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
108 Novel food packaging techniques indicators at least at the prototype stage(Kramer and Farquhar, 1976). The Artech, the Check Spot Co(Vancouver, WA)(US patent 2, 971, 852)and the Tempil (s Plainfield, NJ)indicators could be classified as CTl. The I-Point(Malmo Sweden), the Bio-Medical Sciences(Fairfield, N)(US patents 3, 946, 611 and 4,042, 336)and the 3M Co (St Paul, MN) indicators were TTI. The Tempil indicator could function as a Cttl. It involved a change to a red colour and subsequent movement when exposed above the critical temperature. The l-Point was an enzymatic TTl, and the 3M, a diffusion based TTI By the end of the 1970s, however, very little commercial application of the TTI had been achieved. Research and development activity subsided temporarily, noted by a decrease in the relevant publications and in the new TTI models introduced. However, the better systems remained available and development continued, aiming at fine tuning and making performance more consistent. In the early 1980s, there were four systems commercially available including the l-Point and the 3M TTI. Andover Labs(Weymouth, MA) marketed the Ambitemp and Tempchron devices up to 1985. Both were for use in frozen food distribution and could be classified as CTTl. Their operation was based on the displacement of a fluid along a capillary 6.5 Current TTi svstems In the last fifteen years three types of Tfi have been the focus of both scientific and industrial trials. They claim to satisfy the requirements of a successful TTI and have evolved as the major commercial types on the market. They are described in detail in the following sections 6.5.1 Diffusion-based TTIs The 3M Monitor Mark(3M Co., St Paul, Minnesota)(US Patent, 3,954,011 1976)is a diffusion based indicator. One of the first significant applications ofT was the use of this indicator by the World Health Organization(WHO)to monitor refrigerated vaccine shipments. The response of the indicator is the advance of a blue dyed ester diffusing along a wick. The useful range of temperatures and the response life of the TTI are determined by the type of ester and the concentration at the origin. Thus the indicators can be used either as ctri with the critical temperature equal to the melting temperature of the ester or as TTI if the melting temperature is lower than the range of temperatures the food is stored at, e.g below oC for chilled storage. The same company has marketed the successor to this TTl: the Monitor Mark Temperature Monitor(Fig. 6. 1)and Freshness Check based on diffusion of proprietary polymer materials (US patent 5,667, 303) A viscoelastic material migrates into a diffusely light-reflective porous matrix at a temperature dependent rate. This causes a progressive change of the light transmissivity of the porous matrix and provides a visual response. The response rate and temperature dependence is controlled by the tag configuration
indicators at least at the prototype stage (Kramer and Farquhar, 1976). The Artech, the Check Spot Co (Vancouver, WA) (US patent 2,971,852) and the Tempil (S Plainfield, NJ) indicators could be classified as CTI. The I-Point (Malmo¨, Sweden), the Bio-Medical Sciences (Fairfield, NJ) (US patents 3,946,611 and 4,042,336) and the 3M Co (St Paul, MN) indicators were TTI. The Tempil indicator could function as a CTTI. It involved a change to a red colour and subsequent movement when exposed above the critical temperature. The I-Point was an enzymatic TTI, and the 3M, a diffusion based TTI. By the end of the 1970s, however, very little commercial application of the TTI had been achieved. Research and development activity subsided temporarily, noted by a decrease in the relevant publications and in the new TTI models introduced. However, the better systems remained available and development continued, aiming at fine tuning and making performance more consistent. In the early 1980s, there were four systems commercially available including the I-Point and the 3M TTI. Andover Labs (Weymouth, MA) marketed the Ambitemp and Tempchron devices up to 1985. Both were for use in frozen food distribution and could be classified as CTTI. Their operation was based on the displacement of a fluid along a capillary. 6.5 Current TTI systems In the last fifteen years three types of TTI have been the focus of both scientific and industrial trials. They claim to satisfy the requirements of a successful TTI and have evolved as the major commercial types on the market. They are described in detail in the following sections. 