Time-temperature indicators(TTIs) P. S. Taoukis, National Technical University of Athens greece and T.P. Labuza, University of Minnesota, USA 6.1 Introduction The modern food industry is called on to deliver seemingly contradictory market demands. On the one hand consumers want improved safety and sensory quality together with increased nutritional properties, extended shelf-life and convenience in preparation and use. On the other they want food with a traditional, wholesome image, with less processing and fewer additives In achieving safer and better quality food scientists and manufacturers apply intense optimisation and control of all the production and preservation parameters and additionally explore and benefit from innovative techniques to ensure safety and reduce food deterioration. Novel packaging such as active packaging is among such innovative tools. Producers and regulators rely on the development and application of structured quality and safety assurance systems based on prevention through monitoring, recording and controlling of critical parameters through the entire product life cycle. These systems should include the post-processing phase and ideally extend to the consumers table. The Iso 9001: 2000 quality management standard(Iso 9001: 2000; ISO 15161: 2001), widely adopted by the food industry, emphasises documented procedures for storage, handling and distribution. The globally recommended Hazard Analysis and Critical Control Point(HACCP)safety assurance system also focuses on this phase(93/43/EEC; Codex, 1997; US Federal Register 1996). Certain stages of the chill chain are recognised as important critical control points( CCPs) for minimally processed chilled products such as modified-atmosphere packaged and other ready-to-eat chilled pro Monitoring and controlling these CCPs is seen as essential for safety Research and industrial studies show that chilled or frozen distribution and
6.1 Introduction The modern food industry is called on to deliver seemingly contradictory market demands. On the one hand consumers want improved safety and sensory quality, together with increased nutritional properties, extended shelf-life and convenience in preparation and use. On the other they want food with a traditional, wholesome image, with less processing and fewer additives. In achieving safer and better quality food scientists and manufacturers apply intense optimisation and control of all the production and preservation parameters and additionally explore and benefit from innovative techniques to ensure safety and reduce food deterioration. Novel packaging such as active packaging is among such innovative tools. Producers and regulators rely on the development and application of structured quality and safety assurance systems based on prevention through monitoring, recording and controlling of critical parameters through the entire product life cycle. These systems should include the post-processing phase and ideally extend to the consumer’s table. The ISO 9001:2000 quality management standard (ISO 9001:2000; ISO 15161: 2001), widely adopted by the food industry, emphasises documented procedures for storage, handling and distribution. The globally recommended Hazard Analysis and Critical Control Point (HACCP) safety assurance system also focuses on this phase (93/43/EEC; Codex, 1997; US Federal Register, 1996). Certain stages of the chill chain are recognised as important critical control points (CCPs) for minimally processed chilled products such as modified-atmosphere packaged and other ready-to-eat chilled products. Monitoring and controlling these CCPs is seen as essential for safety. Research and industrial studies show that chilled or frozen distribution and 6 Time-temperature indicators (TTIs) P. S. Taoukis, National Technical University of Athens, Greece and T. P. Labuza, University of Minnesota, USA
104 Novel food packaging techniques handling very often deviate from recommended temperature conditions. Since temperature largely constitutes the determining post-processing parameter for shelf-life under good manufacturing and hygiene practices, monitoring and controlling it is of central importance. The complexity of the problem highlighted when the variation in temperature exposure of single products within batches or transportation sub-units is considered. Ideally, a cost-effective way to monitor the temperature conditions of food products individually, throughout distribution, is required to indicate their real safety and quality. This requirement could be fulfilled by Time Temperature Integrators or Indicators(TTIs). TTIs can be classified as active packaging. A TTI based system could lead to effective quality control of the chill chain, optimisation of stock rotation and reduction of waste, and provide information on the remaining shelf-life of product units. A prerequisite for the application of TTis is the systematic study and kinetic modelling of the role of temperature in determining shelf-life. Based on reliable models of