Development of D- and Z-values for L. monocytogenes and Escherichia coli O157:H7 in cheesemilk to reduce pathogen risks in cheese manufacture

Obj 1. Prepare 5 to 6 strains of L. monocytogenes (representing serotypes 1/2a, 1/2b, and 4b, and including those isolated from cheese, milk, environment, or clinical isolates) either known to be heat resistant or isolated from outbreaks. Obj 2. Conduct inoculation and thermal death time studies. Obj 3. Validate lethality for each pathogen in milk using three temperature-time combinations. Obj 4. Calculate D- and Z-values and build temperature-time tables for use by cheese makers to target desired pathogen kill. Abstract: Specialty, artisanal and farmstead cheeses represent growth opportunities to the dairy industry. Instead of using fully pasteurized milk, some producers choose to heat treat or “Thermize” the milk used for cheesemaking and then rely on the 60-day aging rule. They treat them as ‘raw milk cheeses’ for microbial safety and regulatory purposes. The purpose of this study is to provide guidance on the level of heat treatment needed to reduce the risk of select pathogens in cheeses where the 60 day aging rule can be applied. It does not apply to high moisture, fresh cheeses where raw milk is not allowed such as high moisture, soft cheeses. Laboratory studies have shown that even low levels of E. coli O157:H7 (e.g. 10 cells/ml of cheesemilk) can survive months in laboratory prepared Gouda and stirred-curd Cheddar made with raw milk; foodborne illness outbreaks associated with raw milk Gouda confirm that the risk extends to commercial production. We propose to conduct thermal inactivation studies of L. monocytogenes and shiga-toxin producing E.coli to develop measurements of thermal resistance. These values (D and Z-values) will be the basis for developing user-friendly temperature-time tables or other tools for cheesemakers to choose combinations of thermal parameters to deliver desired lethality of pathogens.