Modelling the behaviour of Listeria monocytogenes in cheese

إنجليزي. Cheese is a complex biological system where environmental dynamics take place between pH, moisture, water activity and temperature. Bacteria growing in or on cheese behave differently depending on the cheese types or in the processing step. In this work, the behaviour of Listeria monocytogenes during cheesemaking and ripening was studied given a milk contamination or surface cross-contamination scenario. When milk was contaminated, L. monocytogenes initiated growth with a different probability during cheesemaking compared to milk or laboratory media, indicating that models built from laboratory media would not accurately predict the behaviour of L. monocytogenes in farmhouse cheese. Lower limits of growth at aw values of 0.94-0.96, depending on contamination level, were identified using an Ordinary Logistic Regression model. In the range tested, the pH showed no effect on growth initiation. Furthermore, L. monocytogenes presented considerable differences in behaviour in pasteurised or raw milk during cheesemaking and ripening. In contrast with the pasteurised milk cheeses, raw milk did not support growth of L. monocytogenes during cheesemaking but during ripening, growth occurred only in raw milk cheeses. The growth observed was modelled with a Logistic Cardinal model. When the contamination was present on the surface only, Listeria was inactivated in cheeses made with pasteurised milk (with pH < 5.4) during ripening and when aw decreased considerably. The inactivation was modelled with a Log-linear response surface model with aw as the factor with higher effect. In contrast, cheese made with raw milk or with pH around neutrality supported the growth of L. monocytogenes and was modelled with the Logistic Cardinal model with pH as the most influencing environmental factor on the growth rate. In summary, the need for food models has been demonstrated and pH and aw were the most significant variables conditioning the behaviour of L. monocytogenes in the various cheese scenarios.

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