10 Time-Temperature Indicators J.D

10 Time-temperature indicators J.D. SELMAN

Time-temperature indicators are part of the developing interest in intelligent packaging, and there has been considerable interest in small temperature indicators (TIs) and time-temperature indicators (TTIs) for monitoring the useful life of packaged perishable products. There are over 100 patents extant for such indicators based on a variety of physico-chemical principles; however, widespread commercial use has been very limited for a number of reasons. For example, TTIs must be easily activated and then exhibit a reproducible time-temperature dependent change which is easily measured. This change must be irreversible and ideally mimic or be easily correlated to the 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 not respond unless some temperature threshold has been exceeded, while full history indicators respond independent of a temperature threshold. This chapter reviews some of the physico-chemical principles utilised by different types of indicator, and discusses the various issues concerning their application, including consumer interests. Similar principles are being used in indicator systems for validating heat processes, and some of the latest research directions are highlighted.

10.1 Introduction

Time-temperature indicators are one example of intelligent packaging, and interest in this is growing because of the need to provide food manufacturers, retailers and consumers alike with assurances of integrity, quality and authenticity. Other intelligent product quality indicators might include microwave doneness indicators, microbial growth indicators, and physical shock indicators. No microbial growth indicators are commercially available yet, but they are likely to be based on the detection of volatile microbial metabolites such as CO2, alcohols, acetaldehyde, ammonia and fatty acids. Tamper evidence and pack integrity indicators are perhaps the most well developed category. The most familiar types include the physical barriers such as plastic heat shrink sleeves and neck bands; tape and label seals; and paper/plastic/foil inner seals across the mouth of a container. More sophisticated 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 zipper seal. Secondary tamper evident features rely on subtle devices based on chemical reactions, biological markers, and concealing techniques. Some that are now commercially available utilise pattern adhesive labels and tapes, solvent soluble dyes and encapsulated dyes, optically variable films and holographic tear tapes. A number of other developments are on the horizon, including the application of smart cards within caps, magnetically coded closures and electrochemical devices. However, gas sensing dyes are the most advanced, especially for modified atmosphere packs. For example, a CO2 sensing dye could be incorporated into the laminated top web film of a modified atmosphere pack, and this could be designed to change colour when the CO2 level falls below a set concentration. In the area of product authenticity and counterfeiting, there is a large range of intelligent package devices which are being developed for use in various industrial sectors. Some of these will be applicable 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 ink scattering, and so on. There is continuing interest in the monitoring of temperature in the food distribution chain from factory to the consumer, and temperature monitoring and measurement, particularly of chilled foods, have been discussed by others (Woolfe, 1992). As part of the approach to assuring product quality through temperature monitoring and control, attention has focused on the potential use of indicators. Temperature indicators may either display the current temperature or respond to some predefined threshold temperature such as a freezing point or a chill temperature such as 80C. TTIs usually utilise a physico-chemical mechanism that responds to the integration of the temperature history to which the device has been exposed. Many different types of indicator have been devised over the years and general reviews have been 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 monitor one 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 important requirements for indicators, including: • Ease of activation and use. - Indicator may need to be stored and stabilised below threshold temperature for several hours before use • Response to temperature or to cumulative effect of time and temperature. • 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 temperatures due to increasing biochemical reaction rates. Such increasing reaction rates are often measured in terms of Q10 (the ratio of the rate at one temperature 0 to that at a temperature 10 C lower). For many chemical reactions Q10 has a value around 2, i.e., the reaction rate approximately doubles for each 100C temperature rise. As different foods lose quality at different rates, it may therefore be important that the indicator reaction has an activation energy that is similar to that of the food deterioration (Taoukis and Labuza, 1989a; 1989b). This is important for two reasons: firstly, the deterioration rates of stored foods follow similar patterns, although Q10 values may be higher, say from 3 to 20; and secondly, chemical reactions can be used in indicator systems so that by design the reaction rate can be made similar to that of the rate of deterioration of the food. Tables of product activation energies or Q10 values have been given by Hu (1972) for ambient shelf-stable foods, by Schubert (1977) and Olley (1978) for frozen products, and by Labuza (1982), and Hayakawa and Wong (1974) for the scientific evaluation of shelf-life.

