Influence of solute, pH, and incubation temperature on recovery of heat-stressed Wallemia sebi conidia

Abstract

The influences of glucose, sorbitol, and NaCl in a basal enumeration medium at water activities a(w) from 0.82 to 0.97 on colony formation by sublethally heat-stressed Wallemia sebi conidia were determined. Over this a(w) range, glucose and sorbitol had similar effects on recovery, whereas at an a(w) of 0.82 to 0.92, NaCl had a detrimental effect. Colony diameters were generally largest on media containing sorbitol and smallest on media containing NaCl. Maximum colony size and viable population of heat-stressed conidia were observed on media at an a(w) of ca. 0.92. When the recovery incubation temperature was 20°C, the number of uninjured conidia detected at an a(w) of 0.82 was reduced compared with the number detected at 25°C, while at 30°C, the number recovered at an a(w) of 0.97 was reduced. The effect on heat-stressed conidia was magnified. This suggest that W. sebi conidia may be more tolerant of a(w) values higher than the optimum 0.92 when the incubation temperature is decreased from the near optimum of 25°C and less tolerant of a(w) values greater than 0.92 when the incubation temperature is higher than 25°C. The sensitivity of heat-stressed conidia increased as the pH of the reovery medium was decreased from 6.55 to 3.71. W. sebi conidia dispersed in wheat flour at a(w) values of 0.43 and 0.71 and stored for up to 65 days at both 1 and 25°C neither lost viability nor underwent sublethal desiccation or temperature injury. These results indicate that media routinely used to enumerate molds in foods and feeds are not suitable for enumerating heat-stressed W. sebi conidia.