food product shelf-life and the kinetics of TTI response, the effect of temperature can be monitored, recorded and translated from production to the consumer's table 6.2 Defining and classifying TTIs a time temperature integrator or indicator(TTD) can be defined as a simple expensive device that can show an easily measurable, time-temperature dependent change that reflects the full or partial temperature history of a food product to which it is attached(Taoukis and Labuza, 1989 ). The principle of TTl operation is a mechanical, chemical, electrochemical, enzymatic or microbiological irreversible change usually expressed as a visible response, in the form of a mechanical deformation, colour development or colour movement he rate of change is temperature dependent, increasing at higher temperatures The visible response thus gives a cumulative indication of the storage conditions that the tti has been exposed to. The extent to which this response corresponds to a real time-temperature history depends on the type of the indicator and the physicochemical principles of its operation. Indicators can thus be classified according to their functionality and the information they convey An early classification system introduced by Schoen and Byrne(1972) separated devices into six categories. Byrne(1976)revised this classification, realising that the main functional difference is whether the indicator responds above a preselected temperature, or responds continuously thus giving nformation on the cumulative time-temperature exposure. He proposed three types 1. defrost indicators 2. time-temperature integrators 3. time-temperature integrators/indicators A similar scheme recognised three categories(Singh and Wells, 1986)
handling very often deviate from recommended temperature conditions. Since temperature largely constitutes the determining post-processing parameter for shelf-life under good manufacturing and hygiene practices, monitoring and controlling it is of central importance. The complexity of the problem is highlighted when the variation in temperature exposure of single products within batches or transportation sub-units is considered. Ideally, a cost-effective way to monitor the temperature conditions of food products individually, throughout distribution, is required to indicate their real safety and quality. This requirement could be fulfilled by Time Temperature Integrators or Indicators (TTIs). TTIs can be classified as active packaging. A TTI based system could lead to effective quality control of the chill chain, optimisation of stock rotation and reduction of waste, and provide information on the remaining shelf-life of product units. A prerequisite for the application of TTIs is the systematic study and kinetic modelling of the role of temperature in determining shelf-life. Based on reliable models of food product shelf-life and the kinetics of TTI response, the effect of temperature can be monitored, recorded and translated from production to the consumer’s table. 6.2 Defining and classifying TTIs A time temperature integrator or indicator (TTI) can be defined as a simple, inexpensive device that can show an easily measurable, time-temperature dependent change that reflects the full or partial temperature history of a food product to which it is attached (Taoukis and Labuza, 1989). The principle of TTI operation is a mechanical, chemical, electrochemical, enzymatic or microbiological irreversible change usually expressed as a visible response, in the form of a mechanical deformation, colour development or colour movement. The rate of change is temperature dependent, increasing at higher temperatures. The visible response thus gives a cumulative indication of the storage conditions that the TTI has been exposed to. The extent to which this response corresponds to a real time-temperature history depends on the type of the indicator and the physicochemical principles of its operation. Indicators can thus be classified according to their functionality and the information they convey. An early classification system introduced by Schoen and Byrne (1972) separated devices into six categories. Byrne (1976) revised this classification, realising that the main functional difference is whether the indicator responds above a preselected temperature, or responds continuously thus giving information on the cumulative time-temperature exposure. He proposed three types: 1. defrost indicators 2. time-temperature integrators 3. time-temperature integrators/indicators. A similar scheme recognised three categories (Singh and Wells, 1986): 104 Novel food packaging techniques
