COMPARISON OF A QUADRATIC RESPONSE-SURFACE MODEL AND A SQUARE ROOT MODEL FOR PREDICTING THE GROWTH-RATE OF YERSINIA-ENTEROCOLITICA

Polynomial equations, relating the growth rate of Yersinia enterocolitica to temperature (0-25-degrees-C) and pH (4.5-6.5) in a liquid medium were constructed for four different acidulants. The logarithm of the time for a 100-fold increase in bacterial numbers could be represented by a quadratic response surface function of pH and temperature. The interactions between pH and temperature on growth rate were found to be additive. Values for a 2 log cycle increase in growth derived from the model were in good agreement with experimental values. Predictions from the quadratic model and from a square root model were compared with experimental values in laboratory media and UHT milk. The mean square error (MSE) for the quadratic response surface model was smaller than that for the square root model in 81% of cases. In UHT milk the square root model increasingly underestimated growth rate, as the temperature decreased and would 'fail dangerous' if used for predictive purposes. This indicated that the response surface model is more reliable for predicting the growth of Y. enterocolitica under conditions of sub-optimal temperature and pH.