Study On The Influence Of Lactic Acid Bacteria On The Flavor And Antioxidant Activity Of Fermented Soymilk

Fermented soymilk has obtained more and more concerns because of its nutrition and health-care function.However,the off-flavor of fermented soymilk has become the key issue to be solved in the field of fermented soymilk.In this paper,the influence of lactic acid bacteria(LAB)on the flavor and antioxidant effect of fermented soymilk was investigated.The results provide support for development of high-quality of fermented soymilk.The main results ineludes:1.Isolation,screening and identification of lactic acid bacteria.135 strians were isolated from 20 samples of tradition fermented products,including 91 cocci isolates and 44 bacilli isolates.12 strians of LAB were screened from 50 strians which were selected randomly by fermentation and storage stability in soymilk.The identification result showed that 2 strians were identified as Lactobacillus rhamnosus,1 strian was identified as Lactobacillus helveticus,5 strians were identified as Streptococcus thermophilus,2 strians were identified as Enterococcus durans and 2 strians were identified as Enterococcus faecium.2.Study on the flavor of fermented soymilk.The enzymes activities of 5 S.thermophilus and the fermentation characteristics,flavor compounds,sensory evaluation of fermented soymilk by 5 S.thermophilus were measured,and the correlation among each other showed that the sensory evaluation of fermented soymilk was significant positive correlated witha-galactosidase(a-Gal),and flavor compounds of 2,3-butanedione,2-heptanone,2-nonanone,was significant negative correlated witha-acetolactate decarboxylase(a-ALDC),and flavor compounds of hexanol,2,4-decadienal,2-pentylfuran.Simultaneously,the acidity of fermented soymilk was significant positive correlated with?-galactosidase(?-Gal)and lactate dehydrogenase(LDH).Through the analysis of physical and chemical indicators,fermentation characteristics,sensory evaluation,flavor compounds and the changing trend of fermented soymilk/milk mixture by S.thermophilus grx90,the results showed that the most important compounds contributed to the off-flavor of fermented soymilk were 1-octen-3-ol,hexanol,2-heptenal and 2,4-decadienal.According to comprehensive evaluation,S.thermophilus grx90 is the preferred strain of soymilk fermentation.3.Study on optimization technology of fermented soymilk.Using S.thermophilus grx90 as the fermentation strain,the optimum preparation conditions of fermented soymilk were as follow:the ratio between soybean and water was 1:8,the proportion of soymilk and milk was 7:3,the amount of sucrose and glucose was 4%and 2%,pasteurization at 90-95? for 15min,and the fermentation condition was 42 ? for 4h with a inoculation rate of 4%.The analysis result of product quality showed that the total protein,fat,non-fat milk solids,titration acidity and the number of valid bacteria in fermented soymilk were above 2.8%,2.2%,8.5%,55°T and108CFU/mL.4.Study on antioxidant effect of fermented soymilk.In vitro antioxidant activity of soymilk(SM),lactic acid bacteria/soymilk mixture(SMC),fermented soymilk by S.thermophilus grx90(FSM)and pasteurized fermented soymilk(PFSM)were evaluated.The result showed that FSM exhibited 32.76%2,2-diphenyl-1-picrylhydrazyl(DPPH)-scavenging activity at 0.2g/mL,which was slightly lower than the effect shown by ascorbic acid at 2?g/mL,Fe2+-chelating effect of FSM was 31.89%which was slightly higher than the effect shown by EDTA at 0.02mg/mL,and FSM had the most potent capacity in the four soymilk samples.In vivo,as to acute alcoholic liver injury model mice,FSM obviously reduced serum alanine transaminase(ALT)and aspartate transaminase(AST)levels,elevated liver tissue superoxide dismutase(SOD)and glutathione perioxidase(GSH-Px)activities,and malondialdehyde(MDA)content was decreased.