10.2 Indicator systems

There are a variety of physico-chemical principles that may be used for indicators, including melting point temperature, enzyme reaction, polymerisation, corrosion, and liquid crystals. Using these systems, many indicators give one of three responses: colour change, movement, or both colour change and movement. A variety of patents have been recorded and some of these are summarised in Table 10.1; a number of types of labels are discussed below. Liquid crystal graduated thermometers may be familiar to some (e.g. those manufactured by Liquid Crystal Devices Ltd., Ruislip, UK), and they can be engineered in different ways, e.g. as a sticky-backed paper label (Avery Label Systems Ltd., Maidenhead, UK) or designed to show selected temperatures as with the Hemotemp II (Camlab, Cambridge, UK). The Table 10.1 Some recent patents - Cold chain monitoring systems Thaw Indicators - Based on Ice Melting Bigand, F.M. French Patent 2626-668A 29.01.88 This device reveals an indicator when the frozen liquid thaws Fauvart, J. French Patent 2616-596A 06.01.89 This is a defrost indicator which consists of blotting paper that becomes coloured by afroze n aqueous dye when it thaws Gradient, F. French Patent 2641-61IA 09.01.89 A defrost indicator for frozen foods; it uses a windowed packaging system to observe change of shape due to thawing Holzer, W. W. German Patent 3716-972A This device makes use of an ice tablet and an empty chamber which 20.05.87 will fill up with water if the temperature rises Holzer, W. W. German Patent 3731-268A This device consists in developing frozen hemispheres of ice on the 17.09.87 surface. When these thaw they lose their shape KAO Corp. This device consists of an evaluation indicator which is stable when Japanese Patent 0031-809 21.07.82 frozen but separates on thawing Levin, D. This indicator uses an irreversible change of state system: once a British Patent 2209-396A 04.09.87 temperature change occurs it is recorded Minnesota Mining MFG European Patent 310-428A This consists of a microporous sheet which becomes wetted when the 02.10.87 liquid thaws. The process is irreversible and operates quickly Mitsubishi Heavy Ind. KK Japanese Patent 2021-229A Use of vegetable leaves to indicate thawing - green colour turns to 08.07.88 black; irreversible on thawing Perez Martinez, F. European Patent 2002-585A This device is a sealed unit containing ice which changes shape on 10.03.87 thawing Perinetti, B. French Patent 2625-599A 28.01.88 Sphere of ice suspended in the centre of a capsule Toporenko, Y. French Patent 2626-072A 20.01.88 This device has a geometrically shaped column of ice coloured with phosphorescent material at the centre. Loss of geometry indicates thawing Uberai, B.S. French Patent 2441-076A 23.12.88 Solvent/membrane indicator; when solvent melts colour is developed Wanfield-Druck KaId W. German Patent 2824-903C Bi-metal strip flexes to display colour to indicate critical temperature 13.10.88 reached Table 10.1 Continued Electrochemical Time-Temperature Devices

Grahm, I. World Patent 9004-765A 24.10.88 Temperature history indicating label; the electrodes of a galvanic circuit Also US Patent 4929-020A form a temperature-responsive device Johnson Matthey US Patent 4804275 14.02.89 Tungsten trioxide electrode/weak acid Dry Diffusion in Gels

Toppan Printing KK Japanese Patent 1141-973A This is a time indicator to show the expiry of foods started at ambient 28.11.87 temperature. The device consists of a dye diffusing into a gel; the rate is determined by time and temperature Toppan Printing KK Japanese Patent 1250-090A Twin lapse display. Dye diffusion in agar. With retarder, e.g. albumin 03.12.87 Chemical Reactions

