Time Temperature Indicators

Time-Temperature-Indicators belong to the group of intelligent packagings. Intelligent packaging systems are systems which monitor the condition of a product, detect changes in it and show its actual condition. They are usually applied in the form of labels on the packaging and thereby act as an indicator. These indicators are able to detect and display certain influences on the package and thus on the packaged product. Product information and information about the product history can thus be transmitted in a simple and fast way. Intelligent packaging is applied in the food sector as well as for the transport of valuable art objects, pharmaceuticals and other fragile and sensitive products.

A time-temperature indicator is defined as a small instrument, with which a time-and temperature-depending change can be visualized and thus the full or partial temperature history of a product can be reflected. Its change bases on a physical, enzymatic, chemical or microbiological reaction and will be expressed by an irreversible colour change or a colour movement along a scale. The extent to which this change occurs, stands in relation to the duration and amount of temperature increase. The visible reaction of the indicator is thus a sign of the storage conditions, which was exposed to the product and changing faster at high temperature increase and correspondingly slower at a lower temperature increase [TAOUKIS, P.S. (2001].

Today's most widely used and generally accepted classification provides a categorization of three types of indicators [KREYENSCHMIDT, J., LETTMANN, T. und PETERSEN, B. (2005)]:
  • Critical Temperature-Indicator – CTI
  • Critical Time-Temperature-Indicator – CTTI
  • Time-Temperature-Indicator – TTI


Critical Temperature-Indicators (CTI)

CTI are indicators which detect an exceedance or undercut of a certain indicator- dependent threshold temperature and make it visible. By exceeding a critical temperature, they respond with a time delay and with an irreversible colour change (Period ranges from a few minutes to several hours). They provide no information about the duration, amount and frequency of exceeding temperature, but only visualize the fact that the critical temperature was exceeded. If the threshold temperature is not exceeded, no colour change occurs and the indicator retains its original colour. Thus, they serve as a warning that physico-chemical and biological reactions could have led to a critical change in the quality of the product. This type of indicators applies especially in frozen or refrigerated products, in which already a unique exceedance or undercut of temperature can cause denaturation in the product [TAOUKIS, P.S. (2001]. This kind of indicators will not be treated further – are not content of this project.

Critical Time-Temperature-Indicators (CTTI)

In contrast to the CTI, the CTTI is able to reflect the cumulative time-temperature history of a product above a indicator dependent threshold temperature. If this temperature is not exceeded, analogous to the CTI, no colour change of the indicator takes place. They are used to display critical temperature variations in the cold chain [TAOUKIS, P.S. (2001].

Time-Temperature-Indicators (TTI)

These types of indicators indicate the total time-temperature history of a product, regardless of the exceeding of a threshold temperature. After activation of these indicators the time-and temperature-dependent colour change starts and provides with the continuous change of colour a statement about the present condition of the product. Through this continuous change of colour it is possible to deduce also the average temperature during the entire TUL-Process. Due to the correlation with the kinetic of spoilage of the product can furthermore be made a statement about the quality of the product [TAOUKIS, P.S. (2001]. They serve thus as an additional sell-by date and are primarily used for chilled products with limited shelf life.

Besides the classification by type of information on colour change, indicators can also be classified by the type of mode of action. The decisive factor is the principle of the colour change, which is based on a photochemical, enzymatic, microbiological, diffusion or polymer-based response. A detailed description of the mode of action can be found in the specific TTI descriptions.

Requirements for an ideal TTI:
  • Realibility
  • Changes of indicator are correlated with the degree of spoilage and the remaining shelf life of the product
  • Simple, clear and irreversible indication
  • Each indicator provides consistent values with the same changes
  • Low costs
  • Easy adjustment of product-specific differences
  • Easy integration in packaging and high-speed packaging processes
  • Easy and visible activation
  • Long shelf life before activation and easy storage conditions
  • Resistance to environmental influences such as light, air pollutants and humidity as well as mechanical influences such as pressure and friction
  • Not hazardous to health
  • Simple and understandable information devices for all target groups (producers, trade and consumer)
  • Detected change can be measured both visually and electronically


Correlation of the TTI to the food deterioration

Information about the durability of a product is currently being given almost exclusively by the expiration date. This conventional way only illustrates the stability at recommended storage conditions, but not the actual freshness of the product. TTI, however, are able to view the actual freshness. Starting point for an accurate statement about the condition of the food is the adaptation of the kinetic behaviour of the TTI to the deterioration kinetic of the food. Both are correlated to each other [KREYENSCHMIDT, J., et al. (2003)]. Analogous to the modeling of the deterioration kinetic of the food the kinetic behaviour of the indicator has to be analyzed. Starting with the choice of appropriate parameters, the time-temperature behaviour is studied and presented using this same mathematical model as for the food products. The activation energy represents here the energy needed to initiate the colour change reaction and can be changed by the choice of substances used. To be suitable for monitoring the quality of a food, food and TTI have approximately the same activation energy. Only then the TTI can represent the deterioration kinetic of the food and inform about its freshness [KREYENSCHMIDT, J., et al. (2003)].

Sources:

KREYENSCHMIDT, J., et al. (2003):
Weiterentwicklung von Methoden zur Überprüfung der Einhaltung der Kühlkette bei frischen Lebensmitteln. Bonn : Landwirtschaftliche Fakultät der Universität Bonn, Schriftreihen des Lehr- und Forschungsschwerpunktes USL, 2003. Nr.107.

KREYENSCHMIDT, J. et al. (2003):
Erprobung eines Temperatur-Zeit-Indikators im Rahmen von HACCP-Konzepten bei der Verpackung, Lagerung und dem Transport von Frischprodukten. Bonn: Landwirtschaftliche Fakultät der Universität Bonn, Schriftreihen des Lehr- und Forschungsschwerpunktes USL, 2003. Nr.106.

TAOUKIS, P. S. (2001):
Modelling the use of time-temperature indicators in distribution and stock rotation. L. M. M. Tijskens (Hrsg.), M. Hertog (Hrsg.) & B. M. Nicolai (Hrsg.), Food Process Modelling (S. 402–431). Cambridge: Woodhead Publishing Limited.