Time-temperature indicators(TTls) 105 L abuse indicators 2. partial temperature history indicators 3. full temperature history indicators(an alternative nomenclature for time temperature integrators) A three-category classification will be used in this chapter(Taoukis et al, 1991) 6.2.1 Critical temperature indicators (CTD) CTI show exposure above(or below) a reference temperature. They involve a time element (usually short; a few minutes up to a few hours) but are not intended to show history of exposure above the critical temperature. They merely indicate the fact that the product was exposed to an undesirable temperature for a time sufficient to cause a change critical to the safety or quality of the product. They can serve as appropriate warning in cases where physicochemical or biological reactions show a discontinuous change in rate Good examples of such cases are the irreversible textural deterioration that happens when phase changes occur(e.g, upon defrosting of frozen products or freezing of fresh or chilled products). Denaturation of an important protein above the critical temperature or growth of a pathogenic microorganism ar other important cases where a CtI would be useful. The critical temperature term is preferred rather than the used alternative ' that is too limiting The term abuse t be misleading as undesirable changes can happen at temperatures which are not as extreme or abusive as the term implies and which are within the acceptable range of normal storage for the product in question. 6.2.2 Critical temperature/time integrators(CTTD) CTTI show a response that reflects the cumulative time-temperature exposure above a reference critical temperature. Their response can be translated into an equivalent exposure time at the critical temperature. They are useful in indicating breakdowns in the distribution chain and for products in which reactions, important to quality or safety, are initiated or occur at measurable rates above a critical temperature. Examples of such reactions are microbial growth or enzymatic activity that are inhibited below the critical temperature. 6.2.3 Time temperature integrators or indicators(TtD TTI give a continuous, temperature dependent response throughout the products history. They integrate, in a single measurement, the full time-temperature history and can be used to indicate anaverage temperature during distribution and possibly be correlated to continuous, temperature dependent quality loss reactions in foods. In the remainder of this chapter, the term Tti will refer to this type of indicator, unless otherwise noted. A different method of classification sometimes used is based on the principle of the indicators' operation. Thus, they
1. abuse indicators 2. partial temperature history indicators 3. full temperature history indicators (an alternative nomenclature for timetemperature integrators). A three-category classification will be used in this chapter (Taoukis et al., 1991). 6.2.1 Critical temperature indicators (CTI) CTI show exposure above (or below) a reference temperature. They involve a time element (usually short; a few minutes up to a few hours) but are not intended to show history of exposure above the critical temperature. They merely indicate the fact that the product was exposed to an undesirable temperature for a time sufficient to cause a change critical to the safety or quality of the product. They can serve as appropriate warning in cases where physicochemical or biological reactions show a discontinuous change in rate. Good examples of such cases are the irreversible textural deterioration that happens when phase changes occur (e.g., upon defrosting of frozen products or freezing of fresh or chilled products). Denaturation of an important protein above the critical temperature or growth of a pathogenic microorganism are other important cases where a CTI would be useful. The ‘critical temperature’ term is preferred rather than the used alternative ‘defrost’ that is too limiting. The term ‘abuse’ might be misleading as undesirable changes can happen at temperatures which are not as extreme or abusive as the term implies and which are within the acceptable range of normal storage for the product in question. 6.2.2 Critical temperature/time integrators (CTTI) CTTI show a response that reflects the cumulative time-temperature exposure above a reference critical temperature. Their response can be translated into an equivalent exposure time at the critical temperature. They are useful in indicating breakdowns in the distribution chain and for products in which reactions, important to quality or safety, are initiated or occur at measurable rates above a critical temperature. Examples of such reactions are microbial growth or enzymatic activity that are inhibited below the critical temperature. 6.2.3 Time temperature integrators or indicators (TTI) TTI give a continuous, temperature dependent response throughout the product’s history. They integrate, in a single measurement, the full time-temperature history and can be used to indicate an ‘average’ temperature during distribution and possibly be correlated to continuous, temperature dependent quality loss reactions in foods. In the remainder of this chapter, the term TTI will refer to this type of indicator, unless otherwise noted. A different method of classification sometimes used is based on the principle of the indicators’ operation. Thus, they Time-temperature indicators (TTIs) 105