Badische Tabakmanuf W. German Patent 3907-683A Time-temperature indicator based on colour development with time 09.03.89 when two chemicals are brought into contact, e.g. amino compounds, hydroquinones, quinones and nitro compounds Bramhall, J.S. US Patent 4825-447A 21.09.87 This sytem comprises liposomes containing a quenched fluorescent dye. Thefluorescenc e is released by lysis when the product temperature fluctuates. It measures positive and negative temperature deviations Lifelines Tech. Inc. US Patent 4892-677 19.12.84 Diacetyiene monomer which polymerises to a dark compound, the intensity of which depends on time-temperature exposure Rame, P. French Patent 2613-069A 25.03.88 A thermal inertia temperature indicator which reacts at a certain preset threshold temperature. It is enclosed in a transparent case. It does not react to short temperature changes Three S Tech BV Japanese Patent 1012-237A This device consists of a microcapsule layer containing an achromatic 22.06.87 lactone compound pigment precursor and solvent. The sheet indicates the time elapsed at 50C temperature intervals 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. When frozen, the liquid inside the ampoule freezes, causing it to break. If the temperature rises to -4°C, the liquid thaws and flows out, staining the backing paper. Chillchecker operates by means of a meltable, dyed compound contained in a porous reservoir (Thermographic Measurements Ltd., Burton, UK). In the inactivated form, a domed indicator paper is separated from a reservoir by a small distance. When the dome is pressed, the two materials come into contact, 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 the Thawalert, a self-adhesive label (18 mm in diameter) which utilises temperature sensitive paints chosen to respond at a variety of threshold freezing and chilling temperatures. The above types are based on simple colour development; others quantify the change. Ambitemp (Andover Monitoring Systems Corp., Andover, USA) was a time-temperature integrator which functioned with a fluid that has a specific melting point related to the product to be monitored. Under abuse conditions the melted liquid moves along the capillary tube. Tempchron (Andover Laboratories Inc., South Weymouth, USA) was a more recent version of Ambitemp which gave a read-out in degree minutes that could be interpreted from a chart. Although these two did semi-quantify the changes, their size and cost did not meet the further important requirements for the indicators to be simple, small and inexpensive. 3M Monitormark indicators consist of a paper blotter pack and track separated by a polyester film layer (3M Packaging Systems, Bracknell, UK). Incorporated into the paper blotter pad are chemicals of very specific melting points and a blue dye. The indicator is designed as an abuse indicator which yields no response unless a predetermined temperature is exceeded. The response temperature of the indicator is therefore the melt point of the chemical used. To activate this partial history indicator, the polyester film layer is removed, allowing the melted chemical and dye to diffuse irreversibly along the track. The higher the temperature above the response level, the faster the diffusion occurs along the track. If the temperature falls below the response level of the tag, then the reaction stops. Each indicator has five distinct windows which allow an estimate of exposure time above present values to be made. Before use the indicator has to be preconditioned by storing at a temperature several degrees below the response temperature of the indicator, so that at the start of the reaction the chemical/dye mix is solid. Response of the indicator is measured by the progression of the blue dye along the track, and this is complete when all five windows are blue. An indicator tag labelled 51, for example, would indicate a response temperature (melt temperature) of 5°C with a response time of 2 days. This response refers to the time taken to complete blue colour for all five windows at a constant 2°C above the response temperature of the tag. Similarly, response times of 7 days and 14 days are available on tags, with response temperatures 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 independently of temperature threshold (I Point A/B, Malmo, Sweden). The device consists of a two-part material, one part containing an enzyme solution, the other a lipid substrate and pH indicator. To activate, the seal between the two parts of the indicator is broken and the contents become mixed. As the reaction proceeds, the lipid substrate is hydrolysed and a pH change results in colour change through four colour increments (0-3, green to red). This reaction is irreversible and will proceed faster as temperature is increased and slower as temperature is reduced. Each label has a colour scale to be used as a matching reference, which can also be expressed as a percentage of set time-temperature tolerance (TTT) elapsed (colour 1: 80% TTT; colour 2: 100% TTT; colour 3: 130% TTT). These labels have been the subject of several studies (Byrne, 1976; Blixt and Tiru, 1977; Blixt, 1984; Singh and Wells, 1987; Grisius et ai, 1987; Ballantyne, 1988; Taoukis and Labuza, 1989). An alternative I Point indicator (type B) is also available. Each indicator model is provided with the same time-temperature characteristics as type A, but the difference occurs in the colour change interval. In model B only two visible colours are seen: green and yellow. Only in the final 5% of preset TTT (95-100%, time to colour in type A) does the indicator change from green to yellow. So, whilst responding to the temperature history, the indicators actually remain green for most of the storage life. The development of a yellow colour then indicates product approaching the end of its shelf-life. This single colour change was designed to reduce variability in colour determination by different personnel, which was a common com- plaint with type A models. A range of indicators (A and B with varying TTT) 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 to 14.3 kcal/g mole (Wells and Singh, 1988c). The biochemical solutions must be accurate; results may tend to become less reproducible at longer intervals. Using the same technology, I Point have made a freezer indicator. Another enzyme