell et al., 1996), arising from dripthat stains fat during prolonged storage. Initially such fat has an acceptable but pinky hue.With time, the staining myoglobin/haemoglobin pigment is oxidized to the brown metforms, which appear green or gray. The color is green or gray and not brown because per-ceived fat color is determined by the combination of light reflected from, and absorbed by,

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the fat surface and immediate subsurface layers. Saturated carbon dioxide packaging doesnot offer a fat color advantage over vacuum packaging (Bell et al., 1996).

B. Modified Atmosphere Packaging

For red meats, high-oxygen MAP systems utilize atmospheres containing approximately20% to 30% carbon dioxide, 60% to 80% oxygen, and up to 20% nitrogen. The elevatedoxygen concentration enhances the bright red meat color and the elevated carbon dioxideconcentration inhibits the growth of aerobic spoilage microorganisms.

High oxygen concentrations in display packs enhance meat color by increasing thethickness of the oxymyoglobin surface layer. At the same time the metmyoglobin layer liesdeeper in the meat. The time taken for that layer to reach the surface is increased, so dis-play life is extended.

Unlike overwrapped trays, high oxygen display packs use a film with high gasbarrier properties, to prevent the gases equilibrating with the ambient atmosphere.The modified atmosphere display packs consist of deep high barrier trays that are gasflushed before an upper high barrier film or lid is sealed in place. However, the pack at-mosphere tends to change during display because the oxygen is lost to respiration andcarbon dioxide is highly soluble in meat. The absorption of carbon dioxide can lead topack collapse. The pack atmosphere remains reasonably stable and the pack shape ismaintained when the ratio of pack volume to meat volume exceeds approximately 3 to 1.The use of high oxygen with high carbon dioxide effectively doubles the color stabilityand time to spoilage over that achieved using ambient atmosphere overwrapped packs(Table 5).

High-oxygen MAP, which provides a chilled product life of only 5 to 10 days, is notsuitable for prolonged storage of meat. Its suitability for display packaging is determinedas much by commercial merchandising strategies as by the preservative capability of thepackaging. The excessive space occupied by deep tray packs, compared to net weight ofmeat sold, tends to restrict MAP packaging to high value products catering to the upper endof the market. As discussed previously, the rate of discoloration is inversely related to tem-perature, so the importance of display cabinet temperature management cannot be overem-phasised.

X. TWO-PHASE PACKAGING

In two (double)-phase packs, the gaseous environment surrounding the meat is changed be-tween storage and display to combine the long storage life of saturated carbon dioxidepackaging with the retail display requirement for oxygen-rich atmospheres. Such systemsare ideal for centralized preparation of product for retail display.

A. Mother Packs

These are the simplest two-phase packaging systems, consisting of retail-ready packs in-side an outer preservative pack. The retail-ready packs may be overwrapped trays or liddedpacks. In both cases, retail films must be highly gas permeable, first, to allow the meat con-tact with the carbon dioxide preservative atmosphere and later, on removal from the motherpack, to allow atmospheric oxygen to bloom the meat. While a simple outer bag could beused to contain the carbon dioxide atmosphere, the inner packs would be free to movewithin the pack. Such movement could damage both inner and outer packs. Consequently,



most proprietary mother pack systems employ a semi-rigid outer container to protect andrestrain the inner packs.

B. Gas-Exchange Packs

In gas-exchange packs, the storage carbon dioxide atmosphere is replaced by one enrichedin oxygen before product is put on display (Ho et al., 1995). The two-phase packs consistof deep-lidded gas-impermeable packs fitted with some means (valve or injection septum)for exchanging gas atmospheres. For storage, the packs are gas flushed with carbon diox-ide and sealed. To prepare the pack for retail display, the carbon dioxide atmosphere is re-placed with a high-oxygen carbon dioxide atmosphere, for example 65% oxygen, 35% car-bon dioxide. The disadvantage of gas-exchange systems is that each retailer must havegassing equipment.

C. Removable Top Web Packs

For display, part or all of the top web (film) is removed from a gas-flushed deep-tray packto allow the high carbon dioxide atmosphere to escape and oxygen (in air) to contact theproduct, which is often secured to the bottom of the deep tray by an oxygen-permeable vac-uum skin (Boghossian et al., 1995). The use of the inner vacuum skin pack not only holdsthe product firmly within the relatively large outer pack but also helps to contain drip.

