Screening Of High Quality Dairy Yeast For Milk Beer Producing And Studies On Active Dry Yeast Preparation

Milk beer is an initiated new type of milk beverage both in domestic and international,which is prepared by two steps fermentation of lactic acid bacteria and yeast.Milk beer is good for human health.Yeast is the key strain in the manufacture of milk beer,which can provide the main flavor substances such as alcohols,acids,esters and so on.At present,the milk beer products produced by commercially available saccharomyces cerevisiae are not able enough to meet the needs of consumers in terms of flavor and taste;some enterprises have improved the quality of milk beer beverage through the use of selected high-quality dairy yeast,but it is not conducive to the long-term development of the milk beer industry due to the lack of high-quality active dry yeast from dairy source.Therefore,the research hotspots on milk beer beverage has been gradually changed into the selection of high quality yeast from dairy source and the preparation of active dry yeast in recent years.In this study,we first screened out a high quality dairy yeast for milk beer producing from Kefir grains,and identified the species,which provided a good yeast for the preparation of active dry yeast.Subsequently,on the basis of shake flask fermentation,the optimum medium composition and the optimum medium culture conditions were studied by using the method of combination of single factor,Box-Behnken design and orthogonal test.At the end of this study,the preparation of active dry yeast by using the method of vacuum freeze-drying was studied,which provided the basis for the industrial milk beer application of dairy yeast active dry yeast.The main results are summarized as follows:Firstly,a high quality dairy yeast strain for milk beer producing was screened out.17 dairy yeasts were isolated from fresh Kefir grains from Tibet,Xinjiang,Heilongjiang and other areas.The stepwise screening included: physicochemical data and microbial colony number of samples fermented by each dairy yeast;the genetic stability of each yeast;sensory evaluation and volatile profile of samples fermented by each dairy yeast;principal components analysis was carried out using physicochemical parameters,sensorial attributes and the volatile profile of samples fermented by each dairy yeast.The results showed that the MJ1 was to be the most suitable yeast based on the assessment criteria,which was identified as Kluyveromyces marxianus strain by using the method of 5.8S rDNA molecular biology.Secondly,the optimal proliferative medium composition and culture conditions of Kluyveromyces marxianus were optimized.The method of single factor and response surface design were used to optimize the medium composition,and the best medium composition was sucrose 66.37 g/L,yeast powder 28.3 g/L,corn syrup 15.61 g/L,KH2PO4 4.13 g/L and MgSO4 0.3g/L;single factor and orthogonal experimental design were used to optimize the culture conditions,optimal culture conditions was 30?,160 r/min and pH 6.0.Thirdly,conditions of the preparation of MJ1 active dry yeast produced by vacuum freeze-drying method were determined,and the optimum formula of vacuum freeze drying protective agent was obtained.The optimum centrifugation condition of MJ1 was 6000 r/min,15 min by comparing the centrifugal yield.;moisture content of freeze-dried powder was 2.9±0.08% after vacuum freeze drying for 24 hours;optimal combination of protective agents was 5% sucrose,5% mannitol and 7.5% skim milk powder through the method of factor and orthogonal experimental design,MJ1 survival rate of freeze-dried powder was about 74.63±1.17%;-20?was more suitable for store up Kluyveromyces marxianus active dry powder than 4?;fermented milk beer samples produced by active dry yeast had good flavor and taste.