Occurrence and Growth of Yeasts in Yogurts

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Abstract
Yoghurts purchased from retail outlets were examined, for the presence of yeasts by being plated onto oxytetracycline malt extract agar. Of the 128 samples examined, 45% exhibited yeast counts above 103 cells/g. A total of 73 yeast strains were isolated and identified as belonging to the genera Torulopsis, Kluyveromyces, Saccharomyces, Candida, Rhodotorula, Pichia, Debaryomyces and Sporobolomyces. T. candida and K. fragilis were the most frequently isolated species, followed by S. cerevisiae, R. rubra, K. lactis and T. versatilis. The growth of yeasts in yoghurts was related to the ability of the yeasts to grow at refrigeration temperatures, to ferment lactose and sucrose, and to hydrolyze milk casein. Most yeast isolates grew in the presence of 100 .mu.g of sorbate and benzoate preservatives/ml. Higher yeast counts from yogurts were obtained when the yoghurts were plated onto oxytetracycline malt extract agar than when they were plated onto acidified malt extract agar.

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