Consumers are increasingly searching for preservative-free and "natural" food options. Confectionery products with low water activity (aw) are typically considered resistant to contamination with filamentous fungi; however, several xerophilic fungi can cause considerable economic losses due to spoilage. Very few studies have been published addressing filamentous fungi that are tolerant to low aw in food products containing all nutrients essential for filamentous fungi growth, including sugars, carbohydrates, proteins, and fats, but not preservatives. The objective of this study was to evaluate the effect of aw on the thermal tolerance and survival of one strain of Aspergillus pseudoglaucus and two strains of Aspergillus fischeri. The decimal reduction time ( D-value) and the temperature needed for a 1-log change in D-value ( z-value) in a low-acid (pH 6) confectionary model comprising evaporated milk and fructose adjusted to aw of 0.70, 0.75, and 0.80 were determined by creating thermal death time curves. Experiments were performed in triplicate. A. fischeri had higher thermal tolerance than did A. pseudoglaucus. An interaction between aw and temperature was found; however, it was not linear with respect to aw. A. fischeri had the highest thermal tolerance for 0.75 aw at 90°C ( D-values of 11.5 to 34.8 min) compared with 0.70 and 0.80 aw, for 0.80 aw or 0.75 aw depending on the strain at 92°C (4.3 to 17.1 min), and for 0.75 aw at 94°C (2.4 to 7.7 min). A. pseudoglaucus had the highest thermal tolerance for 0.70 aw at 78°C ( D-value of 4.9 min), 80°C (1.7 min), and 82°C (0.8 min). The data from this study will be useful for determining parameters for thermal processing of low-aw confectionary products to control the growth of xerophilic fungi.
Keywords: Thermal resistance; Water activity.