Loss of Viability by Listeria monocytogenes in Commercial Bovine Pepsin-Rennet Extract

Abstract

Loss of viability by Listeria monocytogenes strains California, V7, and Scott A in commercial bovine pepsin-rennet extract was determined during storage for 56 d at 7°C. Four levels (103 to 106/ml) of L. monocytogenes were added to the coagulant, and McBride listeria agar was used to determine numbers of survivors. Selected colonies thought to be L. monocytogenes were confirmed biochemically. Samples also were tested during and after completion of cold enrichment (up to 8 wk at 4°C). Coagulant inoculated with 103 to 104L. monocytogenes/ml usually was free of viable cells of the pathogen after 28 d and sometimes after 14 d, as determined by direct plating and cold enrichment. When the inoculum was 105 to 106 cells/ml, samples of coagulant usually were free of viable L. monocytogenes after 42 d and sometimes after 28 d. The three strains of L. monocytogenes behaved similarly, although strain California was somewhat less hardy in the environment of the coagulant than were the other two strains. Bovine pepsin-rennet extract, before inoculation, was free of L. monocytogenes (direct plating and cold enrichment).