THE EFFECT OF PASTEURIZATION ON LISTERIA MONOCYTOGENES

Using the so-called holding technique of pasteurization (61.7 °C., 35 minutes) it was shown that when known numbers of viable Listeria monocytogenes are added to samples of fresh sterile milk, organisms may be recovered whenever the initial number of bacteria added is 5 × 104/ml. or greater. No significant differences in heat resistance among representatives of various Listeria monocytogenes was observed. After 48 hours at room temperature (22 °C.) survivors multiply in such "pasteurized" milk and viable bacterial counts reach 108 per ml. without producing any grossly detectable changes in the milk or suspicious odors.