Research On The Fermentation Characters Of The Selected Yeast And Lab And Development Of Kefir From Mixed Milk

In this paper, the yeast and lactic acid bacteria strains were ascertained to develop the kefir of consistent and controlled quality from soya milk and cow’s milk and also fermentation mechanism of the product was studied simultaneously. Accordingly, the changes of microflora, organic acids and volatile organic compounds were analysed in order to investigate the interaction between the microorganisms. It was expected to develop the products achieving the aim of complementation between animal and vegetable nutrients. Meanwhile, it was beneficial to not only reduce the cost, but also improve the flavor of traditional kefir and expand the application of soybean. The main contents and results were listed as following:The viable counts, total acid, sugar and ethanol in the fermented milk were detected by intervals during the fermentation in order to screen out the suitable yeast strains. From the observations, it was concluded that S. cerevisiae Yl grew well in mixed milk, produced the high amount of total acid, had the relatively strong ability of decomposition of sugar, produced the suitable amount of ethanol and the flavour profile of the fermented milk was good. Therefore, S. cerevisiae Y1was selected as the best yeast strain for fermentation.This paper investigated the variation of viable yeast and bacterial counts, total acid, viscosity, organic acids and volatile organic compounds during the fermentation of the yeast and lactic acid bacteria strains and analyzed the interaction between the microorganisms by correlating the microflora with the changes of other substances so as to choose the appropriate LAB strains which shows the synergistic effect with S. cerevisiae Y1.It was indicated that interactions with a general tendency between LAB and yeasts in the co-cultures were not observed, instead punctual strain-specific effects were noted. The results emphasized the yoghourt culture (Lactobacillus delbreckii ssp bulgaricus B1&Streptococcus thermophilus T5) was the most suitable one among the LAB strains.The fermentation conditions of kefir were optimized through fractional factorial and central composite designs. The results indicated that the optimal conditions were the ratio of cow-milk and soymilk was7:3, the inoculum of the LAB and yeast were7.001og10cfu/mL and5.001og10cfu/mL respectively, the addition amounts of sucrose were1.6%.the fermentation temperature was22℃and the time was22h.The characteristics of final product were:pH(4.32), titratable acidity(0.73%), ethanol(1398.13μg/g), viscosity(2789.68mPa-s), respectively.The basic mechanism of the kefir was examined. The results showed that Streptococcus thermophilus T5grew fast during fermentation after4h.Thus produced the lactic acid which provided the well growth circumstances for Lactobacillus delbreckii ssp bulgaricus B1and S. cerevisiae Y1with the increasing viable counts during8-16h. However, Streptococcus thermophilus T5was conspicuously inhibited after16h. The viable counts of Lactobacillus delbreckii ssp bulgaricus B1was increasing during the whole fermentation because it could resist the high amount of acidity and ethanol in the fermented milk. In the early stage, S. cerevisiae Yl grew slowly, while it produced large amount of ethanol and acetaldehyde during anaerobic fermentation. It could be observed that the galactose, pyruvate and other substances by LAB metabolism may promote the growth and metabolism of S. cerevisiae Y1. The variation of the viable counts of microorganisms showed close relationship to their corresponding metabolic products during fermentation.