6.5.1 Diffusion-based TTIs The 3M Monitor MarkÕ (3M Co., St Paul, Minnesota) (US Patent, 3,954,011, 1976) is a diffusion based indicator. One of the first significant applications of TTI was the use of this indicator by the World Health Organization (WHO) to monitor refrigerated vaccine shipments. The response of the indicator is the advance of a blue dyed ester diffusing along a wick. The useful range of temperatures and the response life of the TTI are determined by the type of ester and the concentration at the origin. Thus the indicators can be used either as CTTI with the critical temperature equal to the melting temperature of the ester or as TTI if the melting temperature is lower than the range of temperatures the food is stored at, e.g., below 0ºC for chilled storage. The same company has marketed the successor to this TTI: the Monitor Mark Temperature Monitor (Fig. 6.1) and Freshness Check, based on diffusion of proprietary polymer materials (US patent 5,667,303). A viscoelastic material migrates into a diffusely light-reflective porous matrix at a temperature dependent rate. This causes a progressive change of the light transmissivity of the porous matrix and provides a visual response. The response rate and temperature dependence is controlled by the tag configuration, 108 Novel food packaging techniques
Time-temperature indicators (TTIs) 109 666 Fig. 6.1 Diffusion based TTI the diffusing polymers concentration and its glass transition temperature and can be set at the desirable range( Shimoni, Anderson and Labuza, 2001). The TTI is activated by adhesion of the two materials. Before use these materials can be stored separately for a long period at ambient temperature 6.5.2 Enzymatic ttis The VITSAB Time Temperature Indicator is an enzymatic indicator. It is the uccessor of the 1-Point Time Temperature Monitor(VITSAB A B, Malmo Sweden). The indicator is based on a colour change caused by a ph decrease which is the result of a controlled enzymatic hydrolysis of a lipid substrate (US Patents 4,043, 871 and 4, 284, 719). Before activation the indicator consists of two separate compartments, in the form of plastic mini-pouches. One compartment contains an aqueous solution of a lipolytic enzyme, such as pancreatic lipase The other contains the lipid substrate absorbed in a pulverised Pvc carrier and suspended in an aqueous phase and a pH indicator mix. Glycerine tricapronate (tricaproin), tripelargonin, tributyrin and mixed esters of polyvalent alcohols and organic acids are included in substrates Different combinations of enzyme-substrate types and concentrations used to give a variety of response lives and temperature depender activation, enzyme and substrate are mixed by mechanically breaking the barrier that separates the two compartments. Hydrolysis of the substrate(e.g, tricaproin) causes acid release(e.g, caproic acid) and the ph drop is translated in a colour change of the pH indicator from deep green to bright yellow. Reference starting and end point colours are printed around the reaction window to allow easier visual recognition and evaluation of the colour change(Fig. 6.2). The continuous colour change can also be measured instrumentally (Taoukis and Labuza, 1989). The TTl is claimed to have a long shelf-life if kept chilled before activation 6.5.3 Polymer-based TTls The Lifelines Freshness Monitor and Fresh-Check indicators(Lifelines Inc Morris Plains, NJ) are based on a solid state polymerisation reaction (US Patent, 3, 999, 946 and 4, 228, 126)(Fields and Prusik, 1983). The TTi function
the diffusing polymer’s concentration and its glass transition temperature and can be set at the desirable range (Shimoni, Anderson and Labuza, 2001). The TTI is activated by adhesion of the two materials. Before use these materials can be stored separately for a long period at ambient temperature. 6.5.2 Enzymatic TTIs The VITSAB Time Temperature Indicator is an enzymatic indicator. It is the successor of the I-Point Time Temperature Monitor (VITSAB A.B., Malmo¨, Sweden). The indicator is based on a colour change caused by a pH decrease which is the result of a controlled enzymatic hydrolysis of a lipid substrate (US Patents 4,043,871 and 4,284,719). Before activation the indicator consists of two separate compartments, in the form of plastic mini-pouches. One compartment contains an aqueous solution of a lipolytic enzyme, such as pancreatic lipase. The other contains the lipid substrate absorbed in a pulverised PVC carrier and suspended in an aqueous phase and a pH indicator mix. Glycerine tricapronate (tricaproin), tripelargonin, tributyrin and mixed esters of polyvalent alcohols and organic acids are included in substrates. Different combinations of enzyme-substrate types and concentrations can be used to give a variety of response lives and temperature dependencies. At activation, enzyme and substrate are mixed by mechanically breaking the barrier that separates the two compartments. Hydrolysis of the substrate (e.g., tricaproin) causes acid release (e.g., caproic acid) and the pH drop is translated in a colour change of the pH indicator from deep green to bright yellow. Reference starting and end point colours are printed around the reaction window to allow easier visual recognition and evaluation of the colour change (Fig. 6.2). The continuous colour change can also be measured instrumentally (Taoukis and Labuza, 1989). The TTI is claimed to have a long shelf-life if kept chilled before activation. 6.5.3 Polymer-based TTIs The Lifelines Freshness MonitorÕ and Fresh-CheckÕ indicators (Lifelines Inc, Morris Plains, NJ) are based on a solid state polymerisation reaction (US Patent, 3,999,946 and 4,228,126) (Fields and Prusik, 1983). The TTI function Fig. 6.1 Diffusion based TTI. Time-temperature indicators (TTIs) 109