based time-temperature indicator has been experimentally developed by Boeriu et ah (1986). This is based on enzymic reactions taking place many orders of magnitude faster in liquid paraffins than in solid ones. The device works as a thaw indicator by triggering off an enzymic colour reaction when the solid paraffin melts. Lifelines' Fresh-Scan labels provide a full-history TTI, again showing a response independently of a temperature threshold. The Lifelines system consists of three distinct parts: a printed indicator label incorporating polymer compounds that change colour as a result of accumulated temperature exposure; a microcomputer with an optical wand for reading the indicator; and software for data analysis (Lifelines Technology Inc., Morris Plains, USA). The indicator label consists of two distinct types of bar code. The first is the standard bar code, providing information on product and indicator type, and the second is the indicator code containing polymer compound that irreversibly changes colour with accumulated temperature exposure. The colour change is based on polymerisation of diacetylenic monomers, which proceeds faster at higher temperatures, leading to more rapid darkening of the indicator bar (Fields and Prusik, 1983,1986; Byrne, 1990). Initially, reflectance of the indicator code is high (approximately 100%), subsequently falling during storage as the reaction proceeds and the colour darkens. Once manufactured, Lifelines' labels immediately start reacting to environmental temperature. Therefore, to maintain high initial reflectance values, indicators must be stored at temperatures of - 200C and below. Studies have found that the colour changes correlate well with quality loss in tomatoes and UHT milk, with activation energies for the indicators ranging from 17.8 to 21.3 kcal/g mole (Wells and Singh, 1988a, 1988b). The portable hand-held computer reads both the bar codes and the indicator codes. The software package has been designed to correlate reflectance measurements to predetermined time-temperature characteristics. Data from the hand-held computer are transferred to a host computer, product freshness measurements are entered into the system, and a comparison is made between the product freshness curve and the response kinetics of the Lifelines labels (ZaIl et al., 1986; Krai et ai, 1988). A mathematical model can then be prepared to compensate for the differences in reaction rates of indicators and product degradation and allow prediction of product quality from one indicator reading. Trials at Campden and Chorleywood Food Research Association found these labels to be more reliable than I Point indicator labels (Ballantyne, 1988). The Lifelines Fresh-Check indicator has been developed for the consumer in a simple visual form (Anon., 1989). A small circle of polymer is surrounded by a printed reference ring. The polymer, which starts out lightly coloured, gradually deepens in colour to reflect cumulative temperature exposure. Again, the higher the temperature, the more rapidly the polymer changes. Consumers may then be advised on the pack not to consume the product if the polymer centre is darker than the reference ring, regardless of the use-by date (Fields, 1989). Once again the required polymer response can be engineered. During the last two years several American companies have 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 proper refrigerated conditions. However, use is still limited by the lack of response to short periods of temperature abuse, and the polymerisation reaction is influenced to some extent by light. The latest types are light-protected by a red filter. There is at present considerable interest in these indicators, for example for fresh eggs where short time-temperature rises may not directly affect quality. Lifelines Inc. also claim good correlation with the quality life of cooked ready meals, fresh chicken and yoghurt. During 1991, Lifelines continued to evaluate their polymer-based indicators used in both the food and pharmaceutical industries, and their Fresh-Check label has been trialled in some of the department stores of the French company Monoprix, where they have been applied to over a dozen types of chilled retail products (Monoprix, 1990). The most prominent of the indicators to date have been the three referred to above, i.e., 3M Monitormark, the I Point type, and the Lifelines Fresh-Scan and Fresh-Check. These have been the subject of a number of independent validation tests, and the test systems and references are given in Table 10.2. Marupfroid (Paris, France) has developed a partial history freezer label based 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 hazard warning area visible externally. If thawing has occurred, the red dye moves along the label and exposes a warning printed in hydrophobic white ink. One very important point must be highlighted here, and that is that all other indicators are placed on the outside of a pack and therefore respond to the environmental temperature. The packaging itself may provide the food with some insulation from the environment and the food temperature will therefore lag behind any changes in outside temperature. In the case of this label, the indicator system is placed inside the pack but with its response change visible externally. Johnson Matthey has patented a system based on the corrosion of an indicator strip (US Patent, 1989). It consists of a film of electrochromic material (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 temperature sensitive and results in a colour boundary which moves down the strip at a rate governed by the temperature. The indicator can be engineered to respond to short total times and shows some promise in this respect, and the potential exists for miniaturisation of such indicators. Oscar Mayer Foods Corp. (Madison, USA) have developed a quality freshness indicator. This is based on pH-sensitive dyes in contact with a dual reaction system which simultaneously produces acid and alkali to maintain a constant pH. When one of the substrates becomes depleted, a rapid pH change occurs, resulting in a sharp visual colour change (green to pink). A rise in temperature causes a shift in the equilibrium and the colour changes. Table 10.2 Validation tests on time-temperature indicators Model System test Reference

