Modelling the behaviour of Listeria monocytogenes in cheese

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 thecheese types or in the processing step. In this work, the behaviour of Listeria monocytogenes duringcheesemaking and ripening was studied given a milk contamination or surface cross-contaminationscenario. When milk was contaminated, L. monocytogenes initiated growth with a different probabilityduring cheesemaking compared to milk or laboratory media, indicating that models built fromlaboratory 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 identifiedusing an Ordinary Logistic Regression model. In the range tested, the pH showed no effect on growthinitiation. Furthermore, L. monocytogenes presented considerable differences in behaviour inpasteurised or raw milk during cheesemaking and ripening. In contrast with the pasteurised milkcheeses, raw milk did not support growth of L. monocytogenes during cheesemaking but duringripening, growth occurred only in raw milk cheeses. The growth observed was modelled with a LogisticCardinal model. When the contamination was present on the surface only, Listeria was inactivated incheeses made with pasteurised milk (with pH < 5.4) during ripening and when aw decreasedconsiderably. The inactivation was modelled with a Log-linear response surface model with aw as thefactor with higher effect. In contrast, cheese made with raw milk or with pH around neutralitysupported the growth of L. monocytogenes and was modelled with the Logistic Cardinal model with pHas the most influencing environmental factor on the growth rate. In summary, the need for food modelshas been demonstrated and pH and aw were the most significant variables conditioning the behaviourof L. monocytogenes in the various cheese scenarios.