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United States Department of Agriculture

Agricultural Research Service

Related Topics

Research Project: THERMAL INACTIVATION OF SHIGA-TOXIN PRODUCING ESCHERICHIA COLI (STEC) IN FROZEN GROUND BEEF

Location: Food Safety and Intervention Technologies

2013 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.


3.Progress Report:

Monitored the survival of Escherichia coli O157:H7 (ECOH) and non-O157:H7 Shiga toxin producing E. coli (STEC) in 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 multi-strain cocktail of STEC of ECOH (ca. 7.0 log CFU/g). Patties were pressed (ca. 2.54 cm thick, ca. 300 grams) and then refrigerated (4 degrees C, 18-24 h), or frozen (-18 degrees C, 3 weeks), or frozen (-18 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 60.0 degrees to 76.6 degrees C. In general, cooking ground beef patties that were refrigerated, frozen, or frozen/thawed to internal temperatures of 71.1 degrees and 76.6 degrees C was effective for eliminating ca. 5.1 to 7.0 log CFU/g of ECOH and STEC. In related studies, we monitored survival of ECOH and STEC in 3-gram portions of flattened (ca. 1.0 mm thick) ground beef inoculated (ca. 7.0 log CFU/g) separately with a single strain of E. coli serotypes O111:H-, O45:H2, O103:H2, O104:H4, O121:H19, O145:NM, O26:H11, and O157:H7. Inoculated wafers of beef were submerged in a water bath and heated to an internal temperature of 54.4 degrees, 60 degrees, or 65.6 degrees C for up to 90 min. D-values, that being the time it takes to reduce 90% of the pathogen, ranged from 13.5 to 32.6 min, 0.7 to 1.2 min, and 0.05 to 0.2 min at 54.4 degrees, 60.0 degrees, and 65.6 degrees C, respectively. In addition, we observed log CFU/g reductions of ca. 2.7 to 6.7 log CFU/g at 54.4 degrees C after 90 min, ca. 3.3 to 6.0 log CFU/g at 60.0 degrees C after 4 min, and 1.5 to 5.8 log CFU/g at 65.6 degrees C after 0.26 min. Thus, cooking times/temperatures effective for inactivating ECOH in ground beef were equally effective against the seven STEC strains investigated herein.


Last Modified: 8/19/2014
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