COMPARISON OF MEDIA FOR ENUMERATING OSMOTOLERANT YEASTS IN ORANGE JUICE CONCENTRATES

Enumeration of osmotolerant yeasts in high-sugar food products can be improved by using diluents and recovery media formulated to closely re semble their natural environment. Heat and freeze-stressed yeasts ofte n exhibit decreased tolerance to acetic pH and selective agents, thus requiring optimum recovery conditions for enumeration. The study repor ted here was done to evaluate three methods for their suitability to e numerate osmotolerant yeasts from fifty Venezuelan orange juice concen trates. Yeasts were enumerated by surface plating serially diluted sam ples on plate count agar containing 52% (w/w) sucrose (PCAS 52) and in cubating at 32-degrees-C for 6 days; pour plating in yeast extract mal t extract agar containing 52% (w/w) sucrose (YEMS 52) and incubating a t 32-degrees-C for 6 days and pour plating in potato dextrose agar (PD A), pH 3.5, and incubating at 25-degrees-C for 5 days. Mean population s of yeasts recovered were: 4.2 X 10(5), 5.3 X 10(4) and 7.5 X 10(3) c fu/ml, respectively, on PCAS 52, YEMS 52 and PDA. High recovery on PCA S 52 is attributed to avoidance of osmotic shock and possible secondar y lethal effects of high temperature, which may be associated with pou r plating techniques. Significantly (P < 0.05) higher population were recovered on PCAS 52 than on YEMS 52 and PDA. Predominant osmotolerant yeasts were Saccharomyces bisporus var. mellis, recently named Zygos- sacharomyces rouxii (Kreger-van Rij W.J.W. (1984) The Yeasts: A Taxono mic Study).