Fate of Shiga Toxin-Producing O157:H7 and Non-O157:H7 Escherichia coli Cells within Refrigerated, Frozen, or Frozen Then Thawed Ground Beef Patties Cooked on a Commercial Open-Flame Gas or a Clamshell Electric Grill

Both high-fat and low-fat ground beef (percent lean:fat = ca. 70:30 and 93:7, respectively) were inoculated with a 6-strain cocktail of non-O157:H7 Shiga toxin-producing Escherichia coli (STEC) or a five-strain cocktail of E. coli O157:H7 (ca. 7.0 log CFU/g). Patties were pressed (ca. 2.54 cm thick, ca. 300 g each) and then refrigerated (4 degrees C, 18 to 24 h), or frozen (218 degrees C, 3 weeks), or frozen (218 degrees C, 3 weeks) and then thawed (4 degrees C for 18 h or 21 degrees C for 10 h) before being cooked on commercial gas or electric grills to internal temperatures of 60 to 76.6 degrees C. For E. coli O157:H7, regardless of grill type or fat level, cooking refrigerated patties to 71.1 or 76.6 degrees C decreased E. coli O157:H7 numbers from an initial level of ca. 7.0 log CFU/g to a final level of <= 1.0 log CFU/g, whereas decreases to ca. 1.1 to 3.1 log CFU/g were observed when refrigerated patties were cooked to 60.0 or 65.5 degrees C. For patties that were frozen or freeze-thawed and cooked to 71.1 or 76.6 degrees C, E. coli O157:H7 numbers decreased to ca. 1.7 or <= 0.7 log CFU/g. Likewise, pathogen numbers decreased to ca. 0.7 to 3.7 log CFU/g in patties that were frozen or freeze-thawed and cooked to 60.0 or 65.5 degrees C. For STEC, regardless of grill type or fat level, cooking refrigerated patties to 71.1 or 76.6uC decreased pathogen numbers from ca. 7.0 to <= 0.7 log CFU/g, whereas decreases to ca. 0.7 to 3.6 log CFU/g were observed when refrigerated patties were cooked to 60.0 or 65.5 degrees C. For patties that were frozen or freeze-thawed and cooked to 71.1 or 76.6 degrees C, STEC numbers decreased to a final level of ca. 1.5 to <= 0.7 log CFU/g. Likewise, pathogen numbers decreased from ca. 7.0 to ca. 0.8 to 4.3 log CFU/g in patties that were frozen or freeze-thawed and cooked to 60.0 or 65.5 degrees C. Thus, cooking ground beef patties that were refrigerated, frozen, or freeze-thawed to internal temperatures of 71.1 and 76.6uC was effective for eliminating ca. 5.1 to 7.0 log CFU of E. coli O157:H7 and STEC per g.