Lifelines Tomato firmness (10-200C) Wells and Singh (1988a) Fresh-Scan Microbial growth in pasteurised milk (0-50C) Grisius et al. (1987) Fresh-Check Green tomato maturity (10-200C) Wells and Singh (1988b) UHT sterilised milk (5-37°C) Wells and Singh (1988b) Fruit cake Wells and Singh (1988b) Lettuce Wells and Singh (1988b) Pasteurised milk (pallet) Malcata (1990) Milk, cream and cottage cheese Chen and ZaIl (1987a) Orange juice Chen and ZaIl (1987b) UHT milk freshness ZaHetal. (1986) Orange juice concentrate (frozen) Krall et al. (1988) Fresh produce (chilled) Krall et al (1988) Hamburger patties Singh and Wells (1986) UHT milk freshness (21-45°C) Taoukis and Labuza (1989a) Orange juice (7.2°C) Taoukis and Labuza (1989b) Response to isothermal conditions (4-300C) WeUs and Singh (1988c) Response to non-isothermal conditions (4-300C) Fields (1985) Response to temperature (0-370C) Fields (1985) Response to temperatures (5°C and 100C) Ballantyne (1988)

I Point Green tomato maturity (10-200C) Wells and Singh (1988b) UHT sterilised milk (5-37°C) Wells and Singh (1988b) Fruit cake Wells and Singh (1988b) Lettuce WeUs and Singh (1988b) Table 10.2 Continued Model System test Reference

Pasteurised whole milk (00C, 5°C and 100C) Grisius et al. (1987) Hamburger rancidity (frozen) Wells et al. (1987) Hamburger rancidity Singh and Wells (1985a) Strawberries (- 12 to + 350C) Singh and Wells (1987) Seafood salad (pallets) (- 20 to - 100C) Singh and Wells (1985b) Codfis h(frozen ) (pallets) Olsson (1984) Steak, beef patties, macaroni cheese (pallets) (- 20 to + 300C) Olsson (1984) Pizza (- 20 to + 300C) Kramer and Farquhar (1977) Milk (4.4-100C) Mistry and Kosikowski (1983) Response to isothermal conditions (4-300C) Taoukis and Labuza (1989a) Response to non-isothermal conditions (4-300C) Taoukis and Labuza (1989b) Response to isothermal conditions Wells and Singh (1988c) Response to isothermal conditions (- 18 to + 5°C) Wells and Singh (1985) Response to isothermal conditions (+ 2C, + 100C, - 12°C, -100C) Ballantyne (1988)

3M Monitormark Hamburger rancidity (>- 17°C) Wells et al. (1987) Singh and Wells (1986) Wells and Singh (1985) Steak, beef patties and macaroni cheese (pallet loads) (- 23.4 to - 15°C) Kramer and Farquhar (1977) Milk (4.4-100C) Mistry and Kosikowski (1983) Response to isothermal conditions (4-300C) Taoukis and Labuza (1989a) Response to non-isothermal conditions (4-300C) Taoukis and Labuza (1989b) Response to isothermal conditions (4 - 1O0C) Ballantyne (1988)

Unspecified (two models) Response to isothermal conditions Arnold and Cook (1977) Imago Industries (La Ciotat, France) have launched their re-usable thermomarker. This is solid and relatively large (88 x 53 mm), and the principal element in its makeup is a shape memory alloy. The alloy effectively 'memorises' two distinct shapes associated with predefined temperatures. In the device itself, a spring made of shape memory alloy changes size according to predetermined temperatures within a programmed range. This in turn activates a system which ejects different coloured balls that signal the reaching of the various temperature thresholds. A patent from Microtechnic (Germany) apparently uses the alignment of two magnets as an indication of the thawing of a frozen food. At the point of freezing, two magnets are held unaligned in a small liquid container. However, if the liquid thaws, then the attraction by the opposite poles of the magnets will promote movement and the two magnets come together, indicating that thawing has occurred. Albert Browne (Leicester, UK) make cold chain indicators which can produce either an abrupt change of colour (yellow to blue) at its end point, or a more gradual change depending on its application. They have specialised in thermal indicators for many years and are now promoting their time-temperature cold chain indicators in both the food and pharmaceutical industries. Food Guardian (Blandford, UK) have begun to promote their label which has a thermometer profile. The label indicates the time on the scale for which the temperature has been above the designated temperature. Senders (London) have developed a threshold label for application to large boxes and pallets, and this consists of both a warning indicator that the temperature is getting too high, and a second indicator showing the need for rejection. Courtaulds Research (Coventry, UK) have considered developing a temperature-sensitive colour in acetate film. This could be used to detect when a product is fully defrosted and ready for cooking, assuming no storage abuse. Bowater Labels (Altrincham, UK) have recently launched their Reactt TTI self-adhesive label for monitoring freezing and chilling distribution temperatures (Pidgeon, 1994). The labels remain inert until activated, then change from blue to red to reveal underlying graphics when preset time/temperature limits are exceeded. Trigon Industries Ltd. (Telford, UK) has also just launched its Smartpak label, which is self-activating before use and shows an irreversible colour change to reveal an underlying symbol warning. For example, the Smartpak 1812 label self-activates when it is frozen below -18°C, and subsequently indicates the temperature rising above -12°C. In the case of microwaveable products, research has shown that for microbiological and other quality criteria, all points within the food should be reheated to an equivalent of 700C for 2 min. To date only two doneness indicators are available. That from 3M (Bracknell, UK) uses a thermochromic 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-change principle described earlier. Other devices are being developed at this time, although the challenge of measuring and correlating cold point temperatures with overall pack temperatures remains considerable. Risman (1993) refers to the gel indicator technique developed at the Swedish Food Research Institute for assessing the reheating performance of domestic microwave ovens for ready meals.

