Purpose: The objective of this study was to evaluate the effect of aw on the thermal tolerance and survival of osmophilic, spoilage fungi Eurotium repens and Neosartorya fischeri.
Methods: The decimal reduction time (D-value) and the temperature needed to cause a 1-log change D-value (z-value) in a confectionery model food, comprised of evaporated milk and fructose adjusted to 0.70, 0.75 and 0.80 aw, was determined by creating thermal death time curves with inoculated product at three different temperatures. Experiments were performed in triplicate.
Results: N. fischeri exhibited greater thermal tolerance than E. repens. A non-linear interaction between aw and temperature was observed in N. fischeri survival experiments. N. fischeri pairwise tests between water activities and temperatures showed significant differences (P<0.01) between aw of 0.70, 0.75 and 0.80 at all temperatures (90°C, 92°C and 94°C) except at 94°C, between 0.75 and 0.80 aw. E. repens demonstrated the greatest thermal tolerance at 0.70 aw at 78°C (4.873±0.576 min), 80°C (1.645±0.011 min) and 82°C (1.144±0.019 min). In general, the heat tolerance of E. repens increased as the water activity decreased however pairwise tests showed no significant difference between D-values at 78°C, 80°C or 82°C between 0.75 and 0.80 aw.
Significance: Our results will be useful for determining limits for thermal processing of low water activity confectionery products to control the growth of osmophilic fungi and to extend shelf life, without the use of preservatives.