Kefir is a traditional beverage that is produced by fermented milk with kefir grains, Kefir grains are complex mixture of bacteria, yeasts, and polysaccharides produced by this micro flora. The initial amount of kefir grains used to produce kefir drinks affects the quality of the beverage in terms of pH, viscosity and microbial characteristics of the final beverage. In this study, the effects of different levels of starter (4, 6 and 8%, containing Streptococcus thermophilus, Lactobacillus delbrueckii subsp. Bulgaricus, Bifidobacterium), yeast Saccharomyces cerevisiae (2, 3 and 4%) and storage time (24, 48 and 72 hours after inoculation) on the physicochemical and microbiological properties of kefir beverage was studied using a central composite design and response surface methodology, in order to produce a product with higher probiotic bacteria than yeast. Physicochemical properties including acidity, pH, ethanol, apparent viscosity and flow behavior parameters, as well as microbial counting was performed. According to the results of the tests, the amount of yeast had the most effect on apparent viscosity and by increasing the amount of yeast and during of storage time, the apparent viscosity and consistency coefficient decreased. In addition, the viscosity increased with increasing starter rates. Regarding the amount of ethanol, the amount of produced ethanol increased by increasing the amount of yeast and during the storage time due to the increase in the amount of yeast and finally the microbial count decreased during storage. Optimization results showed that the sample containing 2% yeast and 4% starter in time 38 hours after inoculation had the lowest amount of ethanol and was suitable for rheological properties.

Keywords: Kefir Starter Yeast Storage time Physicochemical properties

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