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 Packaging 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
to that at a temperature 100C 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 afrozenaqueous 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
20.05.87
This device makes use of an ice tablet and an empty chamber which
willfillup with water if the temperature rises
Holzer, W.
W. German Patent 3731-268A
17.09.87
This device consists in developingfrozenhemispheres of ice on the
surface. When these thaw they lose their shape
Japanese Patent 0031-809 21.07.82
This device consists of an evaluation indicator which is stable when
frozen but separates on thawing
British Patent 2209-396A 04.09.87
This indicator uses an irreversible change of state system: once a
temperature change occurs it is recorded
Minnesota Mining MFG
European Patent 310-428A
02.10.87
This consists of a microporous sheet which becomes wetted when the
liquid thaws. The process is irreversible and operates quickly
Mitsubishi Heavy Ind. KK
Japanese Patent 2021-229A
08.07.88
Use of vegetable leaves to indicate thawing - green colour turns to
black; irreversible on thawing
Perez Martinez, F.
European Patent 2002-585A
10.03.87
This device is a sealed unit containing ice which changes shape on
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
13.10.88
Bi-metal stripflexesto display colour to indicate critical temperature
reached
KAO Corp.
Levin, D.
Table 10.1 Continued
Electrochemical Time-Temperature Devices
Grahm, I.
World Patent 9004-765A 24.10.88
Also US Patent 4929-020A
Temperature history indicating label; the electrodes of a galvanic circuit
form a temperature-responsive device
Johnson Matthey
US Patent 4804275 14.02.89
Tungsten trioxide electrode/weak acid
Toppan Printing KK
Japanese Patent 1141-973A
28.11.87
This is a time indicator to show the expiry of foods started at ambient
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
03.12.87
Twin lapse display. Dye diffusion in agar. With retarder, e.g. albumin
Badische Tabakmanuf
W. German Patent 3907-683A
09.03.89
Time-temperature indicator based on colour development with time
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.
Thefluorescenceis 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
22.06.87
This device consists of a microcapsule layer containing an achromatic
lactone compound pigment precursor and solvent. The sheet indicates
the time elapsed at 50C temperature intervals
Dry Diffusion in Gels
Chemical Reactions
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 complaint 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 redcoloured 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
Lifelines
Fresh-Scan
Fresh-Check
I Point
System test
Reference
0
Tomato firmness (10-20 C)
Microbial growth in pasteurised milk (0-50C)
Green tomato maturity (10-200C)
UHT sterilised milk (5-37°C)
Fruit cake
Lettuce
Pasteurised milk (pallet)
Milk, cream and cottage cheese
Orange juice
UHT milk freshness
Orange juice concentrate (frozen)
Fresh produce (chilled)
Hamburger patties
UHT 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)
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)
Green tomato maturity (10-200C)
UHT sterilised milk (5-37°C)
Fruit cake
Lettuce
Wells
Wells
Wells
WeUs
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)
and
and
and
and
Singh
Singh
Singh
Singh
(1988b)
(1988b)
(1988b)
(1988b)
Table 10.2 Continued
Model
System test
Reference
0
3M Monitormark
Unspecified (two models)
0
Pasteurised whole milk (0 C, 5°C and 10 C)
Hamburger rancidity (frozen)
Hamburger rancidity
Strawberries (- 12 to + 350C)
Seafood salad (pallets) (- 20 to - 100C)
Codfish(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 conditions
Response to isothermal conditions (- 18 to + 5°C)
Response to isothermal conditions (+ 2C, + 100C, - 12°C, -100C)
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)
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)
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)
