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 resemble their natural environment. Heat and freeze-stressed yeasts often exhibit decreased tolerance to acetic pH and selective agents, thus requiring optimum recovery conditions for enumeration. The study reported here was done to evaluate three methods for their suitability to enumerate osmotolerant yeasts from fifty Venezuelan orange juice concentrates. Yeasts were enumerated by surface plating serially diluted samples on plate count agar containing 52% (w/w) sucrose (PCAS 52) and incubating at 32 degrees C for 6 days; pour plating in yeast extract malt extract agar containing 52% (w/w) sucrose (YEMS 52) and incubating at 32 degrees C for 6 days and pour plating in potato dextrose agar (PDA), pH 3.5, and incubating at 25 degrees C for 5 days. Mean populations of yeasts recovered were: 4.2 x 10(5), 5.3 x 10(4) and 7.5 x 10(3) cfu/ml, respectively, on PCAS 52, YEMS 52 and PDA. High recovery on PCAS 52 is attributed to avoidance of osmotic shock and possible secondary lethal effects of high temperature, which may be associated with pour 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 Zygosacharomyces rouxii (Kreger-van Rij W.J.W. (1984) The Yeasts: A Taxonomic Study).