106 Novel food packaging techniques can be categorised as mechanical, chemical, enzymatic, microbiological, polymer, electrochemical, diffusion based, etc 6.3 Requirements for TtIs The requirements for an effective TTI are that it shows a continuous change, the ate of which increases with temperature and which does not reverse when temperature is lowered. There are a number of other desirable attributes for a successful indicator. An ideal TTI would have all the following properties It exhibits a continuous time-temperature dependent change The change causes a response that is easily measurable and irreversible The change mimics or can be correlated to the food's extent of quality deterioration and residual shelf-life It is reliable, giving consistent responses when exposed to the same temperature conditions · It has low cost. It is flexible, so that different configurations can be adopted for various temperature ranges(e.g, frozen, refrigerated, room temperature)with useful response periods of a few days as well as up to more than a year It is small, easily integrated as part of the food package and compatible with a high-speed packaging process It has a long shelf-life before activation and can be easily activated It is unaffected by ambient conditions other than temperature, such as light, humidity and air pollutants e It is resistant to normal me abuse and its response cannot be altered It is non-toxic, posing no threat in the unlikely situation of product contact It is able to convey in a simple and clear way the intended message to its target, be that distribution handlers or inspectors, retail store personnel or consumers Its response is both visually understandable and adaptable to measurement by lectronic equipment for easier and faster information, storage and 6.4 The development of TTIs The drive for development of an effective and inexpensive indicator dates from the time when the importance of temperature variations to final food quality during distribution became apparent. Initially, the interest was focused on frozen foods. The first application of adevice to indicate handling abuse dates from World War II when the US Army Quartermaster Corps used an ice cube placed inside each case of frozen food. Disappearance of the cube indicated
can be categorised as mechanical, chemical, enzymatic, microbiological, polymer, electrochemical, diffusion based, etc. 6.3 Requirements for TTIs The requirements for an effective TTI are that it shows a continuous change, the rate of which increases with temperature and which does not reverse when temperature is lowered. There are a number of other desirable attributes for a successful indicator. An ideal TTI would have all the following properties: • It exhibits a continuous time-temperature dependent change. • The change causes a response that is easily measurable and irreversible. • The change mimics or can be correlated to the food’s extent of quality deterioration and residual shelf-life. • It is reliable, giving consistent responses when exposed to the same temperature conditions. • It has low cost. • It is flexible, so that different configurations can be adopted for various temperature ranges (e.g., frozen, refrigerated, room temperature) with useful response periods of a few days as well as up to more than a year. • It is small, easily integrated as part of the food package and compatible with a high-speed packaging process. • It has a long shelf-life before activation and can be easily activated. • It is unaffected by ambient conditions other than temperature, such as light, humidity and air pollutants. • It is resistant to normal mechanical abuse and its response cannot be altered. • It is non-toxic, posing no safety threat in the unlikely situation of product contact. • It is able to convey in a simple and clear way the intended message to its target, be that distribution handlers or inspectors, retail store personnel or consumers. • Its response is both visually understandable and adaptable to measurement by electronic equipment for easier and faster information, storage and subsequent use. 6.4 The development of TTIs The drive for development of an effective and inexpensive indicator dates from the time when the importance of temperature variations to final food quality during distribution became apparent. Initially, the interest was focused on frozen foods. The first application of a ‘device’ to indicate handling abuse dates from World War II when the US Army Quartermaster Corps used an ice cube placed inside each case of frozen food. Disappearance of the cube indicated 106 Novel food packaging techniques
