CONVENIENT MODEL TO DESCRIBE THE COMBINED EFFECTS OF TEMPERATURE AND PH ON MICROBIAL-GROWTH

A new model in which the maximum microbial specific growth rate (mu(ma x)) is described as a function of pH and temperature is presented. The seven parameters of this model are the three cardinal pH parameters ( the pH below which no growth occurs, the pH above which no growth occu rs, and the pH at which the mu(max) is optimal), the three cardinal te mperature parameters (the temperature below which no growth occurs, th e temperature above which no growth occurs, and the temperature at whi ch the mu(max) is optimal), and the specific growth rate at the optimu m temperature and optimum pH. The model is a combination of the cardin al temperature model with inflection and the cardinal pH model (CPM). The CPM was compared with the models of Wijtzes et al. and Zwietering et al. by using previously published data sets. The models were compar ed on the basis of the usual criteria (simplicity, biological signific ance and minimum number of parameters, applicability, quality of fit, minimum structural correlations, and ease of initial parameter estimat ion), and our results justified the choice of the CPM. Our combined mo del was constructed by using the hypothesis that the temperature and p H effects on the mu(max) are independent, An analysis of this new mode l with an Escherichia coli O157:H7 data set showed that there was a go od correspondence between observed and calculated mu(max) values. The potential and convenience of the model are discussed.