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-timetemperature 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 selfindication that this has occurred, and no reasonable possibility of preactivation, 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 timetemperature 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
(Manchester, UK)
Autoclave Tape
White to black (stripes)
121°C for 10-15 min and 134°C for 3-4 min for
fully developed colour change
3M Industrial Tapes and Adhesives
(Manchester, UK)
Thermometer Strips
Silver to black
Immediately temperature reached
Albert Browne Ltd. (Leicester, UK)
TST
Yellow to mauve
Set to 121°C for 15 min or 134°C for 5.3 min
Albert Browne Ltd. (Leicester, UK)
Steriliser Control
Tube
Red to green
Steam autoclaves - colour change over 100-1800C
for a range of exposure times
Dry heat = 1600C for 120 min to 1800C for 12
min
Ashby Technical Products Ltd. (Ashby
de Ia Zouch, UK)
ATP Irreversible
Temperature
Indicators
Silver to black
Self-adhesive segmented labels giving colour
change when temperature exceeds set point by
1°C
Cardinal Group (Tiburon, CA, USA)
Easterday
Black to red
Set at 2400C 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)
(Temperature Indicators Ltd., Wigan,
UK Agent)
Cook-Chex
Purple to green
Irreversible indicator, eight ranges selectable, semiintegrators using chromium chloride complex for
different temperatures (110-126.70C) and times
(0-150 min) calibrated against spore destruction
PyMaH Corp. (Flemington, NJ, USA)
(Temperature Indicators Ltd., Wigan,
UK Agent)
SteriGage
Thermalog S
A blue colour front diffuses along
a transparent window of an
accept/reject band
The presence of saturated steam lowers the melting
point of a chemical tablet
Diffusion of the blue colour front has been
calibrated against spore destruction (B.
stearothermophilus) over a range of timetemperature combinations
Reatec AG (Switzerland) (Barbie
Engineering, Twickenham, UK Agent)
Reatec
White to black
Immediately temperature is reached: 54.4-1040C
Table 103 Continued
Manufacturer
Trade name
Colour
Change characteristics
Redpoint (Swindon, UK)
Spectratherm
From light blue to a colour in the
spectrum donating maximum
temperature
Liquid crystal colour change immediately
temperature is reached
S.D. Special Coatings (Barking, UK)
Temperature Tabs
Spirig Earnest (Germany) (Cobonic
Ltd., Surrey, UK Agent)
Celsistrip
Celsidot
Celsipoint
Celsiclock
Irreversible colour labels, 40-2600C; lacquers,
40-lOloC; reversible strips, 40-700C
SteriTec (Colorado, USA)
(Temperature Indicators Ltd., Wigan,
UK Agent)
SteriTec (Colorado, USA)
(Temperature Indicators Ltd., Wigan,
UK Agent)
Thermindex Chemicals & Coatings
Ltd. (Deeside, Clwyd, UK)
White to black
Immediately temperature is reached: 40-2600C
Mauve to green
Three-stage semi-integrator using chromium
chloride
Brown to black
Selected precise time and temperature, 121 to
134°C
White to black
Adhesive strips 40-2600C
For crayons and paints, a range of
colours dependent on temperature
reached
Reversible and irreversible inorganic pigment
colour change either immediately temperature is
reached or after a few min exposure, 50-10100C
Integraph
Cross-checks
Thermindex
Thermographic Measurements Ltd.,
Burton, S. Wirral, UK (Temperature
Indicators Ltd., Wigan, UK, European
Agent)
Pasteurisation
Check
White to black or white to red
Immediately temperature reached: 71, 77, 82°C and
88°C ratings ± 1°C. Other temperature ratings on
request
Thermographic Measurements Ltd.
(Burton, S. Wirral, UK)
Thermax
Silver grey to black
Adhesive strips, irreversible colour change paints,
37-2600C
Thermographic Measurements Ltd.
(Burton, S. Wirral, UK)
Autoclave Indicator
Red to green
Autoclave ink. Change set for 30 min at 116°C or
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-dihydroxy-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
Response
Origin
Application
Location
Chemical
Biological
Dispersed
Volume average
Physical
Single
Multi
Intrinsic
Extrinsic
Permeable
Isolated
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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