Screening And Application Of Streptococcus Thermophilus Suitable For Soymilk Fermentation

Fermented soymilk has been extensively studied for its rich nutrition and unique healthy functions.The defects of quality and flavor caused by its beany flavor have not been solved yet.In this study,based on fermentation and storage properties,functional substances,volatile flavor metabolites and sensory evaluation,Streptococcus thermophilus strains suitable for soymilk fermentation were screened.Further,the screened strains in combination with Lactobacillus plantarum P-8 were used for fermentations.On basis of results,the conclusions are made:(1)Soymilks were fermented by Streptococcus thermophilus S10,Streptococcus thermophilus XJ 57-3,Streptococc1s thermophilus S 32-5,Streptococcus thermophilus GW 45-4 and Streptococcus thermophilus GW 21-4,respectively.These strains can complete fermentations within 10 h(pH is less than 4.60).At the end of fermentation,content of soy isoflavones in the Streptococcus thermophilus S10 group was the highest;during storage,the S10 group had appropriate acidity,stable viable count and high functional substance content.(2)Volatile flavor metabolites in five fermented soymilk samples were analyzed.The results show that in the S10 group,the content of the soy-flavored metabolite,such as hexanal and(E,E)-2,4-decadienal decreased during the fermentations.At 1 d of storage,the contents of hexanal and(E,E)-2,4-decadienal fell by 99%and 89%,respectively,and the contents of key flavor metabolites such as acetaldehyde,diacetyl,etc,were higher.The results of electronic tongue and electronic nose test showed that the S10 group samples were stable and the results of sensory evaluation were consistent with it during storage,which indicate that Streptococcus thermophilus S10 has potential application.(3)The soymilks fermented by Streptococcus thermophilus S10 and Lactobacillus plantarum P-8(S10+P-8)was compared with those fermented by Streptococcus thermophilus S10 group,commercial group A,commercial group B and commercial group C.The acidity and other fermentation characteristics were similar between the S10+P-8 group and the commercial group.At the end of fermentation,the soy isoflavone content in the S10+P-8 group was higher.During storage,the acidity and viscosity of S10+P-8 group and commercial group A are similar,and better than other groups.Specifically,they had a balanced acidity and good stringiness,and the functional substance content are higher.As the probiotic starter cultures,the number of viable cells of Lactobacillus plantarum exceeded 1×109 CFU/mL.The results of electronic tongue and electronic nose test showed that S10+P-8 group,Streptococcus thermophilus S10 group and commercial group A had good stability,and the sensory tasting score was consistent with the above results.In this study,Streptococcus thermophills S10 with excellent soymilk fermentation properties was screened.Further,Streptococcus thermophilus S10 in combination with Lactobacillzs plantarum P-8 was used for soymilk fermentation.It indicates that S10 has a potential application as starter cultures.