Fate of Listeria monocytogenes During the Manufacture of Mozzarella Cheese

Abstract

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.