110 Novel food packaging techniques A圆 Fig 6.2 Enzymatic TTI based on the property of disubstituted diacetylene crystals(r-C=C-C C-R) to polymerise through a lattice-controlled solid-state reaction proceeding via 1, 4-addition polymerisation and resulting in a highly coloured polymer. During polymerisation, the crystal structure of the monomer is retained and the polymer crystals remain chain aligned and are effectively one dimensional in their optical properties(Patel and Yang, 1983). The response of the TTI is the colour change measured as a decrease in reflectance The Freshness Monitor consists of an orthogonal piece of laminated paper the front face of which includes a strip with a thin coat of the colourless diacetylenic monomer and two barcodes, one about the product and the other identifying the model of the indicator. The Fresh-Check version, for consumers. is round. and the colour of the 'active centre of the tti is compared to the reference colour of a surrounding ring(Fig. 6.3). The laminate has a red or yellow colour so that the change is perceived as a change from transparent to black. The reflectance of the Freshness Monitor can be measured by scanning with a laser optic wand and stored in a hand-held mea supplied by the TTI producer. The response of Fresh Scan can be device visually evaluated in comparison to the reference ring or continuously ured by a portable colorimeter or an optical densitometer. Before use, the ndicators,active from the time of production, have to be stored deep frozen fresh- Check③ ● Fig 6.3 Polymer based TTI
is based on the property of disubstituted diacetylene crystals (RÿC = CÿC = CÿR) to polymerise through a lattice-controlled solid-state reaction proceeding via 1,4-addition polymerisation and resulting in a highly coloured polymer. During polymerisation, the crystal structure of the monomer is retained and the polymer crystals remain chain aligned and are effectively one dimensional in their optical properties (Patel and Yang, 1983). The response of the TTI is the colour change measured as a decrease in reflectance. The Freshness Monitor consists of an orthogonal piece of laminated paper the front face of which includes a strip with a thin coat of the colourless diacetylenic monomer and two barcodes, one about the product and the other identifying the model of the indicator. The Fresh-CheckÕ version, for consumers, is round, and the colour of the ‘active’ centre of the TTI is compared to the reference colour of a surrounding ring (Fig. 6.3). The laminate has a red or yellow colour so that the change is perceived as a change from transparent to black. The reflectance of the Freshness Monitor can be measured by scanning with a laser optic wand and stored in a hand-held device supplied by the TTI producer. The response of Fresh Scan can be visually evaluated in comparison to the reference ring or continuously measured by a portable colorimeter or an optical densitometer. Before use, the indicators, active from the time of production, have to be stored deep frozen where change is very slow. Fig. 6.2 Enzymatic TTI. Fig. 6.3 Polymer based TTI. 110 Novel food packaging techniques
Time-temperature indicators(TTIs) 111 6.6 Maximising the effectiveness of Ttis Despite the potential of TTIs to contribute substantially to improved food distribution, reduce food waste and benefit the consumer with more meaningful shelf-life labelling, their application up to now has not lived up to the initial expectations. The main reasons for the reluctance of food producers to adopt the TTI have been eliability Cost is volume dependent, ranging from 2 to 20 US cents per unit. If other questions are resolved, cost-benefit analysis should favour use of TTIs. The reliability question has its roots in the history of indicators, due partly to lack of sufficient data, both from studies and from the suppliers. Initial attempts at using TTI as quality monitors were not well designed and hence unsuccessful Re-emerging discussions by regulatory agencies to make TTI use mandatory before the underlying concepts were understood and their reliability demonstrated, resulted in resistance by the industry and may have hurt TTT application up to the present time. Current TI systems have achieved high standards of production quality assurance and provide reliable and reproducible responses according to the specifications stated. Testing standards have been issued by the bsi and can be used by tti manufacturers as well as TTI users(BS 7908: 1999) 6 The question of applicability, however, has been the most substantial hurdle TtI use. earlier studies have been ineffective in establishing a clear methodology on how the TTI response can be used as a measure of food quality The initial approach was to assume an overall temperature dependence curve(or zone) for the shelf-life of a