Thermal death of Escherichia coli O157 in cattle feeds

Aims: To determine if the temperatures used in feed manufacture are likely to destroy Escherichia coli O157. Methods and Results: Two commercial feeds were ground and inoculated with E. coli O157 cells. The feeds were heated to 50, 55, 60, 65 or 70°C. Heating produced quadratic survivor curves, with rapid initial decreases. The survival characteristics of E. coli O157 differed in the two feeds. The reductions observed in one feed may not have been due to heat alone. There was evidence that indigenous anti-E. coli O157 factor(s) in one feed acted with the heat and contributed to the observed rates of bacterial death. Heating at 70°C for 20 or 120 s resulted in approx. 1·3 and 2·2 log reductions in E. coli O157 numbers respectively. Lesser reductions were observed at lower temperatures. Conclusions: The time/temperature combinations used in commercial pelleting processes would not effectively kill high numbers of E. coli O157. Significance and Impact of the Study: This is the first study to look at the survival of E. coli O157 strains after heat treatment within concentrated animal feed. The study provides information on the likely risk of E. coli O157 surviving the animal feed manufacturing process.