An active-packaging variant of this system, which reduces the overall pack size, em-ploys an oxygen absorption/carbon dioxide generation system to establish and maintain thecarbon dioxide storage atmosphere. Such systems offer advantages over gas flush systemsas they allow the volume of the preservative atmosphere to be reduced without compro-mising bacterial growth inhibition. As carbon dioxide is absorbed by the meat, more is gen-erated to maintain the saturated state. Furthermore, the consistent attainment of extremelylow residual oxygen concentrations ensures that meat color degradation during chilled stor-age is minimized. At display, the upper gas-impermeable web with the gas-absorption/gen-eration sachet attached is removed, and the meat in the oxygen-permeable inner pack is ableto bloom.

XI. CHILLED MEAT PACKAGING REQUIREMENTS

The gas barrier properties of packaging films used in the various preservative and displaypacks are summarized in Table 7. The information given in Table 7 is for guidance only.Specific customized, or proprietary, systems may achieve performance specifications us-ing films of higher or lower oxygen transmission rates. With storage packagings, higheroxygen transmission rates reduce effective product life (Table 3). For display packagingsthat use atmospheric oxygen to bloom the meat, the use of lower permeability films maycompromise display life by accelerating metmyoglobin formation.

XII. FROZEN MEAT PACKAGING

At commercial frozen meat storage temperatures (less than �12°C), microbial spoilageis completely stopped but meat is still subject to deterioration through desiccation andchemical changes, such as oxidative rancidity and deterioration in color. Apart from pro-tecting frozen product from physical damage and contamination, the major requirement offrozen packaging is control of water loss. Frozen meat, particularly individual table cuts,

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can be sold frozen in their transportation packs, so that packaging must also be effective inpresenting the product to the consumer.

A. Types of Packaging for Frozen Meat

Boneless beef and lower-value cuts of other species destined for further processing are bulkpacked in 27 kg cardboard cartons within low-density polyethylene liner bags. This pack-aging is cheap, provides good moisture barrier properties, is rugged, and still retains filmflexibility at temperatures approaching �40°C. However, the high oxygen permeability ofthe packaging film results in short quality product life because of color deterioration andoxidative rancidity.

Polyethylene bags may also be used for frozen carcasses. Traditionally, lamb car-casses were frozen in stockinet bags. Demands by regulatory authorities for better hygienicprotection saw stockinet bags replaced by polyethylene bags. This created product handlingproblems, because cold polyethylene is slippery, making handling and stacking difficult, or


Table 7 Oxygen Permeability of Packaging Film Required for Storage and Display of ChilledMeats

Packaging Oxygen Oxygen transmissiontype permeability (ml.m�2.atm�1.day�1 at 23°C)a Comments

StorageVacuum Low 40 to �15 The lower the

transmission the betterto retard metmyoglobinformation

Carbon dioxide Very low Not measurable Requiresoxygen-impermeablemetallized film oractive packaging thatincludes and oxygenscavenger system andultra high barrierplastic films

DisplayOverwrap High 10,000 to �20,000 Oxygenation of

myoglobin to bloommeat requires ingress ofatmospheric oxygen;very high permeabilityfilms are used withground meat

Modified Low �15 Oxygenation ofatmosphere myoglobin and

inhibition of bacterialgrowth are achieved byatmosphere retainedwithin the pack

a At chill storage temperatures, oxygen transmission rates will be lower and very much lower at temperatures be-low 0°C.


even dangerous. The initial solution was to develop special rough-surface polyethylenebags. This solved the stacking/handling problem but proved too expensive. The final solu-tion was polyethylene bags with stockinet outers. (Removal of stockinet in the cause of im-proved hygiene was unpopular because customers had used stockinet for a variety of pur-poses, including wash cloths for automobiles.)

Another method employs a plastic film that is heat shrunk onto the product in a heattunnel. Shrink wrapping is cheaper than vacuum packaging but because of its poor appear-ance is more suitable for product destined for further processing, such as frozen cutting.

