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.

Inactivation of food spoilage fungi by ultra violet (UVC) irradiation.

The effect of ultraviolet irradiation (254 nm, UVC) on Aspergillus flavus, Aspergillus niger, Penicillium corylophilum and Eurotium rubrum was investigated using three different exposure techniques. Survival was determined for spores suspended in liquid medium after 1, 2 and 3 min UVC exposure at 4644 J/m(2)/min. The same UVC dose was applied to spores on the surface of agar plates for 5, 10, 15, 30, 60 and 120 s. Spores of A. niger were dried onto a membrane filter, then exposed to UVC treatment. In the liquid medium, treatments from 1-3 min significantly (P<0.001) reduce the number of viable spores. On the surface of agar plates, after a 15 s exposure, a 80-99% reduction of viable spores was observed for all species except A. niger, for which the reduction was only 62%. For spores dried onto filter membranes, a 3.5 log(10) reduction was achieved for A. niger after 180 s exposure. These observations suggest that UVC irradiation can effectively inactivate spores of A. flavus, P. corylophilum, E. rubrum and A. niger but the efficacy of UVC radiation against fungal spores varies significantly according to methods of exposure to the irradiation, and among genera.