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Appl. Environ. Microbiol. doi:10.1128/AEM.02717-07
Copyright (c) 2008, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Development of a microbial Time Temperature Indicator (TTI) prototype for monitoring microbiological quality of chilled foods

Laboratory of Food Chemistry & Biochemistry, Department of Food Science and Technology, School of Agriculture, Aristotle University, Thessaloniki, Greece; Laboratory of Food Hygiene & Microbiology, Department of Food Science and Technology, School of Agriculture, Aristotle University, Thessaloniki, Greece

^* To whom correspondence should be addressed. Email: kkoutsou{at}agro.auth.gr.

	Abstract

A Time Temperature Indicator (TTI) system based on the growth and metabolic activity of a Lactobacillus sakei strain was developed for monitoring food quality throughout the chilled food chain. In the designed system, an irreversible color change of a chemical chromatic indicator (from red to yellow) progressively occurs due to the pH decline as a result of microbial growth and metabolism in a selected medium. The relation of the TTI response (color change) to the growth and metabolic activity (glucose consumption, lactic acid production, pH decrease) of L. sakei was studied. In addition, the temperature dependence of the TTI kinetics has been investigated isothermally, in the range of 0 to 16 °C, and modeled with a system of differential equations. At all temperatures tested, the pH and color change of the TTI followed closely the growth of L. sakei, with the end point (time at which a distinct visual color change to the final yellow was observed) of the TTI coinciding with a population level of 10⁷-10⁸ cfu/ml. The end point decreased from 27 days at 0 °C to 2.5 days at 16 °C, yielding an activation energy of 97.7 kJ/mol which was very close to the activation energy of L. sakei growth rate in the TTI substrate (103.2 kJ/mol). Furthermore, experiments conducted on the effect of inoculum level showed a negative linear relationship between the level of L sakei inoculated in the system medium and the end point of the TTI. For example, the end point at 8 °C ranged from 6 to 2 days for inoculum levels varying between 10¹ and 10⁶ CFU/ml, respectively. The above relationship provides the ability of easily adjusting the TTI end point at a certain temperature according to the shelf life of the food product of concern by using an appropriate inoculum level of L sakei. The microbial TTI prototype developed in the present study could be used as an effective tool for monitoring shelf life during distribution and storage of food products that are primarily spoiled by lactic acid bacteria or other bacteria exhibiting similar kinetic response and spoilage potential. Apart from the low cost, the main advantage of the proposed TTI is that its response closely matches the quality loss of a food by simulating the microbial spoilage process in such an environment.