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

Abstract

Reduction in water activity ( a w) is used as a microbiological hurdle to prevent food spoilage. To minimize the levels of salt and sugar, which are commonly used to reduce a w, the potential of food structure as a microbiological hurdle needs to be assessed. The concept of matric potential ( Ψ m) is used to measure the effect of food structure on water movement. This study reports the effect of reduced a w and reduced Ψ m 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 ( t G), 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 a w and Ψ m conditions for each species, i.e. 0.95 a w and 2.5% agar for E. herbariorum and 0.98 a w and 2.5% agar for A. niger. As a w decreased from 0.90 to 0.85 a w, t G 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), t G of A. niger was significantly extended at 0.90 a w; however, t G remained the same for A. niger at 0.85 a w, and for E. herbariorum at 0.80, 0.85 and 0.90 a w. This study demonstrated that the germination time for non-xerophilic and xerophilic fungi was extended by reduced a w, however the effect of reduced Ψ m was limited.