Effect of reduced water activity and reduced matric potential on the germination of xerophilic and non-xerophilic fungi

Reduction in water activity (aw) is used as a microbiological hurdle to prevent food spoilage. To minimize the levels of salt and sugar, which are commonly used to reduce aw, the potential of food structure as a microbiological hurdle needs to be assessed. The concept of matric potential (Ψm) is used to measure the effect of food structure on water movement. This study reports the effect of reduced aw and reduced Ψm on the germination of xerophilic fungi (represented by Eurotium herbariorum) and non-xerophilic fungi (represented by Aspergillus niger) on model glycerol agar media. Germination curves were plotted with the percentage of germinated spores against time. The germination time (tG), which is defined as the time at which 50% of the total viable spores have germinated, was estimated using the Gompertz model. Total viable spores was defined as those spores that were able to germinate under the optimum aw and Ψm conditions for each species, i.e. 0.95 aw and 2.5% agar for E. herbariorum and 0.98 aw and 2.5% agar for A. niger. As aw decreased from 0.90 to 0.85 aw, tG increased significantly for both the xerophilic fungi and non-xerophilic species at equivalent matric potential values. When matric potential was reduced from −12 kPa (2.5% agar) to −38 kPa (12.5% agar), tG of A. niger was significantly extended at 0.90 aw; however, tG remained the same for A. niger at 0.85 aw, and for E. herbariorum at 0.80, 0.85 and 0.90 aw. This study demonstrated that the germination time for non-xerophilic and xerophilic fungi was extended by reduced aw, however the effect of reduced Ψm was limited.