EFFECTS OF THERMAL-STRESS AND REDUCED WATER ACTIVITY ON CONIDIA OF ASPERGILLUS-PARASITICUS

Conidia of A. parasiticus (an aflatoxin producer) were harvested from an agar medium, centrifuged, and resuspended in 0.05% Tween-80. The suspension was subjected to a thermal stress of 51.degree. C for up to 4 h. Viability and thermal injury were evaluated by the use of 2 plating media, yeast extract agar and yeast extract agar + 10% NaCl. Evidence of injury was demonstrated by the difference in colony counts on the 2 media. The number of injured cells increased as the heating time continued. Colonies from injured conidia developed more slowly than colonies from unheated conidia. The effect of a reduced water activity (aw) was evaluated by comparing the ability of unheated and heated conidia to grow in media of differing water activities. A most probable number technique was used to measure growth. Media of reduced aw limited the ability of injured conidia to outgrow as compared to unheated conidia. Continued storage of injured conidia in media of reduced aw eventually resulted in a loss of colony-forming ability. Solutes used to lower aw were sodium chloride, glycerol, and sucrose. This mold is of interest to the food industry because of its ability to grow under a wide variety of product storage conditions and because of its ability to produce carcinogenic secondary metabolites known as aflatoxins.