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.