Fate of Listeria monocytogenes During the Manufacture and Ripening of Parmesan Cheese

¹ Department of Food Science, and The Food Research Institute, University of Wisconsin-Madison, Madison 53706

Parmesan cheese was made from a mixture of pasteurized whole and skim milk that was inoculated to contain ca. 10⁴ to 10 ⁵ cells of Listeria monocytegenes/ml. Curd was cooked at 51°C (124°F) for ca. 45 min. During cheese making, maximum numbers of L. monocytogenes appeared just before cooking; at this point, the increase over initial numbers was a .61 to 1.0 order of magnitude. During cooking of curd, the average decrease in numbers of L. monocytogenes was a .22 order of magnitude. During cheese ripening, numbers of L. monocytogenes decreased almost linearly and faster than reported for other hard cheeses. Listeria monocytogenes strain California died faster than did strain V7. Listeria monocytogenes were not detected in cheese after 2 to 16 wk of ripening, depending on the strain of the pathogen and the lot of cheese. Parmesan cheese made in this study was not a favorable medium for survival of L. monocytogenes.

Key Words: Listeria monocytogenes • fate • Parmesan cheese

Submitted on June 8, 1990
Accepted on July 30, 1990