Purpose: The objective of this research is to determine the efficacy of heat treatments on Escherichia coli O157:H7, O145, O45, O26, O111, O103, and O121 in Tryptic Soy Broth.
Methods: Tryptic Soy Broth inoculated with individual strains of STEC (target 108 – 109 cfu/mL) were subjected to three separate heat treatments mimicking cooking temperatures for beef (60, 65.5, and 71 °C). Effectiveness of heat treatment was measured in 30 s increments for several minutes. Samples were plated onto Tryptic Soy Agar and incubated at 37°C for 24 hours. The reduction in log cfu/mL was measured over time.
Results: The O145 and O45 serogroups were more heat stable while the O26 and O111 serogroups appear heat labile. At 60°C, O157:H7 was reduced by 6.5 log cycles. However, O145 and O45 were reduced by 2.2 and 1.4 log cycles. The O45 serogroup had a decreased reduction compared to O157:H7 at all temperatures, while O145 had decreased reductions at 60 and 71°C. At 65.5°C, O157:H7 was reduced 6.07 log cycles. All serogroups were reduced comparably with the exception of O111 which was reduced 10.9 log cycles. For the 71°C temperature, O157:H7 was reduced 6.41 log cycles. The O26 serotype did not tolerate heat treatment and experienced a 10.37 log cycle reduction.
Significance: Consumers follow the USDA recommended minimum internal cooking temperatures in order to effectively reduce E. coli O157:H7 and this research illustrates that some O groups survive temperatures that typically eliminate O157 in a broth model. Further research is needed to determine if the same reduction in pathogen load is observed when the bacteria are added to beef.