Mutation Breeding,Fermentation Preparation And Probiotic Function Of High-yield Extracellular Polysaccharide Streptococcus Thermophilus Strains

Probiotic lactic acid bacteria are closely related to the physiological and metabolic activities of consumers and animals.In the hosts,the microbial flora can be optimized,and the immune system can be stimulated to regulate growth and development,and prevent and treat diseases.Streptococcus thermophilus is essential in the preparation of dairy products.The strain can be used in fermented milk,cheese,butter and other dairy products.The strain not only produces bacteriocin,but also produces metabolites such as lactic acid and polysaccharide.And its important pollution-free natural substances.At present,people have learned that lactic acid bacteria polysaccharide can enhance the body immunity,regulate blood pressure balance,reduce blood lipids,enhance physical fitness and other functions.Therefore,studying the extracellular polysaccharide EPS（Extracellular Polysaccharide）produced by Streptococcus thermophilus has practical theoretical research value.In this experiment,the extracellular polysaccharide Streptococcus thermophilus Q4isolated and screened from the yak milk of the Qinghai-Tibet Plateau was used as the research object.After the growth characteristics test,the high-yield EPS mutant strain was selected by physico-chemical compound mutagenesis.On the basis of the mutagenized strain,the response surface of the strain high-yield EPS was optimized,and the functional yak milk Direct Vat Starter（DVS）was prepared.The results of the study are as follows:1.This study used the Q4 of Streptococcus thermophilus isolated from the yak milk of the Qinghai-Tibet Plateau to screen for Q4F8 high-yield EPS mutations by ultraviolet（UV）and diethyl sulfate（DES）through single mutagenesis and complex mutagenesis.Strain,and the strain had good genetic stability.The extracellular polysaccharide was up to 150.33 mg/L,which was 80.95%higher than the initial strain,and the EPS content increased significantly（p<0.01）.At the same time,the efficiency of individual mutagenesis was lower than the efficiency of compound mutagenesis.2.Based on GM₁₇-based medium,the medium matrix and conditions were optimized by single factor screening test,Placket-Burman Design（PBD）test and Box-Benhnken Design（BBD）test optimization method.The results showed that the main order of factors affecting the synthesis of EPS by Streptococcus thermophilus was pH>culture temperature>lactose mass fraction.The optimum process parameters were pH 6.90,culture temperature 42℃and lactose mass fraction 1.5%,and the EPS synthesis amount reached 189.98 mg/L under this condition,which was 2.29 times higher than the original strain.By combining the mutagenesis screening of the original strain Q4 and the culture conditions and medium optimization,the EPS yield of the strain was significantly increased（p<0.01）.At the same time,studies had shown thatStreptococcus thermophilus Q4F8 was superior to plant peptone in its ability to utilize animal peptone.3.The optimal Direct Vat Starter for yak milk was prepared by strain proliferation culture,pre-freezing conditions,single protective agent screening and orthogonal optimization test for the preparation of high value-added yak yoghurt.Studies had shown that the best combination of protective agents for the strain was:skim milk 12%,sodium glutamate 1.0%,glycerol 6%and trehalose 8%,and the vacuum-packed starter had good stability and the strain has better probiotic performance.4.Study on rheological characteristics of yogurt and probiotic characteristics of strains.The apparent viscosity of the mutagenized strain was 57.42%higher than that of the original strain.The fermented milk prepared by Q4F8 was a typical non-Newtonian pseudoplastic fluid;the Q4F8 strain was resistant to gastrointestinal fluid,bile salt and common pathogenic bacteria.The ability was significantly increased（p<0.05）.Streptococcus thermophilus Q4F8 had good rheological properties and probiotic function.In summary,the mutant strain had effectively improved the yield of metabolite EPS by mutagenesis breeding and culture conditions optimization,while the high-yield EPS Streptococcus thermophilus single strain starter had good fermentation characteristics in the yak milk fermentation industry.At the same time,it could provide a theoretical reference for the industrialized preparation and application of functional lactic acid bacteria to better serve the fermentation industry and lactic acid bacteria starter.