2011 Annual Report
1a.Objectives (from AD-416)
Quantify the effect of freezing, thawing, and/or subsequent cooking on the fate of Escherichia coli O157:H7 (ECOH) and other non-verocytotoxigenic E. coli (STEC) in ground beef patties.
1b.Approach (from AD-416)
Ground beef will be purchased from a local retailer, inoculated with a five-strain cocktail of pathogenic E. coli, formed into patties, and held frozen for up to 3 months. The patties will be cooked, with and without prior thawing, on commercial grills and pathogen viability will be quantified. Parameters to be evaluated will include the fat content of ground beef, serotype of E. coli, freezing duration and thawing regimen for patties, cooking temperature, and grill type as detailed in Appendix A. The experimental design was derived from multiple conversations between key players from both FSIS and ARS. Ongoing discussions between ARS and FSIS as to the nature, number, and scope of the project may necessitate that the timeframe for completion be extended and/or the cost be increased.
We comparatively quantified thermal resistance of Escherichia coli O157:H7 (ECOH) and non-O157:H7 Shiga toxin producing E. coli (STEC) in refrigerated, frozen, and thawed ground beef patties cooked on commercial grills. Both high fat and low fat ground beef (percent lean:fat = 70:30 and 93:7, respectively) were purchased from a local butcher and inoculated with genetically-marked, 5-strain cocktails of ECOH or STEC (ca. 7.0 CFU/g). Patties were pressed (ca. 2.54 cm thick, ca. 300 grams) and then either refrigerated (4 deg C, 18 hours) or frozen (4 deg C) or frozen and then thawed at 4 deg for 18 hours or 21 deg C for 10 hours before being cooked on a commercial open-flame gas grill or on a clam-shell electric grill to internal temperatures of 60.0 deg, 65.5 deg, 71.1 deg, or 76.6 deg C. Regardless of the level of fat or type of grill, cooking refrigerated patties to 71.1 deg or 76.6 deg C resulted in a 6.2 to deg 7.0 and 6.1 to deg 6.4 log CFU/g decrease in ECOH and STEC numbers, respectively. When refrigerated patties were cooked at 60.0 deg or 65.5 deg C, ECOH numbers decreased by ca. 4.1 to 5.6 and 4.6 to 6.5 log CFU/g, respectively, whereas STEC numbers decreased by ca. 3.9 to 5.2 and 5.6 to greater than 6.1 log CFU/g, respectively. When frozen or thawed patties were cooked to either 65.5 deg, 71.1 deg, or 76.6 deg C, regardless of the fat level or type of grill, ECOH and STEC numbers decreased by 4.3 to greater than or equal to 6.2 and log CFU/g, whereas a decrease of 2.5 to 4.8 and 3.3 to 5.4 log CFU/g reduction in ECOH and STEC, respectively, was observed after cooking at 60.0 deg C. These results validated that cooking refrigerated and frozen ground beef patties to an internal temperature of greater than or equal to =71.1 deg C is effective for destroying ECOH and STEC and, in turn, lessening the threat of illness associated with this food borne pathogen.