Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 10, 2008
Publication Date: March 1, 2009
Citation: Porto Fett, A.C., Juneja, V.K., Luchansky, J.B., Tamplin, M. 2009. VALIDATION OF COOKING TIMES AND TEMPERATURES FOR THERMAL INACTIVATION OF YERSINIA PESTIS STRAINS KIM5 AND CDC-A1112 IN GROUND BEEF. Journal of Food Protection. 72(3): 564-571. Interpretive Summary: The safety and security of our food supply have been of great concern since the terrorist attacks of 2001 due to the potential for deliberate contamination with biological threat agents. Yersinia pestis, the bubonic plague agent, is a highly-virulent pathogen and a likely threat agent for committing an act of agro-bioterrorism. Although gastrointestinal plague is extremely rare, the disease has been caused by the consumption of contaminated meat. Given the prevailing heightened awareness of food as a target for terrorist activity and the pathogenicity of Y. pestis, it is important to gain a better understanding about the behavior and control of this bacterium, particularly in high volume, higher-risk foods such as ground beef. In this study we evaluated the thermal resistance of Y. pestis inoculated in ground beef and heated in a temperature controlled water bath and cooked on two different commercial grills. When ground beef patties were inoculated with the pathogen and cooked on either a clam shell-style grill or an open flame gas grill to internal temperatures ranging from about 49 to 71 deg C, there was considerable killing of the pathogen. Similar results were obtained when ground beef was heated in the water bath. These findings confirm that, regardless of the cooking method/grill used, ground beef patties should be cooked to the recommended internal temperature of at least 71.1 deg C (160 deg F) for 15 seconds to appreciably decrease the risk of food borne illness due to the consumption of ground beef that may be purposefully contaminated with Y. pestis.
Technical Abstract: The thermal stability of Yersinia pestis inoculated into retail ground beef (25 per cent fat) and heated in a temperature-controlled water bath or cooked on commercial grills was evaluated. Irradiated ground beef (3-g portions) was inoculated with ca. 6.7 log10 CFU/g of Y. pestis strain KIM5 and heated in a water bath stabilized at 48.9 deg C (120 deg F), 50 deg C (122 deg F), 52.5 deg C (126.5 deg F), 55 deg C (131 deg F), 57.5 deg C (135.5 deg F), or 60 deg C (140 deg F). The rate of inactivation increased with increasing temperature; average D-values in ground beef were 192.17 min at 48.9 deg C, 34.38 min at 50 deg C, 17.11 min at 52.5 deg C, 3.87 min at 55 deg C, 1.32 min at 57.5 deg C, and 0.56 min at 60 deg C, and the Z-value was 4.67 deg C (40.41 deg F). In related studies, irradiated ground beef patties (ca. 95 g per patty) were inoculated with ca. 6.0 log10 CFU/g of Y. pestis strains KIM5 or CDC-A1122 and cooked on an open-flame gas grill or on a clam-shell type electric grill to target internal temperatures of 48.9 deg C (120 deg F), 60 deg C (140 deg F), or 71.1deg C (160 deg F). On the gas grill, numbers of strain KIM5 decreased from ca. 6.24 log10 CFU/g to 4.32, 3.51, and equal or less than 0.7 log10 CFU/g at 48.9, 60, and 71.1 deg C, respectively. For strain CDC-A1122, numbers decreased from ca. 6.22 to 3.46 log10 CFU/g at 48.9 deg C, whereas a decrease from ca. 6.22 to equal or less than 0.7 log10 CFU/g was observed at both 60 and 71.1 deg C. On the clam-shell grill, numbers of strain KIM5 decreased from ca. 5.96 to 2.31 log10 CFU/g when the patties were heated to 48.9 deg C, whereas cooking the patties to 60 or 71.1 deg C resulted in a decrease from ca. 5.96 to equal or less than 0.7 log10 CFU/g. Using the clam-shell grill, numbers of strain CDC-A1122 decreased from ca. 5.85 to equal or less than 0.7 log10 CFU/g at all three cooking temperatures. These data confirm that cooking ground beef on an open-flame gas grill or on a clam-shell type electric grill at internal temperature of 71.1 deg C would appreciably lessen the likelihood, severity, and/or magnitude of illness if ground beef patties were purposefully contaminated with Y. pestis.