TTIs are small measuring devices or tags attached to the food package that show in an easy, accurate and precise way a measurable irreversible change, which is time-temperature dependent, and gives an indication on the quality storage conditions of a food. The irreversible change mimics the change produced in a relevant characteristic, or target attribute, related to food undergoing the same variable temperature exposure [21, 34]. TTIs can be classified in mechanical, chemical, electrochemical, enzymatic or microbiological systems, according to the working principle. The irreversibly change, from the time of their activation, is usually expressed as a visible response in the form of a mechanical deformation, color development or color movement. The visible response reflects the full or partial temperature history that the TTI has been exposed too. The rate of change increases at higher temperatures in a manner similar to the most physicochemical reactions [17, 21, 34, 43]. In this regard, a prerequisite for application of TTI is the systematic kinetic modeling of the temperature dependence of shelf-life products and the same equations applied for the quality loss of the food are used for the response of the TTI [17, 34]. Equation 3 is used to relate the effect of temperature on the TTI response and the EA of the response is obtained. Then, by
R. N. Zúñiga and E. Troncoso 140
using Eq. 6 the value of Teff for the TTI is calculated. If the TTI and the food product quality
deterioration reactions have similar temperature dependence, translated into activation energies differing by less than 25 kJ/mol, the same Teff can be used for the food [34]. In
designing an indicator, it may be important that the activation energy of the device is similar to that of the food deteriorations as well as rate of deterioration [18].
The relevant characteristic of the food to be monitored can be any safety or quality attribute of interest, such as microbial load, loss of a specific vitamin, texture, or color [21, 43]. Of major interest are important hidden quality attributes like microbiologic safety. TTIs can be used to monitor the temperature exposure along time to which a perishable product is subjected (e.g., during distribution of a chilled food), from the point of manufacture to the display shelf at the supermarket and as end point indicators readable by the consumer [17, 21, 34]. Principal advantages and disadvantages in the use of TTIs are listed in Table 2.
Table 2. Main advantages and disadvantages of TTIs
Advantages
Ability to quantify the integrated time-temperature impact on a target attribute of individual products without the need for information on the actual temperature history of the product.
Facilitate scheduling of distribution, thus products approaching the end of their shelf-life are moved first at the warehouse and retail level.
Capacity to indicate problem areas in the distribution system so that they could be resolved and it could reduce food waste and ensure that a consistent quality food product reaches the consumer.
Practical advantage in use over certain other types of monitoring equipment of giving a simple and individual indication of temperature abuse, which makes them an attractive addition to assuring safety and quality to the consumer.
Disadvantages
Using only a TTI to monitor food safety loss would result in error if temperature is not the only rate determining factor, as other factors that change with time can be critical.
A single enzymatic reaction could not be representative of the quality spectrum and temperature profile across the package.
Application to the outside of a food pack means that surface temperatures are being used to change the indicators.
Difference in temperature sensitivity between TTI response and spoilage reaction can result in an accumulative error of the response to the real quality loss.
Several patents have been developed on processes which could be used as a basis for TTIs (Figure 1). These include changes with temperature based on melting-point temperature, enzyme reaction, polymerization, electrochemical corrosion, and liquid crystals. Here, we updated the list of TTI patents tabulated by Taoukis [34], listing relevant TTI patents from the year 2000 to the present (Table 3). However, only few types of TTIs are current commercial products for food applications, for instance we can mention: Fresh-Check® from TEMPTIME, based on polymerization reaction (www.freshcheck.com); (eO)TM from
Shelf-Life Calculation and Temperature-Time Indicators 141 CRYOLOG, based on bacterial lactic acid production (www.cryolog.com); and CheckPoint® I and III from Vitsab, based on enzymatic and diffuse reaction leading to pH change (www.vitsab.com). Year 1985 1990 1995 2000 2005 2010 N um be r o f pa te nt s 0 2 4 6 8 10 12 14 16 18
Figure 1. Time evolution of patents related with TTIs. The search was done using the Matheo PatentTM software version 9.61 (Matheo Software, Marseille, France) in the database of patents solicited in USA and Europe, using the following search: ―temperature‖ and ―indicator‖ or ―integrator‖. Most of the patents found belong to the following International Patent Classification: G01K3/04 Detection of temperature changes to compensate the measurement of other variables or compensation of lectures over instruments, taking into account temperature variations.
