10 Time-temperature indicators - Semantic Scholar · 10 Time-temperature indicators J.D. SELMAN Time-temperature indicators are part of the developing interest in intelligent packaging,

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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10 Time-temperature indicators - Semantic Scholar · 10 Time-temperature indicators J.D. SELMAN Time-temperature indicators are part of the developing interest in intelligent packaging,

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