general class of foods, e.g., frozen foods, and aim for an indicator that has a similar temperature dependence curve for the time to reach a specific point on its scale. Such a generalisation has proved insufficient, as even foods of the same type differ significantly in the temperature dependence of the deterioration in their quality. What is needed is a thorough knowledge of the shelf-life loss behaviour of the food system through accurate kinetic models It has been widely assumed that the behaviour of a TTI should strictly atch that of the particular food to be monitored at all temperatures. This approach, even if feasible, is impractical, and requires an unlimited number of TTI models. Instead of a TTI exactly mimicking quality deterioration behaviour of the food product, a meaningful, general scheme of translating TTI response to food status is needed. This should be based on systematic modelling of both the TTI and the food. Advances in modelling are now making this possible. Current developments in this area are reviewed by Taoukis(2001). The following sections discuss how modelling contributes to the practical use of TTIs
6.6 Maximising the effectiveness of TTIs Despite the potential of TTIs to contribute substantially to improved food distribution, reduce food waste and benefit the consumer with more meaningful shelf-life labelling, their application up to now has not lived up to the initial expectations. The main reasons for the reluctance of food producers to adopt the TTI have been: • cost • reliability • applicability. Cost is volume dependent, ranging from 2 to 20 US cents per unit. If other questions are resolved, cost-benefit analysis should favour use of TTIs. The reliability question has its roots in the history of indicators, due partly to lack of sufficient data, both from studies and from the suppliers. Initial attempts at using TTI as quality monitors were not well designed and hence unsuccessful. Re-emerging discussions by regulatory agencies to make TTI use mandatory, before the underlying concepts were understood and their reliability demonstrated, resulted in resistance by the industry and may have hurt TTI application up to the present time. Current TTI systems have achieved high standards of production quality assurance and provide reliable and reproducible responses according to the specifications stated. Testing standards have been issued by the BSI and can be used by TTI manufacturers as well as TTI users (BS 7908:1999). The question of applicability, however, has been the most substantial hurdle to TTI use. Earlier studies have been ineffective in establishing a clear methodology on how the TTI response can be used as a measure of food quality. The initial approach was to assume an overall temperature dependence curve (or zone) for the shelf-life of a general class of foods, e.g., frozen foods, and aim for an indicator that has a similar temperature dependence curve for the time to reach a specific point on its scale. Such a generalisation has proved insufficient, as even foods of the same type differ significantly in the temperature dependence of the deterioration in their quality. What is needed is a thorough knowledge of the shelf-life loss behaviour of the food system through accurate kinetic models. It has been widely assumed that the behaviour of a TTI should strictly match that of the particular food to be monitored at all temperatures. This approach, even if feasible, is impractical, and requires an unlimited number of TTI models. Instead of a TTI exactly mimicking quality deterioration behaviour of the food product, a meaningful, general scheme of translating TTI response to food status is needed. This should be based on systematic modelling of both the TTI and the food. Advances in modelling are now making this possible. Current developments in this area are reviewed by Taoukis (2001). The following sections discuss how modelling contributes to the practical use of TTIs. Time-temperature indicators (TTIs) 111
1 12 Novel food packaging techniques 6.7 Using ttis to monitor shelf-life during distribution TTIs can be used to monitor the temperature exposure of food products during distribution, from production up to the time they are displayed at the < permarket. Attached to individual cases or pallets they give a measure of preceding temperature conditions at selected control points. Information from TTIs can be used for continuous, overall monitoring of the distribution system, leading to recognition and correction of weak links in the chain Furthermore, it serves as a proof of compliance with contractual requirements by the producer and distributor. It can guarantee that a properly handled product was delivered to the retailer, thus eliminating the possibility of unsubstantiated rejection claims by the latter. The presence of the TTI itself would probably improve handling, serving as an incentive and reminder to distribution employees throughout the distribution chain of the importance of proper temperature storage The same TTls can be used as shelf-life end point