Thermal inactivation kinetics of three vegetative bacteria as influenced by combined temperature and pH in a liquid medium

Abstract

The kinetics of thermal inactivation as affected by combined temperature and liquid pH for three vegetative bacteria— Escherichia coli (ATCC 25922), Listeria monocytogenes (SLCC 5764) and Pseudomonas fluorescens (172)— have been studied using published bench-scale data and additional experimentally determined data from the heating of samples in ampoules of thin-walled glass. These bacteria represent common micro-organisms known to grow in solid and liquid foods. Up to six levels of temperature (52, 54, 56, 58, 60 and 62° C) in combination with up to eight levels of pH (4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0 and 7.5) with exposure times ranging from 10 seconds to five minutes, were used in experimental designs to cover the wide biokinetic range of interest. The carrier liquid, a 2 kg m−3 mucilage of Carbopol® 934 was selected because of the stability of its viscosity over the range of temperature and pH values, and its resistance to bacterial growth. The viscosity of this mucilage closely simulates that of a range of liquid foods. The effect of pH on the rate of thermal inactivation was significant for all three bacteria, especially at the lower exposure temperatures. However, because survivor data showed tailing with longer exposure times, the widely held assumption of first-order kinetics for thermal inactivation of vegetative bacteria is not supported. Concave-up tails appear in the data for P. fluorescens and E. coli and both concave-up and concave-down tails appear in the data for L. monocytogenes. Taken together, data for all three bacteria support the necessity for a model formulation for non-linear survivor kinetics as influenced by combined exposure temperature and liquid pH.