<i>Listeria monocytogenes</i>: heat resistance and behaviour during storage of milk and whey and making of Dutch types of cheese

Abstract

The heat resistance of Listeria monocytogenes suspended in milk was determined with a plate pasteurizer and glass capillary tubes. The reduction of the, number of Listeria obtained by the plate pasteurizer was ,,;5 log 10 units at 64.0 °C for 10 s and 6-7 log 10 units at 67.0 °C for 10 s. The heat resistance of freely suspended and phagocyted Listeria was investigated with glass cap­illary tubes. The surviving Listeria were determined with and without resuscitation at 4 °C. Using the resuscitation method it appeared that phagocytosis did increase the heat resistance of one of the two tested Listeria strains by factor 2. With the resuscitation method up to 30 times more Lis­teria could be recovered than without resuscitation. The estimated decimal reduction times for the most heat-resistant strain was about 17 sat 64.0 °C and 8 sat 66.0 °C. Incubation tests of L. monocytogenes in raw and HTST-pasteurized milk (15 s 72 °C) at 4 and 7 °C showed some injury of Listeria during the first 1-2 days. Thereafter growth commenced. In intensively pasteurized milk (30 min 98 °C) Listeria was not injured. Gouda and Maasdam types of cheese were made from Listeria-contaminated milk. During the manufacture Listeria was concentrated in the curd by factor 10, and a limited growth occurred. During 6 weeks of ripening at 13 °C the number of Listeria was almost constant