Screening Of Lactic Acid Bacteria With High Conversion Of Soy Isoflavones And Its Application In Fermented Soymilk

Due to the large population base in China,milk resources are in short supply.In addition,the yogurt market competition is fierce,some people pursue vegetarianism,the development of plant protein drinks has become a new trend,and the research on soy and functional soy products is also in-depth,Lactobacillus fermented soy products are favored by consumers for their unique health effects.It is a new type of nutritious and healthy food that has probiotic properties of lactic acid bacteria and contains soy nutrients.In this study,isoflavone aglycones were used as the starting point,and the sieved L.plantarum 58 was used to convert the bound isoflavone glycosides into free isoflavone aglycones and determ:ine the optimal conversion conditions of soy isoflavones in soymilk.The fermentation characteristics and storage stability of soymilk were studied,focusing on enhancing the antioxidant activity of fermented soymilk and increasing the bioavailability of soy isoflavones,providing an experimental basis for the development of high-quality fermented soymilk with both flavor,nutrition and functional activity.The main studies are as follows:(1)Screening and identification of lactic acid bacteria with high conversion of soy isoflavones.The esculin color experiment was used to screen 140 lactic acid bacteria stored in the Key Lab of Dairy Biological Technology and Safety Control of Jiangsu Province for ?-glucosidase-producing strains,and the enzyme-producing strains were used to ferment soymilk.The ability of lactic acid bacteria to transform soy isoflavones was determined by high performance liquid chromatography,and the acid production rate,acidity,viable count and water holding capacity were compared to obtain the best strain.There were 12 strains producing ?-glucosidase,7 strains of which were over 55 mg/L of soy isoflavones.The content of soy isoflavone aglycone(59.64 mg/L)in fermented soymilk by strain 58 was the highest,which was more than 8 times of soymilk(7.15 mg/L).In addition,the fermentation characteristics such as acid production rate(3.11°T/h),final fermentation acidity(35.01°T),viable count(8.78 logCFU/mL),and water holding capacity(35.10%)of strain 58 were superior to the other 6 strains,so it was determined as the experimental strain for optimizing soy isoflavone conversion conditions.After 16S rDNA sequencing,strain 58 was identified as L.plantarum.(2)Objective to optimize the conditions of soy isoflavones in soymilk transformed by L.plantarum 58.Taking the growth of L.plantarum 58 and ?-glucosidase activity as indicators,the most suitable carbon source for L.plantarum 58 was selected.The growth curve of L.plantarum 58 and the changes in ?-glucosidase activity were determined to select the optimal inoculation time for L.plantarum 58.Soy isoflavone aglycone content was set as an indicator,and the amount of inoculation,lactose content,fermentation temperature,fermentation time were selected as single factors for testing.On this basis,the response surface method was combined to determine the optimal transformation conditions of soy isoflavone.The ?-glucosidase activity(0.66 U/mL)of L.plantarum 58 was the highest in lactose medium,which was better than the medium with sucrose,glucose,fructose,and maltose.The ?-glucosidase activity(0.75 U/mL)reached the best when cultured for 18 h.Through Design-Expert 8.0.6.1 software analysis,a multiple regression model was obtained(Y=68.03-1.56A-0.74B+1.13C+2.10D+0.53AB-0.08AC-0.09AD-0.16BC-0.048BD+0.66CD-4.83A2-3.43B2-2.97C2-4.08D2).The optimal conditions for L.plantarum 58 to transform soy isoflavones are 3.83%inoculation amount,5.75%lactose content,38.14? fermentation temperature,9.83 h fermentation time.Under this condition,the measured content of soy isoflavone aglycon(68.16 mg/L)was 15.07%higher than the content of soy isoflavone aglycon before optimization,which was not significantly different from the response surface prediction value(68.63 mg/L).The quadratic model is scientific and accurate.(3)Study on the antioxidant activity of fermented soymilk.The antioxidant activity of L.plantarum 58 fermented soymilk,soymilk,unfermented soymilk and pasteurized fermented soymilk were compared through indicators such as total antioxidant capacity,DPPH free radical scavenging ability,and ferrous ion chelating ability,then the relationship between antioxidant activity and concentration of fermented soymilk was explored.The total antioxidant capacity,DPPH radical scavenging capacity and ferrous ion chelating capacity of 4 kinds of samples were different.When concentrations are 0.10 g/mL,the total antioxidant capacity(0.27 mmol/L Trolox,0.58 mmol/L FeSO4)of fermented soymilk is the strongest,which is significantly higher than that of the other 3 kinds of samples(P<0.05).When concentrations are 0.04 g/mL,fermented soymilk has the strongest ability to scavenge DPPH free radicals(48.06%),which is equivalent to the scavenging ability of vitamin C at 0.01 g/L concentration.When concentrations are 0.05 g/mL,fermented soymilk has the strongest ferrous ion chelating ability(80.76%),which is equivalent to the chelating ability of EDTA at 0.03 g/L concentration.The total antioxidant capacity,DPPH free radical scavenging capacity and ferrous ion chelating capacity of soymilk were the weakest.The fermentation of L.plantarum 58 enhances the antioxidant activity of soymilk.(4)Study on fermentation characteristics of L.plantarum 58 and stability of soymilk during storage.Soymilk was fermented with L.plantarum 58 under the best conditions to transform soy isoflavones,and then the sensory,physicochemical properties,changes of various indexes during fermentation and storage of soymilk were studied.The fermented soymilk of L.plantarum 58 is milky white,slightly yellow,with good taste,uniform and fine texture,and a trace of whey is precipitated.The protein content is 1.89%,the fat content is 2.01%,the solid content is 12.96%,the acidity is 46.48°T,the viable count is 8.85 logCFU/mL,and the water holding capacity is 35.03%.In addition,the content of lactic acid in fermented soymilk is 5.17 g/L and the content of acetic acid is 0.56 g/L,which helps to improve the storage performance of the product,imparting sourness and refreshing taste.During the low-temperature storage at 4? for 21 days,the stability of fermented soymilk was good,the pH and acidity did not change much,and the viable count remained above 108 CFU/mL.(5)Study on the bioavailability of soy isoflavones in fermented soymilk.The changes of soy isoflavone monomers in soymilk and fermented soymilk during simulated gastrointestinal digestion were measured,and the effects of gastrointestinal digestion and fermentation of L.plantarum 58 on the bioavailability of soy isoflavone monomers in soymilk were explored.After fermentation,the bioavailability of daidzein and genistein increased from 53.18%and 30.75%to 54.61%and 32.68%respectively.The bioavailability of daidzein and genistein in fermented soymilk reached 60.31%and 47.55%respectively.The bioavailability of soy isoflavone in fermented soymilk was significantly improved by L.plantarum 58 fermentation(P<0.05),the bioavailability increased from 36.76%to 40.86%.