Heat resistance of Yersinia enterocolitica grown at different temperatures and heated in different media

In the range of 4–20°C, growth temperature did not influence the heat resistance at 54–66°C for Yersinia enterocolitica at pH 7 in citrate phosphate buffer. However, when cells were grown at 37°C, the D62 increased from 0.044 to 0.17 min. This increase was constant at all heating temperatures tested (z=5.7–5.8). Growth temperature did not influence the proportion of heat-damaged cells after a heat treatment, as measured by their response to a 2% of sodium chloride added to the recovery medium. The sensitivity of heat treated cells to nisin or lysozyme depended on growth temperature: Whereas the number of cells grown at 4°C surviving heat treatment was the same regardless of the presence of 100 IU/ml of nisin or 100 μg/ml of lysozyme in the recovery medium, that of cells grown at 37°C was, in these media, lower. The pH of maximum heat resistance in citrate phosphate buffer was pH 7 for cells grown at 37°C, but pH 5 for those grown at 4°C. In both suspensions the magnitude of the effect of pH on heat resistance was constant at all heating temperatures. For cells grown at 4°C the heat resistance at 54–66°C, in skimmed milk or pH 7 buffer, was the same. For cells grown at 37°C this also applied for heat treatment at 66°C but at 56°C the heat resistance in skimmed milk was higher.