Survival of Listeria monocytogenes in raw milk treated in a pilot plant size pasteurizer.

The survival of Listeria monocytogenes in raw milk treated in a pilot plant size pasteurizer was investigated. Raw milk was inoculated with different initial concentrations of L. monocytogenes and heated at temperatures ranging from 69 degrees to 73 degrees C. Listerias were not isolated from any of the milk samples immediately after thermal treatment. They were isolated, however, from 46.6% of heated samples (none from samples heated at 73 degrees C) after variable periods at refrigeration temperature. The results suggest that a low number of listerias survive some thermal treatments, but a cold enrichment is necessary to repair the thermally injured cells and detect these organisms in milk. The importance of the isolation technique in the recovery of listerias from pasteurized milk samples is discussed.