Validation of Cooking Times and Temperatures for Thermal Inactivation of Yersinia pestis Strains KIM5 and CDC-A1122 in Irradiated Ground Beef

Irradiated ground beef samples (ca. 3-g portions with ca. 25% fat) inoculated with Yersina pestis strain KIM5 (ca. 6.7 log CFU/g) were heated in a circulating water bath stabilized at 48.9, 50, 52.5, 55, 57.5, or 60°C (120, 122, 126.5, 131, 135.5, and 140°F, respectively). Average D-values were 192.17, 34.38, 17.11, 3.87, 1.32, and 0.56 min, respectively, with a corresponding z-value of 4.67°C (8.41°F). In related experiments, irradiated ground beef patties (ca. 95 g per patty with ca. 25% fat) were inoculated with Y. pestis strains KIM5 or CDC-A1122 (ca. 6.0 log CFU/g) and cooked on an open-flame gas grill or on a clam-shell type electric grill to internal target temperatures of 48.9, 60, and 71.1°C (120, 140, and 160°F, respectively). For patties cooked on the gas grill, strain KIM5 populations decreased from ca. 6.24 to 4.32, 3.51, and ≤0.7 log CFU/g at 48.9, 60, and 71.1°C, respectively, and strain CDC-A1122 populations decreased to 3.46 log CFU/g at 48.9°C and to ≤0.7 log CFU/g at both 60 and 71.1°C. For patties cooked on the clam-shell grill, strain KIM5 populations decreased from ca. 5.96 to 2.53 log CFU/g at 48.9°C and to ≤0.7 log CFU/g at 60 or 71.1°C, and strain CDC-A1122 populations decreased from ca. 5.98 to ≤0.7 log 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 to the temperatures and times recommended by the U.S. Department of Agriculture and the U.S. Food and Drug Administration Food Code, appreciably lessens the likelihood, severity, and/or magnitude of consumer illness if the ground beef were purposefully contaminated even with relatively high levels of Y. pestis.