10.3 Indicator application issues and consumer interests

It is generally agreed that there are a number of potential applications for which the above-mentioned indicators could be used regarding the monitoring of various aspects and parts of the chilled and frozen distribution chains (Singh and Wells, 1990). However, the industry has been expressing concern regarding several issues about all types of indicator. TIs and TTIs represent new applications of technology, with little or no history of successful and reliable application, and until recently there has been no standard against which their performance could be assessed. Also, the proliferation of TIs and TTIs now being offered, involving many different forms of indication, is of concern as this is likely to confuse the consumer. Provided these concerns are addressed by a given indicator for a specified product (or range), the potential exists for indicators to be used in several ways, including on pallets or 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 appropriate temperature, which should remain constant. However, there may be several points in the distribution chain where the environmental temperature is raised. Such periods may be short, from a few minutes to several hours. To date, most indicators will not react rapidly enough to respond to such regimes. For example, a Lifelines indicator subject to 24 hours at 5°C, six hours at 100C, and two hours at 200C did not show a response that was significantly different to the control at 5°C (Ballantyne, 1988). Lifelines have done work over the last two years and now claim that a dual chemistry system can be engineered to specifications required. Therefore, there may be some important limitations of some indicators that must be recognised, in particular relating to reliability and reproducibility, sensitivity to short-time- temperature abuse, response to environment temperature but not necessarily food temperature, and cost benefits. For example, in 1988 Lifelines bar code labels cost 30-70p each (scanning system US$20 000), I Point labels 15-2Op each, and the 3M Monitormark about £1.50, for small trial quantities. In 1991, Lifelines' prices in the USA ranged from 7.5 to 3.50 for bar code labels and 3.5 to 1.250 for Fresh-Checks. The latter lower cost related to production runs in excess of 10 million units. To be effective and of value to manufacturer and consumer, TIs and TTIs must provide an indication of the end-life of the product. This should be no less clear and unambiguous to the great majority of the population than the current minimum durability instruction. In particular, some consumers may have difficulty in detecting the difference between two colours, or shades of one colour, where this forms the end point. Related to this, the point at which product life starts can be clearly defined for the purposes of declaring a 'best before' or 'use by' date. It is essential that the start point of the life of 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 legal requirement for a best before and use by date on the pack will continue for the foreseeable future. Therefore, consumer instructions on the pack will need to clearly indicate the action to be taken when there is conflict between end of product life indication as given by the best before and use by date and the TTI. There is also concern that where TIs and TTIs may have a role to play with regard to product quality over life, unsubstantiated claims should not be made regarding any role in relation to safety. TTIs in general do not measure product temperature. Only one commercially available type is known, which is claimed to measure food surface temperature. None is known to measure food centre temperature. Almost all respond to temperatures on the outside of the pack, where there may be some thermal insulation between product and indicator (Malcata, 1990). Measurement at this point may be of value, but the limitations in terms of usefulness and relevance of such measurement need to be made clear to the user and the consumer. A TI or TTI which reflects product temperature would be of far greater value and relevance than one which responds to the temperature on the outer surface of the pack. A TI or TTI also needs to be able to cope with fluctuating temperatures (including elevated temperatures for a short time) and to respond accurately and reproducibly at the extremes of temperature likely to be experienced by the product. A TTI may need to mimic the growth of food spoilage microorganisms, or whatever other time- temperature related factor is liable to affect the quality of the foodstuff, over the full range of temperatures likely to be experienced and when the temperature fluctuates. The quality management of the manufacture, distribution and storage of the TTI and the reproducibility of its performance must be of at least as high an order as the food product it seeks to monitor. In addition, there is concern that the wrong TI or TTI may be applied to a given product. An incorrectly applied date mark is self-evident, at least to the manufacturer at the point of application. As manufacturers may be producing simultaneously a range of products with different predicted lives, they will require a range of TIs or TTIs designed with related performance characteristics. Hence, every indicator should be supplied with a clear indication to the manufacturer, distributor, retailer, and the enforcement authorities of the precise temperature threshold or time-temperature integration to which the indicator will respond. The TI or TTI needs to be no less resistant to malpractice and tampering than is the printed date on the pack. The indicator or the package should self-indicate if removed from the product; at the same time, if