Microbiological Safety of Cheese Made from Heat-Treated Milk, Part II. Microbiology

Abstract

A review of epidemiological literature identified six illness outbreaks transmitted via U.S. produced cheese during 40 years, 1948–1988. During these four decades, the United States cheese industry produced over 100 billion pounds of natural cheese (not including cottage and related varieties). The most frequent causative factor in U.S. and Canadian cheese-related outbreaks was post-pasteurization contamination. Faulty pasteurization equipment or procedures were implicated in one outbreak each in the U.S. and Canada. Use of raw milk was a factor in one outbreak in each country. Inadequate time-temperature combinations used for milk heat treatment were not implicated. The epidemiology of cheese-related outbreaks in the U.S., Canada, and Europe demonstrated that soft surface-ripened cheese, e.g. Camembert and Brie, are at significantly greater risk to transmit pathogens than other cheeses. No outbreaks were linked to hard Italian varieties, e.g. Parmesan, Romano, and Provolone. Varieties such as Cheddar and Swiss were infrequently involved. A variety of pathogens have been isolated from raw milk. Some, including Salmonella, Listeria, and enteropathogenic Escherichia coli can survive and grow in some cheeses. In one of the few published studies of milk heat-treatment for cheesemaking, multistrain or species mixtures of pathogens were inoculated into raw milk at levels of 105/ml which was heat-treated in a commercial HTST pasteurizer — mean holding time 17.6 s, minimum 16.2 s. All strains of Yersinia enterocolitica, Campylobactersp., Escherichia coli 0157:H7, and all but one Salmonella species were destroyed at 65°C (149°F). Salmonella senftenberg (rarely isolated from cheese) was inactivated at 69°C (156.2°F). Listeria monocytogenes in naturally contaminated milk at levels of 104 organisms per ml was inactivated at 66°C (150.8°F); laboratory-cultured inoculum at levels of 105 organisms per ml required 69.0°C (156.2°F). A multiplicity of practices other than pasteurization or heat-treatment contribute significantly to the microbiological safety of cheese. Some, such as milk quality management, lactic culture management, pH control, salt addition, and controlled curing conditions are established technologies. Others represent potential opportunities, such as natural inhibitory substances in milk, antibacterial substances, e.g. nisin and lysozyme.