Screening And Identification Of High-yielding γ-aminobutyric Acid Lactic Acid Bacteria And Their Application In Fermented Soymilk

At present,biosynthesis is the safest method to prepare γ-aminobutyric acid(GABA),but the resources of lactic acid bacteria strains with high GABA production are not sufficient.Relying on the existing strains in the laboratory,this experiment screened lactic acid bacteria strains with high GABA production,analyzed the growth and fermentation characteristics of the strains,and applied them to functional fermented soymilk to investigate the effects of strains combination on GABA formation and flavor quality of fermented soymilk.It provides theoretical basis for the development of rich probiotics resource bank and functional fermented soymilk.The research contents are as follows:(1)Screening and identification of lactic acid bacteria strains with high GABA production.Three strains with high GABA production and effective resistance to gastrointestinal digestion were obtained from 80 strains in the laboratory,namely SMN10-3,SMN12-7,and SMN15-6,and the GABA production contents were 2.01mg/m L,1.48 mg/m L and 1.90 mg/m L,respectively.According to molecular biology identification,SMN15-6 is Pediococcus lactis,and SMN10-3 and SMN12-7 are Lactobacillus fermentum.Further combining the acid-producing ability and GABA yield of three target strains in soymilk,two strains of Lactobacillus fermentum SMN10-3 and Pediococcus lactis SMN15-6 were selected to fermented soymilk.(2)Effects of strains combination on the formation of GABA in fermented soymilk.Lactobacillus fermentum SMN10-3 and Pediococcus lactis SMN15-6 were used to ferment soymilk in different proportions,and found that strains combination could effectively increase the activity of glutamate decarboxylase and the production contents of GABA.SMN10-3: SMN15-6=2:1 group had the highest glutamate decarboxylase activity(62.06U/m L),and the GABA content reached 1.736 mg/m L,which was the best compounding ratio.(3)Effects of strains combination on volatile flavor compounds of fermented soymilk was investigated.The compositions and differences of flavor compounds in soybmilk from different groups were investigated by GC-IMS,electronic nose and amino acid analyzer.A total of 32 volatile flavor compounds were detected in six groups of samples.Significant difference was found in the odor of fermented and unfermented soymilk.The beany flavor compounds such as hexanal and 2-hexenal were not detected after fermentation,while the flavor compounds such as 2-cyclohexene-1-one and 2-ethyl hydroxypropionate were significantly increased,indicating that the beany flavor of soymilk could be moderately reduced by fermentation,and the flavor composition of the samples could be enriched.The fermented soymilk with the ratio of SMN10-3: SMN15-6=2:1 had higher umami and lower bitterness,and had better taste.(4)Storage characteristics of high-yield GABA fermented soymilk.The changes of GABA content,p H value,viable count and taste of fermented soymilk within 28 days were studied.It was found that SMN10-3: SMN15-6=2:1 fermented soymilk had the lowest decrease rate of GABA content,p H value,the viable count,and had good quality stability during storage.The flavor richness of the fermented soymilk was improved after short-term storage(7 days),but the flavor was also lost if the storage time was too long.