Time-temperature indicators (TTIs) 10 mishandling(Schoen and Byrne, 1972). The first patented indicator goes back to 1933(Midgley, 1933). Over a hundred US and international patents relevant to time-temperature indicators have been issued since. During the last 30 years numerous TTI systems have been proposed of which only few reached the prototype and even fewer the market stage. Patents dating up to 1990 are tabulated in the literature(Byrne, 1976; Taoukis, Fu and Labuza, 1991). Byrne (1976) gives an overview of the early indicators and Taoukis(1989)presents a detailed history of TTl. Table 6.1 lists significant recent TTI patents classified according to type and principle of operation. The first commercially available TTI was developed by Honeywell Corp (Minneapolis, MN)(Renier and Morin, 1962). The device never found commercial application, possibly because it was costly and relatively bulky. In the early 1970s, the Us government considered mandating the use of indicators on certain products (OTA, 1979). This generated a flurry of research and development. Researchers at the US Army Natick Laboratories developed a TTI that was based on the colour hange of an oxidisable chemical system controlled by the temperature dependent permeation of oxygen through a film(Hu, 1972). Field testing over a two-year period with the TTI attached to rations showed their potential for use(Killoran, 1976). The system was contracted to Artech Corp(Falls Church, VA)for commercial development. By 1976 Six companies were making temperature Table 6.1 List of recent TTI patents and classification according to type and mode of response Date mentor Principle of operation Patent No 1991 Jalinski. TJ Chemical (TTD US5,182,212 1991 Jalinski. TJ Chemical (TTD US5,085,802 Chemical(CTT Us5,085,801 1991 Swartzel KR Physicochemical (TTD) US5,159,564 1992 Jalinski. T Chemical(CTT EP497459A1 1993 Veitch. RJ Physicochemical (CTD) EP563769A1 1993 Loustaunau. A Physical (CTD) EP6l5614A1 1994 Loustaunal Physical (CTD) US5,460,117 1994 Veitch, RJ Physicochemical (CTD) US5490.476 1995 Prusik. T Physicochemical (TTD) US5709472 1996 Cannelongo, J F. Physical (CTD) USs,779,364 96 Veitch. RJ. Physical (CTD) EP835429A1 Arens. et al Physicochemical (TTD) US5,667,303 1997 Schneider. N Physical (CTD) US6.030.118 1999 Simons. MJ Physicochemical (CTT EP930488A2 2000 Schaten. BB Physical(CTD) EP1053726A2 Prusik. T Physical (CTTD) US6,042,264 Ram. A.T. Chemical (TTD) US6,103,351 Bray, A.V. Physical (TTD) US6,158,38 2001 Simons. MJ Physicochemical (TTD US6.214.623 Qiu,J Physicochemical (TTD 2002 Qiu, J Physicochemical (TTD US6.435.128
mishandling (Schoen and Byrne, 1972). The first patented indicator goes back to 1933 (Midgley, 1933). Over a hundred US and international patents relevant to time-temperature indicators have been issued since. During the last 30 years numerous TTI systems have been proposed of which only few reached the prototype and even fewer the market stage. Patents dating up to 1990 are tabulated in the literature (Byrne, 1976; Taoukis, Fu and Labuza, 1991). Byrne (1976) gives an overview of the early indicators and Taoukis (1989) presents a detailed history of TTI. Table 6.1 lists significant recent TTI patents classified according to type and principle of operation. The first commercially available TTI was developed by Honeywell Corp (Minneapolis, MN) (Renier and Morin, 1962). The device never found commercial application, possibly because it was costly and relatively bulky. In the early 1970s, the US government considered mandating the use of indicators on certain products (OTA, 1979). This generated a flurry of research and development. Researchers at the US Army Natick Laboratories developed a TTI that was based on the colour change of an oxidisable chemical system controlled by the temperature dependent permeation of oxygen through a film (Hu, 1972). Field testing over a two-year period with the TTI attached to rations showed their potential for use (Killoran, 1976). The system was contracted to Artech Corp (Falls Church, VA) for commercial development. By 1976 six companies were making temperature Table 6.1 List of recent TTI patents and classification according to type and mode of response. Date Inventor Principle of operation Patent No 1991 Jalinski, T.J. Chemical (TTI) US5,182,212 1991 Jalinski, T.J. Chemical (TTI) US5,085,802 1991 Thierry, A. Chemical (CTI) US5,085,801 1991 Swartzel, K.R. Physicochemical (TTI) US5,159,564 1992 Jalinski, T. Chemical (CTI) EP497459A1 1993 Veitch, R.J. Physicochemical (CTI) EP563769A1 1993 Loustaunau, A. Physical (CTI) EP615614A1 1994 Loustaunau, A. Physical (CTI) US5,460,117 1994 Veitch, R.J. Physicochemical (CTI) US5,490,476 1995 Prusik, T. Physicochemical (TTI) US5,709,472 1996 Cannelongo, J.F. Physical (CTI) US5,779,364 1996 Veitch, R.J. Physical (CTI) EP835429A1 1997 Arens R. et al. Physicochemical (TTI) US5,667,303 1997 Schneider, N. Physical (CTI) US6,030,118 1999 Simons, M.J. Physicochemical (CTI) EP930488A2 2000 Schaten, B.B. Physical (CTI) EP1053726A2 2000 Prusik, T. Physical (CTTI) US6,042,264 2000 Ram, A.T. Chemical (TTI) US6,103,351 2000 Bray, A.V. Physical (TTI) US6,158,381 2001 Simons, M.J. Physicochemical (TTI) US6,214,623 2001 Qiu, J. Physicochemical (TTI) US6,244,208 2002 Qiu, J. Physicochemical (TTI) US6,435,128 Time-temperature indicators (TTIs) 107