Heat resistance of Listeria monocytogenes in naturally infected and inoculated cow's milk.

A two phase slug flow tubular heat exchanger was used for the thermal inactivation of Listeria monocytogenes in natural infected milk from seven cows. L. monocytogenes serotype 4b inoculated UHT sterilized milk was monitored in a parallel study. The two milks were heated at 71.1 degrees C for holding times of 2, 4, 10, 15, 20 and 30 s. Milk was assayed for survivors immediately after heat treatment and weekly thereafter for 4 weeks during storage at 4 degrees C. No survivors were detected in the naturally infected milk at any of the holding times. Survivors were found at 2 and 4 s in the inoculated UHT milk with initial titres of 8 x 10(2) to 7.1 x 10(3) c.f.u./ml, only after storage at 4 degrees C for 28 days. No survivors were detected for 10 through 30 s holding times.