POSSIBLE BENEFITS OF ADDING YEAST TO PROBIOTiC YOGURT.

Poor survival of probiotic bacteria in yogurt has been well documented. The poor survival 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. 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. In the first study, growth of a probiotic yeast, Saccharomyces boulardii, in association with the bio-yogurt microflora, was studied in commercial bio-yogurt and UHT yogurt. S. boulardii was incorporated into the products and stored at 4°C over a 4-week period, which is the normal time accepted as the shelf-life of yogurt. Lactose utilisation, pH and the production of organic acids were monitored at each interval.

Based on the results in this study, S. boulardii, has the ability to grow in bio-yogurt reaching maximum counts exceeding 10⁷ colony forming units (cfu) per gram. The number of yeast populations was substantially higher in the fruit based yogurt, mainly due to the presence of proportions of sucrose and fructose derived from the fruit. Despite the inability of S. boulardii  to utilize lactose, the yeast species successfully utilized the available organic acids, galactose and glucose derived from the bacterial metabolism of the lactose present.

The investigation was expanded in the second study. Four dairy-associated yeasts, Debaryomyces hansenii, Kluyveromyces marxianus, Yarrowia lipolytica and Issatchenkia orientalis, commonly associated with yoghurt, were isolated and inoculated into yoghurt and related dairy products during processing. The survival and growth of the yeasts were monitored over a 4-week storage period. Sugar utilization, pH 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 per gram. As in the first study, some of the species were unable to utilize lactose. However, they utilized the 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. Thus, the addition of yeasts is promising. However, as in the first study, excessive gas and ethanol production initiated by some yeast species proved to be major constraints which require further investigation.

References:

Lourens-Hattingh, A. & Viljoen B.C., 2001. Growth and survival of a probiotic yeast in dairy products.  Food Research International 34, 791-796.

Lourens-Hattingh, A. & Viljoen B.C., 2002. Survival of dairy-associated yeasts in yoghurt and yoghurt-related products. Food Microbiology 19, 597-604.