The impact of prior growth and food matrix conditions on thermal resistance of <i>L. monocytogenes</i>, a causative agent of the foodborne illness listeriosis, was evaluated in this study.

In an initial study ( Main Study #1), acid induced and non-induced cells of thirteen strains of <i>Listeria monocytogenes</i> were placed into two heating menstrua: pH 3.0, water activity (Aw) 0.987 and pH 7.0, Aw 0.970. In twelve out of twenty-six combinations, non-induced cells were more heat resistant than induced cells. <i>L. monocytogenes</i> strain #201, in a follow-up study ( Main Study #2) using a factorial design to test additional combinations of water activity and pH heating menstrua, non-acid-induced cells generally were more heat resistant than acid-induced cells although the acid-induced cells showed greater thermal resistance in the heating menstrua having the lowest pH values.

An increase in thermal resistance could lead to underestimation of treatments necessary to eliminate potential contamination by <i>L. monocytogenes</i>.