Graduate Thesis Or Dissertation
 

An Exploration of Cheese Safety Parameters and Aging as Mitigation Strategy for Foodborne Pathogens in Cheese

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/qj72pg91m

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  • Pasteurization is an effective method of reducing risks associated with foodborne pathogens in raw milk. Cheese made from raw milk has a higher potential to harbor harmful bacterial pathogens. However, some cheesemakers prefer to use unpasteurized milk to preserve the natural microflora present in the raw milk and to prevent degradation of compounds such as (enzymes, fatty acids, and proteins). As a mitigation strategy following several outbreaks in the 1930s and 1940s, the US government codified a set of guidelines commonly termed “the 60-day rule”, which instructs cheese makers using raw milk to age their cheeses at a temperature of at least 35°F for at least 60 days before packaging and sale to consumers. Despite minimal data existing on the effectiveness of aging at the time, this 60-day aging procedure was considered by FDA and the industry as a reasonable risk reduction strategy for these products. Since this time, several studies have cast doubt on the efficacy of “the 60-day rule” and FDA has communicated a desire to re-visit these regulations. The overall objective of this research was to support a better understanding of the behavior of foodborne pathogens and microbial index organisms in cheese, specifically Escherichia coli and Listeria spp. These studies were focused on two semi-soft cheese types with different salting procedures: Cheddar cheese (dry-salted) and Gouda cheese (wet brined). The body of this research includes a critical review of the existing literature on the survival of these pathogens in cheese as well as experimental laboratory-based research to generate primary data to further our understanding of formulation and aging temperature considerations. A critical review of previous research on the behavior of E. coli and Listeria spp. in Cheddar and Gouda included 15 studies that met our inclusion criteria of being aged and tracking the survival of pathogenic organisms while tracking the differences in aging conditions, cheese pH, salt levels, moisture content and other specific conditions that may impact differences in pathogen survival. Cheddar cheese is a dry salted cheese that is typically aged at refrigeration temperature (~4°C). According to 21 CFR 133.113 Cheddar cheese must have a moisture content of less than 39% and a milk fat solids content of greater than 50%. The aim of this study was to investigate the impact of salt concentration on microbial survival during aging as well as evaluate the behavior of naturally contaminating E. coli and its survival in raw and pasteurized Cheddar. E. coli and Listeria cocktails were prepared and inoculated into fresh, warm curd prior to the time of salting, just prior to molding and pressing. Cheddar cheese curds were salted at 2% and 4% w/w. Cheddar Cheese was pressed and aged at 4°C for up to 176 days and microbial survivors were enumerated through the aging period. E. coli populations declined from initial inoculation rates before stabilizing through the first 60 days and declined slowly through the end of the aging period with faster decline by both strains at higher salt rates, and with E. coli ATCC 25922 dropping below the detection limit at the higher salt rate by day 60. Listeria populations declined at different rates depending on strain, with L. monocytogenes WRLP85 persisting at relatively high levels throughout the aging period. L. innocua ATCC 33090 declined slightly faster at the higher salt rate but only dropped below the detection limit in the final enumeration while persisting in the lower salt rate. Salt concentration at the tested levels was not a significant factor in the survival of E. coli and Listeria spp. in Cheddar cheese during aging. Collectively, the work presented in this thesis presents a critical evaluation of the impact of aging of Cheddar and Gouda on the survival of foodborne pathogens. The body of research, including the primary research in this thesis, supports aging of >60 days as a risk-reduction strategy for these cheeses. This information may support the development of food safety plans as aging is a process control step that does provide risk reduction; however, finished product could still have low levels of pathogenic bacterial survival and may not meet FDA’s “zero tolerance” policy for the presence of foodborne pathogens in food products that are in commerce. Gouda is a wet-brined cheese that is often aged at temperatures above refrigeration (10-20°C). According to 21 CFR 113.187 Gouda must have a moisture content of less than 46% and milk fat solids content of greater than 50%. The aim of our study was to investigate the impact of brining and aging temperatures on the survival of E. coli and Listeria spp. E. coli and Listeria cocktails were prepared and inoculated into fresh, warm curd prior to molding and pressing. Cheeses were brined for 36 hours at 4°C, 10°C, and 15°C, and cheese was vacuum sealed and aged for >60 days at the same brining temperature. E. coli levels saw a significant decline over 60 days, but the populations stabilized and remained at similar levels through the end of the study. Listeria populations declined slower at 4°C through 60 days than at 15°C, but all treatments saw populations decline below the detection limit by the end of the study. Survival of pathogenic strains in saturated brines was also monitored at 4°C and 20°C. Pathogens were rapidly inactivated in salt-saturated brines stored at 20°C, but populations were relatively stable at 4°C. These data indicate that brine management temperature could be an important factor to control cross-contamination in wet-brining operations.
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