Survival of dairy-associated yeasts in yoghurt and yoghurt-related products Original Research Article

The poor survival of probiotic bacteria added to yoghurts is mainly attributed to the low pH of the product environment. Since yeasts have the ability to metabolize organic acids, resulting in a decrease in acidity, the inclusion of yeasts as part of the normal microflora, in association with probiotic bacteria has been suggested with the intention to assure better survival of the probiotic organisms in bio-yoghurt. Furthermore, a commensalistic association between yeasts and lactic acid bacteria exists. In order to understand the potential impact of yeast on probiotic bacteria, it was firstly important to assess the ability of yeast isolates to grow and survive in yoghurt. Accordingly, four dairy-associated yeasts, Debaryomyces hansenii, Kluyveromyces marxianus, Yarrowia lipolytica and Issatchenkia orientalis, associated commonly with yoghurt were isolated and inoculated subsequently into yoghurt and related dairy products during processing. The survival and growth of the yeasts were monitored over a 4-week storage period, the normal time accepted as the shelf-life of yoghurts. pH, sugar utilization and the production of organic acids were determined on a regular basis during the shelf-life to evaluate the possible contribution of the yeasts towards the products. The yeast species were able to survive in bio-yoghurt reaching maximum counts exceeding 107 cfu g−1. Despite the inability of some species to utilize lactose, the yeast species utilized available organic acids, galactose and glucose derived from bacterial metabolism of the milk lactose, as well as possible free fatty acids or free amino acids present in the dairy products. Excessive gas and ethanol production initiated by some yeast species proved, however, to be major constraints.