Vacuum packaging of meat for frozen storage does not differ markedly from that forchilled storage. While oxygen-impermeable films are the norm, oxygen permeable filmsare sometimes used so that the product will bloom. These combined storage and retail readypacks can have a high standard of visual display, provided storage time is short. Whethergas-permeable or barrier films are used is often determined by the customers’ product colorpreference.

XIII. PRODUCT DETERIORATION DURING FROZEN STORAGE

A. Gross Carton Deformation and Breakdown

The unattractive or damaged appearance of cartons of frozen meat, including offals, mayresult from deficiencies in one or more of five areas: handling before freezing, type of pack-aging used, freezing regimen, materials handling, and temperature control (Turczyn, 1980).Not only are these five problem areas equally important to carton condition, but their ef-fects are interrelated: a deficiency in one area will adversely affect how the others provideessential product protection.

Frozen product within the carton contributes to the stacking strength and impact re-sistance of a shipping carton. If product in a carton stack is not completely frozen or thawsduring shipment, the cartons will deform unless extra-strong (more expensive) cartons havebeen used. Similarly, underfilling will compromise stacking strength and impact resistance.Underfilling also contributes to slow freezing and frost formation, as air gaps slow heattransfer and are sites for frost accumulation. If subsequent thawing occurs, the humid con-ditions caused by melting frost will soften and weaken the cardboard carton.

Shipping cartons must be strong enough to prevent side and bottom bulge when theyare filled with unfrozen product prior to freezing. Bulges set during freezing and preventsquare and secure stacking of cartons.

Improper handling will also cause carton damage. Frozen meat or offal cartons arehandled several times and may experience two or three transport modes between freezingand final thawing for use. Cartoned product can experience impact, vibration, fluctuatingtemperatures and humidities, and large stacking forces. Even strong, well-designed cartonscontaining fully frozen product will break down when exposed to rough handling, and toinadequate blocking, bracing and unitization during transit. Palletization and the use of me-chanical handling equipment such as forklift trucks will eliminate much rough handling.

B. Freezer Burn

During prolonged frozen storage, moisture can be lost. Products lose moisture as a conse-quence of vapor pressure gradients within the product and between the product and the ex-ternal environment. These gradients are caused by temperature fluctuations during storage(Jul, 1969; Anon., 1986). When the product surface is warmer than the core, moisture

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moves toward the surface. Unfortunately, when the temperature gradient is reversed duringcooling, moisture is not easily transferred back to its original location (Reid, 1993). Whenthe air temperature is higher than the product temperature, moisture moves from the prod-uct into the air, resulting in surface desiccation known as freezer burn.

Freezer burn, characterized by changes in the surface appearance of frozen meat oroffal, is the result of sublimation of ice from product surfaces during storage. The desicca-tion resulting from this sublimation appears as grey patches on the product surface. Fluc-tuating storage temperatures accelerate the onset of freezer burn. Freezer burn occurs mostrapidly at higher storage temperatures when the vapor pressure of the ice in frozen productexceeds the relative humidity of the air in a freezer.

Freezer burn can lead to accelerated lipid oxidation. The open structure at severelydesiccated surfaces provides a large surface area for interaction with oxygen. As well, wa-ter loss causes reactants to be concentrated in the remaining free water (Varnam and Suther-land, 1995).

Freezer burn can be reduced if product is suitably packaged in a tight-fitting film thatis impermeable to water and vapor or if the cold store is maintained at a high relative hu-midity, so-called storage over ice. With meat cuts and offals, freezer burn is most com-monly associated with damaged packaging. Loss of pack integrity allows areas of the prod-uct surface to be exposed to the external environment. The use of loose packaging can leadto an intra-package freezer burn condition manifest by frost or “snow” within the pack.

C. Frost Formation

The mechanism of intra-package frosting is the same as freezer burn except that the mois-ture moves from the product to the inside surface of the packaging. Intra-pack snow for-mation cannot occur when a water- and vapor-impermeable film is tightly applied to theproduct surface. However, should a space exist between the product and its packaging,moisture will move into the space and then condense on the inside of the package to pro-duce snow. In a consumer pack, this snow will prevent the buyer from seeing the product.If the product temperature is lower than the environment, then condensation occurs on theproduct (Anon., 1986), to appear as surface ice. Consumers may erroneously, but under-standably, construe surface ice as an underhand method of adding weight to the product.

