The occurrence and diversity of yeasts in commercial yoghurt
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Yeasts have a competitive advantage in yoghurt due to its ability to grow at low pH values and temperatures and therefore are major role players in causing spoilage. The yeasts occurring as natural microflora in commercial yoghurt were isolated and identified according to conventional identification and enumeration techniques. Characteristics of the naturally contaminating yeasts of commercial yoghurt revealed a limited diversity of yeast species. The yeasts most frequently isolated were Kluyveromyces marxianus, Debaryomyces hansenii, Saccharomyces cerevisiae, Saccharomyces exiguus and Yarrowia lipolytica. Rhodotorula spp. occurred at a very low incidence. Since these yeasts play a substantial role in the spoilage of commercial fruit yoghurts,especially when cold storage practices are neglected, the deterioration of yoghurt samples obtained from the manufactures were evaluated at different temperatures for a period of 30 days during this study. Based on the results obtained, the interaction between the yeasts and lactic acid bacteria resulted in a decline in pH values and the stabilization of viable lactic acid bacterial loads. The highest number of yeast populations, up to 104 and 106 cfu/g, was found when yoghurts were exposed to elevated temperatures in the range of 25°C, while lower yeast counts were obtained from samples kept refrigerated at a temperature of 5°C. Populations not less than 103 cfu/ml were generally observed in commercial yoghurt samples. All the isolated yeasts were examined based on relevant key properties that governed their growth and survival in yoghurt. All the yeasts were tolerant to 4-8% NaCI, except for Saccharomyces cerevisiae, and 40 - 80 ppm nitrite. The lipolytic and proteolytic activity appeared variable. The yeasts in 60 samples of Hungarian yoghurt were enumerated and identified according to conventional methods and compared with a similar survey conducted in South Africa. The yeast counts ranged from 103 - 105 cfu/g in both countries whereas the lactic acid bacterial loads showed similar declining values with their maximums ranging from 105 - 106 cfu/g. Despite similar manufacturing procedures, types of yoghurt, and starter cultures, Hungarian yoghurts showed a less diverse population of yeasts. Only Saccharomyces cerevisiae was isolated from yoghurts from both countries. High proportions of Candida parapsilosis, Pichia cactophila and Torulaspora delbrueckii found in Hungarian yoghurts were absent in South African yoghurts.