Biostabilization of Kefir with a Nonlactose-Fermenting Yeast

Abstract

The biostabilization of kefir was studied using sequential fermentation of milk with a nonlactose-fermenting yeast culture and mixed cultures of lactic acid bacteria. To simulate kefir manufacturing, reconstituted skim milk containing three concentrations of glucose (0.4, 0.5 and 1.0%) was fermented first with the yeast culture, followed by lactic fermentation. The yeast fermentation produced ethanol, but the ethanol production did not continue during lactic fermentation with 0.4 and 0.5% of added glucose. However, some continued ethanol production occurred at 1.0% of added glucose. This pattern appeared to be extended to the refrigerated and room storage. During fermentation, all samples showed similar trends in pH reduction, but higher percentages of added glucose caused faster reduction in pH. The titratable acidity showed trends comparable with the pH changes. With 0.4% added glucose, the yeast culture containing Saccharomyces cerevisiae utilized 98% of the added glucose and produced 0.07% ethanol at 31°C during 16h; the mixed lactic cultures utilized 29% of the lactose at 22°C during the additional fermentation of 16h. Little change occurred during storage at 5°C for 7 d. The selected strain of Saccharomyces cerevisiae appeared to be effective in stabilizing kefir products during storage.