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 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.