2012 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.
As part of our programmatic efforts to characterize and control pathogens of concern in raw ground and non-intact beef, 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 or frozen or frozen and then thawed 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, our results validated that cooking refrigerated and frozen ground beef patties to an internal temperature of =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. In related studies, we quantified the effect of heat on the fate of ECOH and STEC in 3-gram portions of flattened (ca. 1.0 mm thick) ground beef inoculated (ca. 10 million cells/g) separately with a single strain of E. coli serotypes O111:H-, O45:H2, O103:H2, O121:H19, O145:NM, O26:H11, and O157:H7. Our data established that cooking times/temperatures (54.4 deg, 60 deg, or 65.6 degC) effective for inactivating ECOH in ground beef are equally effective against the additional STEC strains investigated.