Temperature abuse initiating yeast growth in yoghurt

The occurrence of yeasts in dairy products is significant because they can cause spoilage, effect desirable biochemical changes and they may adversely affect public health. While fermentative and spoilage activities of yeasts at elevated temperatures are well known in many food and beverage commodities, little attention has been given to the specific occurrence and significance of yeasts in dairy products at these temperatures. Since 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 manufacturers were evaluated at different temperatures for a period of 30 days during this study. Total yeast were enumerated, isolated and identified from the yoghurt samples. The highest number of yeast populations, up to 10(5) and 10(6) cfu/g, was found when yoghurts were exposed to elevated temperatures in the range of 25 degreesC, while lower yeast counts (10(3) and 10(4) cfu/g) were obtained from samples kept refrigerated at a temperature of 5 degreesC. 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 most prevalent yeast species isolated, included strains of Kluyveromyces marxianus, Saccharomyces cerevisiae, Saccharomyces exiguus and Yarrowia lipolytica. (C) 2002 Elsevier Science Ltd. All rights reserved.