Purpose: Due to the finding of an increased thermotolerance in some Escherichia coli strains from raw milk cheese, the aim of this study was to investigate the efficacy of different subpasteurization conditions for the reduction of E. coli in raw milk using a pilot plant pasteurizer.
Methods: Nine E. coli strains, including four Shiga toxin-producing E. coli (STEC) isolates, were examined. Eight strains originated from raw milk cheese and one from vat raw milk. To reflect conditions applied by cheese manufacturers, a pilot plant pasteurizer was used (milk heated in continuous flow by passing two plate heat exchangers). After inoculation of whole raw milk, bacterial reductions at heating temperatures between 60 and 70°C (steps of 2.5°C) and holding times of 15, 20, and 25 s were determined.
Results: Six of the nine E. coli strains, including the four STEC strains, were similarly reduced at 60, 62.5, and 65°C, while an increased thermotolerance was observed for the remaining three strains. At 60 and 62.5°C, the reduction of all strains was below 2 log after 25 s. Reductions of at least 5 log after 25 s were observed at 65°C for six strains and at 67.5°C for eight strains. But one strain was reduced after 25 s at 67.5°C by less than 1.0 log.
Significance: For certain E. coli strains, time-temperature combinations above 65°C were required to obtain substantial reductions within the applied thermisation treatment. On the other hand, the examined STEC strains did not show increased thermotolerance.