Modelling the effect of water activity reduction by sodium chloride or glycerol on conidial germination and radial growth of filamentous fungi encountered in dairy foods

Water activity (aw) is one of the most influential abiotic factors affecting fungal development in foods. The effects of aw reduction on conidial germination and radial growth are generally studied by supplementing culture medium with the non-ionic solute glycerol despite food aw can also depend on the concentration of ionic solutes such as sodium chloride (NaCl). The present study aimed at modelling and comparing the effects of aw, either modified using NaCl or glycerol, on radial growth and/or conidial germination parameters for five fungal species occurring in the dairy environment. The estimated cardinal values were then used for growth prediction and compared to growth kinetics observed on commercial fresh cheese. Overall, as compared to glycerol, NaCl significantly increased the fungistatic effect resulting from aw reduction by extending latency and/or reducing radial growth rates of Paecilomyces niveus, Penicillium brevicompactum, Penicillium expansum and Penicillium roqueforti but not of Mucor lanceolatus. Besides, NaCl significantly reduced aw range for conidial germination and delayed median germination time of P. expansum but not of P. roqueforti. Despite these observations, cardinal aw values obtained on glycerol-medium yielded similar predictions of radial growth and germination time in commercial fresh cheese as those obtained with NaCl. Thus, it indicates that, for the studied species and aw range used for model validation, the use of NaCl instead of glycerol as a aw depressor had only limited impact for fungal behavior prediction.