Belinda Chapman

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 (a(w)) 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 (psi(m)). 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 a(w) and 0.95 a(w). The matric potential of GA decreased when the agar concentration increased from 1.5 to 7.5%. When the agar concentration increased at each a(w), 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.

Evaluation of the efficacy of four weak acids as antifungal preservatives in low-acid intermediate moisture model food systems

The potential efficacy of four weak acids as preservatives in low-acid intermediate moisture foods was assessed using a glycerol based agar medium. The minimum inhibitory concentrations (MIC, % wt./wt.) of each acid was determined at two pH values (pH 5.0, pH 6.0) and two a(w) values (0.85, 0.90) for five food spoilage fungi, Eurotium herbariorum, Eurotium rubrum, Aspergillus niger, Aspergillus flavus and Penicillium roqueforti. Sorbic acid, a preservative commonly used to control fungal growth in low-acid intermediate moisture foods, was included as a reference. The MIC values of the four acids were lower at pH 5.0 than pH 6.0 at equivalent a(w) values, and lower at 0.85 a(w) than 0.90 a(w) at equivalent pH values. By comparison with the MIC values of sorbic acid, those of caprylic acid and dehydroacetic acid were generally lower, whereas those for caproic acid were generally higher. No general observation could be made in the case of capric acid. The antifungal activities of all five weak acids appeared related not only to the undissociated form, but also the dissociated form, of each acid.

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

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 (Psi(m)) is used to measure the effect of food structure on water movement. This study reports the effect of reduced a(w) and reduced Psi(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 Psi(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 Psi(m) was limited.