Table 3. Selected list of TTI patents published during the last ten years. The search was done using the Matheo PatentTM software version 9.61 (Matheo Software, Marseille,
France) in the database of patents solicited in USA and Europe, using the following search: “temperature” and “indicator” or “integrator”
Date Inventor Patent name Patent number
2003 Tester, R; Al-Ghazzewi, F Time temperature indicators linked to sensory detection
WO03006941A1 2003 Spevacek, JA Time-temperature integrating
indicator
WO03025530A1 2004 Zweig, SE Electronic time-temperature
indicator and logger
WO2004097357A2 2005 Levy, Y; Haarer, D Time-temperature indicator based
on valence isomerizations
WO2005075978A2 2005 Varlet-Grancher, X Time temperature indicator (TTI)
system
WO2005078402A1 2006 Reichert, H;
Simmendinger, P; Bolle T
Enzyme-based time temperature indicator
WO2006015961A2
R. N. Zúñiga and E. Troncoso 142
Table 3. (Continued)
Date Inventor Patent name Patent number
2006 Azizian, F; Leonard, MW; Herlihy, SL Novel time/temperature indicators WO2006091466A1 2006 Azizian, F; Leonard, MW; Herlihy, SL Time/temperature indicators, their preparation and use
WO2006091465A1 2006 Azizian, F; Leonard, MW; Herlihy, SL Improved time/temperature indicators WO2006091464A1 2007 Craig, V; Senden, TJ; Kugge, C
Time-temperature indicators WO2007012132A1 2007 Barmore, C Carbon monoxide modified
atmosphere packaging having a time temperature indicator
US2007059402A1
2007 Azizian, F; Leonard, MW Improved time/temperature indicators
WO2007035365A1 2007 Weder, C; Kinami, M;
Crenshaw, B
Time-temperature indicators US2007158624A1 2007 Patel, G Time-temperature, UV exposure
and temperature indicator
WO2007117273A2
2007 Ren, FC Enzyme type time-temperature indication card production method
CN101055255A
2008 Feuerstack, M; Jannasch, U
Time temperature indicator for identifying goods
EP1882919A2 2008 Haarer, D; Gueta-
Neyroud, T; Salman, H
Time temperature indicator WO2008083926A1 2009 Haarer, D; Gueta-
Neyroud, T; Salman, H
Time temperature indicator WO2009156285A1 2009 Xinglian X, Guanchong Z;
Nan, Z; Peng N
Enzyme type time-temperature indication card
CN201255668Y 2010 Reichert, H; Huegin, M;
Dueggeli, M; Feiler, L
Time temperature indicator comprising indolenin based spiropyrans
WO2010079114A1
2010 Salman, H; Tenetov, H Time-temperature indicator based on thioalkyl and thioaryl
substituted spiroaromatics
WO2010092030A1
2010 Ying, C; Lixin, L; Zhiye, L
Time-temperature indicator based on lipase reaction CN201512536U 2010 Zhilu, A; Jianxin, L; Meijuan, L; Zhili, P; Xiaorui, S; Na, W; Xinhua, X Diffusible time-temperature indicator, indicator card and preparation method of the indicator card
CN101718597A
Shelf-Life Calculation and Temperature-Time Indicators 143
Date Inventor Patent name Patent number
2010 Wenjie, Y; Xingmin, L; Yuan, A; Danyang, C; Zheng, W; Ruifeng, R
Method for manufacturing microbial time-temperature indicator card
CN101900685A
2011 Lixin, L; Ying, C; Weizhou, Z
Preparation method and application of time temperature indicator based on lipase reaction diffusion
CN102175677A
2012 Huffman, B; Smith, DE; Lentz, CM
Temperature-activatable time- temperature indicator
WO2012050824A1
Utilizing TTIs to predict the extent of quality change in a product based on temperature history provides the link for implementing a quality-based criterion for inventory management [15]. Kouki et al. [8] performed a survey in which the objective was to answer the question of whether the TTI technology can effectively reduce the total operating cost. The researchers formulated an inventory model for perishable foods and derived the operating cost of the model with or without a TTI technology applied to the inventory system. It was showed that for a food product with an average shelf-life of 7 days, TTI can reduce operating costs in a range of 7 to 37%, depending on the variability of the shelf-life. The authors concluded that TTIs can considerably improve the inventory management, but the reduction in total operating cost depending on TTI price.