Growth limits of Listeria monocytogenes as a function of temperature, pH, NaCl, and lactic acid

Models describing the limits of growth of pathogens under multiple constrai nts will aid management of the safety of foods which are sporadically conta minated with pathogens and for which subsequent growth of the pathogen woul d significantly increase the risk of food-borne illness. We modeled the eff ects of temperature, water activity, pH, and lactic acid levels on the grow th of two strains of Listeria monocytogenes in tryptone soya yeast extract broth. The results could be divided unambiguously into "growth is possible" or "growth is not possible" classes. We observed minor differences in grow th characteristics of the two L. monocytogenes strains. The data follow a b inomial probability distribution and may be modeled using logistic regressi on. The model used is derived from a growth rate model in a manner similar to that described in a previously published work (K. A. Presser, T. Ross, a nd D. A. Ratkowsky, Appl. Environ, Microbiol, 64:1773-1779, 1998). We used "nonlinear logistic regression" to estimate the model parameters and develo ped a relatively simple model that describes our experimental data well. Th e fitted equations also described well the growth limits of all strains of L. monocytogenes reported in the literature, except at temperatures beyond the limits of the experimental data used to develop the model (3 to 35 degr eesC). The models developed will improve the rigor of microbial food safety risk assessment and provide quantitative data in a concise form for the de velopment of safer food products and processes.