The fate of Listeria monocytogenes Scott A, was studied in broth, at different aws (by adding NaCl or KCl from 0·0 to 1·4 mol l−1), pHs (from 4·0 to 7·3 by adding lactic acid), and nisin concentrations (from 0 to 100 IU ml−1).Methods and results
Increasing salt and nisin concentrations and decreasing pH resulted in lower growth rates and extended lag phases. At pH 4·5 no growth was observed while in presence of nisin and/or 1 mol l−1 salts of both kinds, L. monocytogenes Scott A was inactivated. Equal-molar concentrations of NaCl or KCl (similar aw), exerted similar effects against L. monocytogenes in terms of lag phase duration, growth or death rate. The growth boundaries of L. monocytogenes Scott A at 5°C were also estimated by growth/no growth turbidity data, modeled by logistic polynomial regression. The concordance of logistic models, were 99·6 and 99·8% for NaCl and KCl, respectively.Conclusions
The growth interfaces derived by both NaCl and KCl models were almost identical. Hence, NaCl can be replaced by KCl without risking the microbiological safety of the product. Increasing nisin concentrations markedly affected the interface resulting in a more inhibitory environment for L. monocytogenes Scott A. Low to medium salt concentrations (0·3–0·7 mol l−1 of either NaCl or KCl) provided a protective effect against inhibition of L. monocytogenes Scott A by nisin.Significance and impact of the study
Modelling the growth boundaries not only contributes to the development of safer food by providing useful data, but can also be used to study interactions between factors affecting initiation of growth of pathogenic micro-organisms.