Application Of Lactobacillus Delbrueckii Subsp.bulgaricus T-50 And Streptococcus Thermophilus S10 Mixed Starter

Lactobacillus delbrueckii subsp.bulgaricus and Streptococcus thermophilus are the main components of fermented milk starter.In this study,Lactobacillus bulgaricus T-50 and Streptococcus thermophilus S10 with good fermentation characteristics were taken as the research objects,and the effects of the compound ratio of the two on the fermentation characteristics of fermented milk at the ratios of 1:10,1:100 and 1:1000,respectively,were studied to determine the optimal fermentation ratio.Non-targeted metabolomics was used to explore the changes of metabolomics when the optimal combination proportion fermentation and single strain fermentation were adopted,and the differential metabolites were analyzed.The fermentation broth with that optimal compound proportion was used as the experimental group and three commercial fermentation broth with different types were used as the control group,and the viscosity was ranked as PYS-V141>YF-L922>JD.By compare the microrheological properties,storage stability and sensory evaluation,the main results were as follows:The experimental results of fermented milk with different compound proportions showed that the fermentation time in the 1:10,1:100,and 1:1000 compound groups was 5.21h±0.23,5.40h±0.15,and 5.59h±0.18,respectively,and there was no significant difference between the groups(P > 0.05).Among them,fermented milk with the ratio of 1:1000 had weak post-acid property,and its viscosity and sensory evaluation scores were significantly higher than those of the other two groups(P < 0.05),which was determined as the optimal combination ratio.The results of differential metabolite enrichment analysis in the single bacteria group and the experimental group showed that the differential metabolites between the single bacteria fermented milk and the fermented milk with the optimal compound ratio mainly included organic acids,amino acids and small peptides.The differential metabolites mainly involved in phenylalanine,tyrosine,tryptophan biosynthesis,phenylalanine metabolism,hydrochloride and nicotinamide metabolism,lemon salt cycle(TCA cycle)and many other complex metabolic pathways,among which,the enriched substances such as succinic acid,tyrosine,and tryptophan were more than those in the single strain,indicating that the combination of the two strains could improve the glycolysis and protein hydrolysis abilities.The micro-rheological results of fermented milk in the experimental group and different control groups during fermentation showed that the gel properties of fermented milk in the experimental group were similar to those in YF-L922 group.During storage,the experimental group and the control group maintained good storage stability.Among them,the viable count,pH,titratable acidity and sensory evaluation scores of fermented milk in the experimental group were not significantly different from those in the YF-L922 group(P > 0.05).Besides,according to the principal component analysis and clustering analysis,the viscosity,water holding capacity and texture properties of fermented milk from these two groups were more similar,and there were significant differences from those of the other two groups(P < 0.05).In summary,the 1:1000 combined fermentation of Lactobacillus bulgaricus T-50 and Streptococcus thermophilus S10 was applied to fermented milk.The fermented milk had excellent fermentation characteristics,medium viscosity,good storage stability and sensory characteristics,and provided a theoretical basis and technical support for the industrial application of the combined starter.