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