Evaluation of solutes for their ability to retard spreading of Eurotium amstelodami on enumeration media

Abstract

The value of media used to enumerate xerophilic fungi in foods is often limited due to overgrowth of slow-growing strains with colonies of Eurotium species. In an attempt to develop a medium which would minimize this problem, NaCl, glycerol and sorbitol were evaluated as solutes to control spreading of colonies of four strains of Eurotium amstelodami on dichloran rose bengal chloramphenicol (DRBC) agar. Recovery of heat-stressed conidia of two strains was enhanced in DRBC agar containing 12% (a w 0·93) and 16% (a w 0·91) NaCl; the other two strains were unaffected in DRBC agar containing 0–16% NaCl, regardless of heat treatment. Colony spreading was retarded on DRBC agar as the NaCl concentration was incrementally decreased from 8 to 0% or increased from 8 to 20%. Glycerol, at concentrations of 18–34%, had no effect on the number of colonies formed by unheated and heat-stressed conidia, but did not control spreading. The presence of 25–60% sorbitol in DRBC agar did not affect the recovery of unheated conidia; however, 50–55% sorbitol resulted in maximum recovery of heat-stressed conidia. DRBC agar containing 50% sorbitol (DRBC–50S, a w 0·90) was judged overall to be the most suitable solute-supplemented enumeration medium for recovering unstressed, heat-stressed and freeze-stressed E. amstelodami from laboratory menstrua and wheat, peanut and cowpea flours.