Fate of Listeria monocytogenes During the Manufacture of Mozzarella Cheese.

Mozzarella cheese was made from a mixture of pasteurized whole and skim milk which was inoculated to contain 104-105 CFU Listeria monocytogenes (strain Ohio, California, or V7) per ml. Temperature of milk was maintained at 40°C (104°F) for 30 min when curd became resilient and the pH reached 5.90-5.93. Populations of L. monocytogenes changed at different rates during the various phases of making Mozzarella cheese. During the early stages of curd formation, numbers of L. monocytogenes were ca. 4-fold greater in curd than in whey. Numbers of L. monocytogenes in freshly cut curd were 25 to 38% greater than in inoculated milk. Cooking curd at 40°C for ca. 30 min caused a decrease of ca. 38% as compared to numbers of the pathogen in curd after cutting. During cheddaring of curd, numbers of L. monocytogenes increased by ca. 25%, over numbers at the end of cooking. Placing of curd in hot water [77°C (170°F)] and stretching for 3-4 min caused complete demise of the pathogen, as determined by our methods. The curd temperature during stretching was 58 to 65°C (136 to 149°F). Results of cold enrichments were all negative for stretched and brined curd. L. monocytogenes failed to survive during the making of Mozzarella cheese as done in this study.