Progress in Conventional Methods for Detection and Enumeration of Foodborne Yeasts
Larry R. Beuchat
Center for Food Safety and Quality Enhancement, Department of Food Science and Technology, University of Georgia, Griffin, Georgia 30223-1797, USA
Article history:
Received September 12, 1997
Accepted November 11, 1998
Key words:
yeasts detection, yeast enumeration, foodborne yeasts, mycological media
Summary:
Genera and numbers of yeasts associated with various types of foods differ according to their ability to tolerate or thrive in environments characterized by wide ranges of temperature, pH, Eh, water activity (aw) and nutrient content. The performance of a particular medium for detecting different ecological groups of yeasts is also affected by the chemical and physical nature of the food environment. No single medium is satisfactory for detecting, isolating and enumerating all genera of yeasts and in all foods. Antibiotic-supplemented media such as di-chloran rose Bengal chloramphenicol agar and tryptone glucose yeast extract chloramphenicol agar are generally superior to acidified potato dextrose agar and other acidified media for enumeration of the vast majority of spoilage yeasts, particularly when cells are in a stressed condition. Dichloran glycerol (18%) agar performs well for enumerating moderately xerotolerant yeasts. Malt extract yeast extract glucose (up to 70%) agar can be used for detecting and enumerating moderate and extreme xerophiles. Lysine agar, Schwarz differential agar and Lin's wild yeast differential agar are used by the brewing industry to differentiate wild yeasts from brewer's strains. Lysine agar is selective for apiculate yeasts and ethanol sulfite yeast extract agar is selective for Saccharomyces. Modified molybdate agar can be used to selectively isolate yeasts in tropical fruits. Preservative-resistant yeasts can be detected on acidified (0.5% acetic acid) tryptone glucose yeast extract agar. The recommended incubation temperature is 25°C, but incubation time between plating and counting colonies ranges from 5 days for determination of general populations of yeasts to 10 days for xerotolerant yeasts. New and improved media for selectively isolating various groups, genera, species and strains of yeasts capable of growing only under specific environmental conditions in specific types of foods and beverages are needed.
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