P2-104 Thermal Resistance of Non-O157 Shiga Toxin-Producing Escherichia coli (STEC) in Irradiated, Raw Ground Beef

Monday, July 27, 2015
Exhibit Hall (Oregon Convention Center)
Daniel Unruh , Kansas State University , Olathe , KS
Randall Phebus , Kansas State University , Manhattan , KS
Harshavardhan Thippareddi , University of Georgia , Athens , GA
Introduction:  The USDA-FSIS declared six serogroups of Shiga Toxin-producing Escherichia coli (STECs) as adulterants in manufacturing trim and raw ground beef product components. Determining the thermal processing parameters (D- and z-values) will assist the beef industry in designing thermal processes to achieve regulatory performance standards.

Purpose: The objective of this study was to determine the thermal processing parameters for the STECs (cocktails of strains of the same serogroup), E. coli O157:H7 and two cocktails of STECs (containing one strain representing each serogroup) in irradiated, raw ground beef.

Methods: Cocktails of strains for each serogroup, cocktails of STECs (USDA-ARS and ATCC), and E. coli O157:H7 were grown individually in tryptic soy broth (TSB) and inoculated into raw ground beef (73% or 93% lean) to obtain ca. 7 log CFU/g. Inoculated ground beef (3 g) was vacuum-packaged in small vacuum bags and stored overnight at 4°C. The samples were submerged in a water bath at 54.4, 60.0, or 65.6°C, removed at prescribed time intervals, and surviving populations were enumerated using non-selective and selective media. Thermal resistance parameters were calculated.

Results: D-values for the six serogroups of STECs ranged from 28.32 to 45.02 min at 54.4°C, 1.96 to 2.94 min at 60.0°C, and 0.12 to 0.21 min at 65.6°C. These D-values of STECs were similar (P > 0.05) to E. coli O157:H7 values determined in this study at the same temperatures. D-values for O45 and O121 in 73% lean ground beef at 60.0°C were higher (P ≤ 0.05) compared to E. coli O157:H7. Fat content of the ground beef (27 vs. 7%) did not affect (P > 0.05) D-values of the STECs.

Significance: Non-O157 STECs have similar heat resistance as E. coli O157:H7 in ground beef, regardless of fat level, and thus current validated thermal processes will be effective to mitigate the risk of STECs in beef.