removed it should damage the packaging in such a way that a fresh indicator cannot be applied without detection. Finally, TIs and TTIs in themselves must 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 concluded recently that a specification was required which could be common to all types of TIs and TTIs, and which could be used by manufacturers of such indicators in order to meet the requirements of the industry and of the consumer. Such a specification would address the basic technical requirements for the performance of such indicators, although it is accepted that commercial reasons may influence the decision to use indicators for a particular application. A joint Ministry of Agriculture, Fisheries and Food (MAFF)/industry working party met during 1991 at the Campden and Chorleywood Food Research Association, and has completed a food industry specification (George and Shaw, 1992). It is hoped that this will provide a basis for indicator manufacturers to design the performance of their indicators to meet the needs of the food industry, and at the same time provide a basis for the users of such indicators to check the indicator performance against their requirements. This specification defines the testing scope for indicator type and application. It refers to the quality management of the indicator manufacture, the indicator compatibility with food, the need for evidence of tamper abuse, and indicator labelling. It then outlines test protocols for indicator response to temperature, including temperature cycling and abuse, and the evaluation of the kinetic constants of the indicator. It covers evaluation of the accuracy of indicator activation point, and the clarity and accuracy of end point determination, and finally simulated field testing. A survey of 511 UK consumers, carried out by the National Consumer Council (MAFF, 1991), indicated that almost all respondents (95%) thought that TTIs were a good idea, but only grasped their concept after some explanation, indicating that substantial publicity or an education campaign would be required. Use of TTIs would have to be in conjunction with the durability date, with clear instructions about what to do when the indicator changed colour. The relationship and possible conflict between the indication 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 TTI response 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 at home, 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 of the blame on the food suppliers. However, the value of TTIs was recognised for raising confidence in retail handling, and improving hygiene practices when food is taken home and stored in refrigerators. It is clear that there is a future for TTIs in monitoring the chill chain. Development of different indicators is still in progress and technical difficulties have to be overcome by carrying out the appropriate tests (George and Shaw, 1992). However, the consumer can appreciate the concept, and the advantages and benefits of increased 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 assessing pasteurisation and sterilisation processes, and some examples of commercially available indicator systems are summarised in Table 10.3. Most of these tend to give qualitative indications. Current research is directed towards evaluating new systems which may give precise quantitative indication. Hendrickx et al (1993) have conducted an extensive review and have classified time-temperature indicators, as shown in Figure 10.1, in terms of working principle, type of response, origin, application in the food material, and location in the food. For biological TTIs, the change in biological activity such as of microorganisms, their spores (viability) or enzymes (activity) upon heating is the basic working principle. The use of inoculated alginate particles is an example of the use of spores (Gaze et al, 1990). Recent studies on enzyme activity have shown potential for the use of a-amylase, using differential scanning calorimetry to measure changes in protein conformation (De Cordt et al, 1994). Brown (1991) studied the denaturation of several enzymes and suggested that an approach which measures the status of a number of enzymes in terms of pattern recognition would be better than using a single enzyme to indicate retrospectively the heat process that had been applied. Brown (1991) also determined the feasibility and potential for ELISA techniques for retrospective assessment of the heat treatment given to beef and chicken. Marin et al (1992) studied the effects of graded heat treatments of 30 min from 40 to 1000C on meat protein denaturation. They measured the remaining antigenic activity of the meat proteins and found this was significantly correlated with the heating temperature. Varshney and Paraf (1990) used specific polyclonal antibodies to detect heat treatment of ovalbumin in mushrooms, and could identify whether the ovalbumin had been heated to lower than 65°C or higher than 85°C. In terms of chemical systems, potential has been shown for correlating the loss 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 Manufacturer Trade name Colour Change characteristics 3M Industrial Tapes and Adhesives Autoclave Tape White to black (stripes) 121°C for 10-15 min and 134°C for 3-4 min for (Manchester, UK) fully developed colour change 3M Industrial Tapes and Adhesives Thermometer Strips Silver to black Immediately temperature reached (Manchester, UK) Albert Browne Ltd. (Leicester, UK) TST Yellow to mauve Set to 121°C for 15 min or 134°C for 5.3 min 0 Albert Browne Ltd. (Leicester, UK) Steriliser Control Red to green Steam autoclaves - colour change over 100-180 C Tube for a range of exposure times Dry heat = 1600C for 120 min to 1800C for 12 min Ashby Technical Products