Extensive frost formation can to account for up to 20% product weight losses (Anon.,1986). As with freezer burn, water movement within a pack during frozen storage is asso-ciated with deleterious changes in product quality. Such changes are minimized if a low,constant storage temperature is maintained.

D. Recrystallization

Recrystallization is a physical phenomenon whereby ice crystals in frozen foods increasein size but diminish in number during storage. The formation of large ice crystals can causeconsiderable tissue damage (Mazur, 1960), leading to increased drip losses on thawing(Martino and Zaritzky, 1988). Recrystallization is temperature dependent. The rate of re-crystallization decreases abruptly at temperatures below about �7°C (Sy and Fennema,1973), and few cases of ice recrystallization have been observed at temperatures below�12°C (Bevilacqua and Zaritzky, 1980). The most common type of recrystallization oc-curring in frozen meat—migratory recrystallization—is associated with temperature fluc-tuation during storage. Small ice crystals melt as the temperature increases, and the wateris then transferred to the surface of larger crystals during the cooling phase. In other words,



the unfrozen water does not renucleate to form new ice crystals (Reid, 1993; Sahagian andGoff, 1996).

E. Freeze-Thaw Cycling

Today, one would expect freeze-thaw cycling problems to be a matter of only historical in-terest. Unfortunately, such temperature abuse can still occur as a result of plant failures orwhere a lack of national or international infrastructures compromises cold chain integrity.Cyclical freezing and thawing results in considerable cell damage and tissue disruption, in-cluding membrane fragmentation; movement of water, solutes, and small particulates in thetissue; and compaction of cell organelles such as mitochondria (Strange et al., 1985a, b;Jones et al., 1986).

F. Rancidity

Frozen storage slows but does not prevent the onset of oxidative rancidity. Generally, thelower the storage temperature, the slower the rate of rancidity onset. Oxidative rancidity oc-curs most rapidly at �2°C to �4°C and virtually ceases below �30°C (Varman andSutherland, 1995). Pork is very susceptible to rancid spoilage and must be stored at �18°Cor below if good eating quality is to be retained for more than about 6 months. Cured porkproducts are especially prone to rancid spoilage, because the salt component of the cure actsas a catalyst for lipid oxidation. With poultry, the onset of rancidity is typically slowed bythe use of oxygen-barrier packaging. The susceptibility of animal fat to oxidation relates tothe degree of unsaturation of the fatty acids, which is higher in poultry and pigs than in ru-minants.

G. Nutrient Loss

Of the principal macronutrients, proteins and fats can readily undergo chemical change dur-ing frozen storage. Changes to proteins result in loss of solubility, meat toughening, andchanges in meat water holding capacity. With fats, oxidation leads to off-flavor develop-ment. However, changes determined by chemical or physicochemical methods are of lim-ited value in estimating the residual nutritional value of these constituents. In practice, dur-ing storage at temperatures below �18°C for periods of a year or more, the nutritionalvalue of macronutrients is not reduced (Anon., 1986).

For meats, losses of vitamins during frozen storage are product dependent. At tem-peratures between �18°C and �12°C, between 10% and 40% of the total vitamins can belost during storage, with thiamine losses being the most prominent (Anon., 1986). Perhapsof more practical importance is the loss of soluble nutrients, including vitamins, associatedwith drip loss during thawing. It is important that any judgment on the significance of nu-trient loss associated with prolonged frozen storage be made in the context of the relativecontribution that the food item makes to the consumers’ daily intake of the nutrient inquestion.

XIV. STORAGE LIFE EXPECTATIONS FOR FROZEN MEAT

The International Institute of Refrigeration (Anon., 1986) define the Practical Storage Life(PSL) of a product as “. . . the period of frozen storage after freezing during which the prod-uct retains its characteristic properties and remains suitable for consumption or the intended

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process.” With frozen meats and offals, the recommended length of storage remains con-troversial because of the influence of packaging, storage temperature, relative humidity,moisture loss during freezing, and variation in the products themselves (Anon., 1994).

