Predictive models developed in cheese for growth of Listeria monocytogenes

EU regulations on the presence of Listeria monocytogenes in certain cheeses are based on the ability of the cheese to support the growth of L. monocytogenes. The aim of this study was, in separate experiments, to simulate milk contamination and environmental cross-contamination of cheese, respectively, with L. monocytogenes and determine the growth rate for each scenario using predictive models. For the milk contamination scenario, pasteurised and raw milk were inoculated with a mix of two strains of L. monocytogenes (strain 6179 a persistent strain isolated at a cheese plant and strain C5 isolated from the farm environment). The pH, lactic acid, water activity (a w ) and numbers of L. monocytogenes were monitored during cheesemaking (5 h). For the cross-contamination scenario, Listeria free commercial smear ripened semi-soft raw milk cheeses were surface inoculated with the same strain mix of L. monocytogenes. During ripening, pH, a w and L. monocytogenes numbers were determined. For pasteurised milk cheesemaking and raw milk cheese ripening conditions, growth of L. monocytogenes was observed. Using cardinal models, the values of the optimum growth rates (μ opt ) were estimated at 1.30 log cfu/h during cheesemaking and 0.28 log cfu/day during ripening. If environmental factors are monitored, it is possible to simulate the behaviour of Listeria and assess the optimal growth rate.