Ltd. (Ashby ATP Irreversible Silver to black Self-adhesive segmented labels giving colour de Ia Zouch, UK) Temperature change when temperature exceeds set point by Indicators 1°C 0 Cardinal Group (Tiburon, CA, USA) Easterday Black to red Set at 240 C for 20 min, ketone based Colour Therm (Surrey, UK) Colour-Therm White to black or red Immediately temperature is reached PyMaH Corp. (Flemington, NJ, USA) Cook-Chex Purple to green Irreversible indicator, eight ranges selectable, semi- (Temperature Indicators Ltd., Wigan, integrators using chromium chloride complex for 0 UK Agent) different temperatures (110-126.7 C) and times (0-150 min) calibrated against spore destruction PyMaH Corp. (Flemington, NJ, USA) SteriGage A blue colour front diffuses along The presence of saturated steam lowers the melting (Temperature Indicators Ltd., Wigan, Thermalog S a transparent window of an point of a chemical tablet UK Agent) accept/reject band Diffusion of the blue colour front has been calibrated against spore destruction (B. stearothermophilus) over a range of time- temperature combinations 0 Reatec AG (Switzerland) (Barbie Reatec White to black Immediately temperature is reached: 54.4-104 C Engineering, Twickenham, UK Agent) Table 103 Continued Manufacturer Trade name Colour Change characteristics Redpoint (Swindon, UK) Spectratherm From light blue to a colour in the Liquid crystal colour change immediately spectrum donating maximum temperature is reached temperature S.D. Special Coatings (Barking, UK) Temperature Tabs Irreversible colour labels, 40-2600C; lacquers, 40-lOloC; reversible strips, 40-700C Spirig Earnest (Germany) (Cobonic Celsistrip White to black Immediately temperature is reached: 40-2600C Ltd., Surrey, UK Agent) Celsidot Celsipoint Celsiclock SteriTec (Colorado, USA) Mauve to green Three-stage semi-integrator using chromium (Temperature Indicators Ltd., Wigan, chloride Integraph UK Agent) SteriTec (Colorado, USA) Brown to black Selected precise time and temperature, 121 to (Temperature Indicators Ltd., Wigan, 134°C Cross-checks UK Agent) Thermindex Chemicals & Coatings White to black Adhesive strips 40-2600C Ltd. (Deeside, Clwyd, UK) Thermindex For crayons and paints, a range of Reversible and irreversible inorganic pigment colours dependent on temperature colour change either immediately temperature is reached reached or after a few min exposure, 50-10100C Thermographic Measurements Ltd., Pasteurisation White to black or white to red Immediately temperature reached: 71, 77, 82°C and Burton, S. Wirral, UK (Temperature Check 88°C ratings ± 1°C. Other temperature ratings on Indicators Ltd., Wigan, UK, European request Agent) Thermographic Measurements Ltd. Thermax Silver grey to black Adhesive strips, irreversible colour change paints, (Burton, S. Wirral, UK) 37-2600C Thermographic Measurements Ltd. Autoclave Indicator Red to green Autoclave ink. Change set for 30 min at 116°C or (Burton, S. Wirral, UK) 15 min at 127°C TLC Ltd. (Deeside, UK) TLC 8 Red to black Organic thermo-chromic ink; colour changes immediately temperature is reached exhibit heat-induced changes. For example, Kim and Taub (1993) have been studying the thermally produced marker compounds 2,3-dihydro-3,5-dihy- droxy-6-methyl-(4H)-pyran-4-one and 5-hydroxymethylfurfural. Both these compounds are produced when D-fructose is heated, and glucose yields only the latter compound. Hence, where a food contains either of these sugars, there is some basis for assessing heat treatment received as the kinetic characteristics make them suitable as markers for bacterial destruction. As before, the kinetic response requirement which a TTI should fulfil can be derived theoretically and should match the response of the target index, such as a spore or a nutrient, when subjected to the same thermal process. Potential exists for multicomponent TTIs in the evaluation of thermal processes (Maesmans et al, 1994). Regarding the origin of the TTI, an extrinsic TTI is a system added to the food, while intrinsic TTIs are intrinsically present in the food. In terms of the

Working principle Biological Chemical Physical

Response Single Multi

Origin Intrinsic Extrinsic

Application Dispersed Permeable Isolated

Location Volume average Single point

Figure 10.1 General classification of time-temperature indicators (after Hendrickx et al, 1993). application of the TTI in the food product, dispersed systems allow the evaluation of the volume average impact, whilst all three approaches (see Figure 10.1) can be used as the basis for single point evaluations. When using intrinsic components as the TTI, the TTI will be more or less evenly distributed throughout the food, and this also eliminates heat transfer limitations. This whole field is currently the subject of a major European collaborative research study co-ordinated by the Centre for Food Science and Technology at the University of Leuven in Belgium.

10.5 Conclusions

The interest in this subject has generated numerous research studies and practical 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 from the application of indicators in aiding the monitoring and assurance of distribution chains. This, in turn, is leading to the development of new indicators that are much more precisely designed to meet the needs of the food industry. In the broader context of time-temperature integration, applications for thermal process assessment are receiving further attention and novel approaches are actively being researched. Such developments will assist in the assurance in and broader introduction of new heat processes such as microwave sterilisation. Overall, it is likely that there will continue to be exciting developments during the next five years.

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