indicators readable by the consumer and attached to individual products. Tests using continuous instrumental readings to define the end point under constant and variable temperatures showed that such end points could be reliably and accurately recognised by panellists (Sherlock et al., 1991). However, for a successful application of this kind there is a much stricter requirement that the t response matches the behaviour of the food. To achieve this the TTI end point should coincide with the end of shelf-life at one reference temperature and the activation energy should differ by less than 10kJ/mol from that of the food. In this way the TTI attached to individually packaged products can serve as active shelf-life labelling instead of, or in conjunction with, open date labelling. The TTI assures the consumer that the product was properly handled and indicates the remaining shelf-life. Consumer surveys have shown that consumers can be very receptive to the idea of using these tTi on dairy products along with the date code( Sherlock and Labuza, 1992). Use of TTI can thus also be an effective marketing tool. Diffusion-based TTls have been used in this way by the Cub Foods Supermarket chain in the USA and polymer-based TTIs by the Monoprix chain in France and the Continent stores in Spain A number of experimental studies have sought to establish correlations between the response of specific TTls and quality characteristics of specific products. Foods have been tested at different temperatures, plotting the response of the TTI v. time and the values of selected quality parameters of the foods before testing the statistical significance of the TTI response correlation to the quality parameters. Foods correlated to TTI include pasteurised whole milk(Mistry and Kosikowski, 1983; Grisius et al., 1987, Chen and zall, 1987) ice cream(Dolan et al., 1985) frozen hamburger(Singh and Wells, 1985) chilled cod fillets(Tinker et al., 1985) refrigerated ready to eat salads( Cambell, 1986)
6.7 Using TTIs to monitor shelf-life during distribution TTIs can be used to monitor the temperature exposure of food products during distribution, from production up to the time they are displayed at the supermarket. Attached to individual cases or pallets they give a measure of the preceding temperature conditions at selected control points. Information from TTIs can be used for continuous, overall monitoring of the distribution system, leading to recognition and correction of weak links in the chain. Furthermore, it serves as a proof of compliance with contractual requirements by the producer and distributor. It can guarantee that a properly handled product was delivered to the retailer, thus eliminating the possibility of unsubstantiated rejection claims by the latter. The presence of the TTI itself would probably improve handling, serving as an incentive and reminder to distribution employees throughout the distribution chain of the importance of proper temperature storage. The same TTIs can be used as shelf-life end point indicators readable by the consumer and attached to individual products. Tests using continuous instrumental readings to define the end point under constant and variable temperatures showed that such end points could be reliably and accurately recognised by panellists (Sherlock et al., 1991). However, for a successful application of this kind there is a much stricter requirement that the TTI response matches the behaviour of the food. To achieve this the TTI end point should coincide with the end of shelf-life at one reference temperature and the activation energy should differ by less than 10kJ/mol from that of the food. In this way the TTI attached to individually packaged products can serve as active shelf-life labelling instead of, or in conjunction with, open date labelling. The TTI assures the consumer that the product was properly handled and indicates the remaining shelf-life. Consumer surveys have shown that consumers can be very receptive to the idea of using these TTI on dairy products along with the date code (Sherlock and Labuza, 1992). Use of TTI can thus also be an effective marketing tool. Diffusion-based TTIs have been used in this way by the Cub Foods Supermarket chain in the USA and polymer-based TTIs by the Monoprix chain in France and the Continent stores in Spain. A number of experimental studies have sought to establish correlations between the response of specific TTIs and quality characteristics of specific products. Foods have been tested at different temperatures, plotting the response of the TTI v. time and the values of selected quality parameters of the foods before testing the statistical significance of the TTI response correlation to the quality parameters. Foods correlated to TTI include: • pasteurised whole milk (Mistry and Kosikowski, 1983; Grisius et al., 1987; Chen and Zall, 1987) • ice cream (Dolan et al., 1985) • frozen hamburger (Singh and Wells, 1985) • chilled cod fillets (Tinker et al., 1985) • refrigerated ready to eat salads (Cambell, 1986) 112 Novel food packaging techniques