Effect of agar concentration on the matric potential of glycerol agar media and the germination and growth of xerophilic and non-xerophilic fungi

The concept of water activity (aw) does not differentiate between water status resulting from the interaction of water with solutes, and that from interaction of water with matrices, which is termed matric potential (Ψm). This study reports the effect of agar concentration (1.5, 3.0, 4.5, 6.0, and 7.5%, w/w) on matric potential of glycerol agar media (GA) and the germination and growth of xerophilic fungi (Eurotium herbariorum and E. rubrum) and non-xerophilic fungi (Aspergillus niger, A. flavus, and Penicillium roqueforti) on GA with or without sorbic acid (0.1–0.4%, w/w) at 0.90 aw and 0.95 aw. The matric potential of GA decreased when the agar concentration increased from 1.5 to 7.5%. When the agar concentration increased at each aw, the radial growth rate of the xerophilic fungi generally increased but the biomass density (biomass per unit area) decreased, whereas the radial growth rate of the non-xerophilic fungi generally decreased but the biomass density was unchanged. In the absence of sorbic acid, the time to germination of each species was similar for all agar concentrations. In the presence of sorbic acid, the time to germination of some species was significantly longer at higher agar concentrations (4.5%–7.5%) than 1.5% agar. This study demonstrated the inhibition effect on germination and growth of non-xerophilic fungi and xerophilic fungi by decreased matric potential resulting from increased agar concentrations, and the different responses of non-xerophilic fungi and xerophilic fungi to water stress from solutes and matrices. The concept of matric potential may be useful in food microbiology to provide a better understanding of fungal growth in complex food matrices.