EFFECT OF HEATING RATE ON THE THERMAL INACTIVATION OF LISTERIA-MONOCYTOGENES

In order to quantify the effect of heating rate on the thermal inactivation of Listeria monocytogenes an accurate means of describing the inactivation kinetics at near instantaneous heating was used. Survivor curves for L. monocytogenes, at near instantaneous heating, were obtained over the temperature range 50-64 degrees C. The use of a linear function to describe the data would have given only a poor approximation of the true inactivation kinetics. With a model based on a logistic algorithm extremely accurate descriptions were made. In processes which had rates of heating less than or equal to 5.0 degrees C min(-1), significant deviations of real kill from predicted kill were observed. Predicted kill assumed that heating rate did not affect the inactivation kinetics of a thermal process. At rates of heating between 5.0 and 0.7 degrees C min(-1) the deviation greatly increased as the rate of heating decreased; approximately a 1.7 x 10(5)-fold difference at 0.7 degrees C min(-1). Maximum thermotolerance was induced at rates of heating less than or equal to 0.7 degrees C min(-1). The increased thermotolerance during slow rates of heating was analogous to the induction of the heat-shock response. The models described in this work allow for confident assessments of safety to be made not only at near instantaneous heating but also when the heating rate varies.