Purpose: Therefore, the objective of this study was to develop a mathematical model to describe the kinetic behavior of Staphylococcus aureus on the cheese formulated at different water activities.
Methods: Low sodium (0.3% NaCl) Cheddar cheese was ground, and NaCl was added and well-mixed to obtain 0.970, 0.975, 0.983, and 0.991 of water activities. The mixture was restructured in 6-well microtiter plates. A mixture (0.1 ml) of S. aureus strains (ATCC13565, ATCC14458, ATCC23235, ATCC27664, and NCCP10826) was inoculated on the surface of the restructured cheese. The samples were then aerobically stored at 7, 15, 25, and 30°C for up to 30 days. The cell counts of total bacteria and S. aureus were enumerated on tryptic soy agar and mannitol salt agar, respectively. The growth data of S. aureus were fitted to the Baranyi model to calculate maximum specific growth rate (μmax; log CFU/g/h) and lag phase duration (LPD; h). The kinetic parameters were fitted to a secondary model. The model performance was evaluated with observed data, and root mean square error (RMSE) was calculated.
Results: In general, total bacterial and S. aureus growth increased (P < 0.05) as storage temperature increased depending on water activity of cheese. For a primary model, μmax values were increased, but LPD values were decreased as storage temperature and water activity increased. The secondary model also well-described the effect of storage temperature and water activity on the kinetic parameters. Moreover, the S. aureus cell counts predicted by the model simulation were very close to the observed S. aureus cell counts on cheese.
Significance: The result indicates that the developed predictive model in this study should be useful in predicting S. aureus on cheese at various water activities.