Modelling the interaction of storage temperature, pH, and water activity on the growth behaviour of Listeria monocytogenes in raw and pasteurised semi-soft rind washed milk cheese during storage following ripening

The objective of this study was to investigate the growth of Listeria monocytogenes in semi-soft rind washed cheese made from raw and pasteurised milk at different storage temperatures (4, 10 and 15°C) over a 28 day period simulating storage following ripening. Changes in water activity (a_w) and pH in cheeses were also monitored during storage. Response surface models were used to model the interaction of storage temperature and time on a_w, pH and L.monocytogenes population. Growth curves were fitted using Baranyi, modified Gompertz and Logistic models at all storage temperatures for both cheeses, and model parameters were statistically analysed. In raw and pasteurised milk cheeses, all models showed a significant (P<0.05) increase in the specific growth rate (SGR, Day^-1) of L.monocytogenes with an increase in storage temperature. A higher SGR was observed for L.monocytogenes in pasteurised milk cheese (0.18-0.85Day^-1) compared to raw milk cheese (0.05-0.37Day^-1) at all storage temperatures studied. Response surface models indicated an increase in the L.monocytogenes population and pH with an increase in storage temperature. However, a decreasing trend in a_w for both cheese types was observed. The predicted regression model parameters for both the raw and pasteurised milk cheese showed a high correlation coefficient R²>0.87. Overall, the L.monocytogenes population increased up to 3 log₁₀cfu^-g^-1 for both cheeses during storage following ripening. The fitted models confirmed different L.monocytogenes growth behaviour between raw and pasteurised milk cheeses, which could support the Food Business Operator in predicting growth during storage following ripening.