The lack of any international agreement either on the method of PSL assessment orthe end point has resulted in different PSLs being reported by different researchers for thesame products. These differences, distinct from those that can reasonably be attributed totime/temperature/tolerance or product/processing/packaging factors (the so-called TTTand PPP factors), reflect differences in assessment protocols. Because of the variability oftaste panels, the use of physical, chemical, or other objective tests for assessing quality at-tributes appears attractive. However, it must not be forgotten that acceptability of the prod-uct to the consumer and the value the consumer will attach to that product is what is im-portant (Jul, 1984). The range of indicative practical storage lives of frozen meat and meatproducts is given in Table 8. As discussed previously, temperature fluctuation can have agreater deleterious effect on storage life than the use of a higher but constant temperature.

From Table 22.8 it can be seen that there is an inherent species difference in respectto frozen storage stability. Susceptibility to oxidative rancidity is a major determinant of ef-fective storage life. Another determinant is the degree of processing, chopping, or grinding.Each reduces effective storage life, almost certainly by increasing cellular damage and in-creasing the surface area exposed to the deleterious effects of oxygen.

XV. SUMMARY

Fresh meat is a highly perishable commodity whose prolonged storage requires chill (0° to�1.5°C) or freezing (less than �12°C) temperatures. At chill storage temperatures, effec-tive storage life is determined by the onset of microbial spoilage, whereas desiccation andchemical changes limit frozen storage life.

Storage life of chilled product is increased through the use of systems that provide anoxygen-free environment within the package. Increasing the carbon dioxide concentrationwithin an anoxic package further enhances storage life. Both vacuum and carbon dioxidepackaging systems demand packaging materials with high oxygen and carbon dioxide bar-rier properties. Under anoxic conditions, meat becomes purple-red, which is unattractive tomost retail-level customers. Consequently, retail display packaging systems must providean aerobic environment to allow the meat to bloom. Aerobic environments can be provided


Table 8 Practical Storage Life for Frozen Meat and Meat Products

Practical storage life (months)

Temperature (°C)

Product �12 �18 �23

Beef 4 to 12 6 to 18 12 to 24Chopped beef 3 to 4 4 to 6 8Pork 2 to 6 4 to 12 8 to 15Pork sausages 1 to 2 2 to 6 3Lamb 3 to 8 6 to 16 12 to 18Veal 3 to 4 4 to 14 8 to 15

Source: Adapted from ASHRAE Refrigeration Handbook (Anon., 1994).


by the use of oxygen-permeable films, allowing the ingress of atmospheric oxygen, or theuse of oxygen-impermeable films to prevent oxygen egress where oxygen enriched modi-fied atmospheres are used. To maximize chilled storage and display life, product should beheld at the lowest temperature that can be sustained without freezing. Chilled storage lifereduces by approximately 10% for every 1°C that the storage temperature exceeds the op-timum of �1.5°C, and color stability during retail display is halved for every 5°C increasein product temperature.

At first sight, frozen storage appears less sensitive to packaging and temperature thanchilled storage. In relative terms this is not the case, given the expectation of much longerpractical storage life. The deleterious effects of oxygen on product color and palatabilitycan be limited through the use of high oxygen barrier packagings or avoided by judiciousselection of storage temperature and relatively short practical storage lives. Similarly, prod-uct desiccation can be prevented if closely applied moisture-impermeable packagings areused. Selection of packaging materials and storage temperature is dictated by the intrinsicstability of the product and the longevity required for marketing that product. In general,product stability is maximized by the use of packaging materials with high gas and waterbarrier properties and as low a storage temperature as can be consistently maintained. Foreffective frozen storage, it is more important that the storage temperature is kept constantthan allowed to fluctuate even if the average fluctuating temperature is lower than the con-stant storage temperature.

There is no best packaging/storage system; rather, there are variously effective pack-aging storage solutions. Judicious selection of an appropriate solution requires integrationof functional requirements and the marketing performance that must be delivered. Theoverriding performance requirement is that the selected system will, in a cost-effectivemanner, deliver product to the user in a safe and wholesome condition with the resilienceto satisfy expectations at each step in the processing distribution chain.

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Dk1792 ch19

Education

frozen meat packaging

successful meat packaging

selfservice packaging

vacuum packaging

nonpreservative packaging

types of packaging

principles of packaging

packaging methods