Modelling the effect of pH, acidulant and temperature on the growth rate of Yersinia enterocolitica

Growth of two pathogenic and one environmental serotype of Yersinia enterocolitica under acidic conditions and at 4 and 25°C was investigated. At both temperatures the maximum growth inhibitory pH depended on the acidulant used and was in the order acetic > lactic > citric > sulphuric. At the lower temperature the maximum growth inhibitory pH was 0.3-0.5 pH units higher than at 25°C. No difference was observed between the behaviour of pathogenic and environmental serotypes in this respect. Measurement of growth at a number of sub-optimal temperatures and pH values showed that the variation of growth rate with temperature could be represented by a square root plot. The effect of different pH values could be incorporated into the model by replacing the regression coefficient b by its relationship with pH. Values of maximum growth inhibitory pH derived from the model were in good agreement with experimental values with the exception of acetic acid.