The occurrence and diversity of yeasts in commercial yoghurt

[摘要] Yeasts have a competitive advantage in yoghurt due to its ability to grow at low pH valuesand temperatures and therefore are major role players in causing spoilage. The yeastsoccurring as natural microflora in commercial yoghurt were isolated and identified accordingto conventional identification and enumeration techniques. Characteristics of the naturallycontaminating yeasts of commercial yoghurt revealed a limited diversity of yeast species. Theyeasts most frequently isolated were Kluyveromyces marxianus, Debaryomyces hansenii,Saccharomyces cerevisiae, Saccharomyces exiguus and Yarrowia lipolytica. Rhodotorulaspp. 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 samplesobtained from the manufactures were evaluated at different temperatures for a period of 30days during this study. Based on the results obtained, the interaction between the yeasts andlactic acid bacteria resulted in a decline in pH values and the stabilization of viable lactic acidbacterial loads.The highest number of yeast populations, up to 104 and 106 cfu/g, was found when yoghurtswere exposed to elevated temperatures in the range of 25°C, while lower yeast counts wereobtained from samples kept refrigerated at a temperature of 5°C. Populations not less than103 cfu/ml were generally observed in commercial yoghurt samples. All the isolated yeastswere examined based on relevant key properties that governed their growth and survival inyoghurt. All the yeasts were tolerant to 4-8% NaCI, except for Saccharomyces cerevisiae, and40 - 80 ppm nitrite. The lipolytic and proteolytic activity appeared variable.The yeasts in 60 samples of Hungarian yoghurt were enumerated and identified according toconventional methods and compared with a similar survey conducted in South Africa. Theyeast counts ranged from 103 - 105cfu/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, Hungarianyoghurts showed a less diverse population of yeasts. Only Saccharomyces cerevisiae wasisolated 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.

[发布机构] University of the Free State