Screening Of Suitable Strains For Fermented Soymilk And Development Of Starter

The main strains of starter cultures on the market are Streptococcus thermophilus and Lactobacillus delbrueckii subsp.bulgaricus.Commercially available starter for soybean yoghurt is relatively rare.Bean yoghurt fermentation has unsuitable strains,serious soy odor,heavy acidity,poor stability and other issues.Based on long-term preliminary experiments,it is found that bean yoghurt fermented by kefir has a lighter smell of beans,milder,moderately sweet and sour taste,and a significant increase in aglycone-type isoflavone content.However,kefir grains are natural starters,and their bacterial structures are relatively complex,which are easily affected by the environment and sources.Therefore,the study of kefir flora,the isolation of microbial resources,the screening of functional strains and suitable strains for fermentation,and the study of stable fermentation agents have important application prospects.This research focused on the analysis of domesticated kefir bacteria,isolation of important strains,screening of fermentation strains,screening high-yield soy isoflavone aglycones bacteria,the analysis of the quality of soybean yogurt.The main results are as follows:(1)Through the metagenomics technology,the domesticated kefir flora were sequenced.The major microorganisms are Lactococcus(74.94%),Lactobacillus(24.54%),Acetobacter(0.15%).30 strains of Lactococcus lactis,6 strains of L.subsp.lactis,11 strains of Lb.plantarum,7 strains of Pediococcus acidilactici,2 strains of Lb.casei,3 strains of Lb.paracasei,and 1 strain of Enterococcus faecalis and 3 strains of Lb.kefiri were isolated from kefir.(2)Lactic acid bacteria fermented soybean yoghurt at 25°C for 24 hours.The fermentation strains were screened according to pH,curd,stick-producing ability and flavor.Three strains of Lb.plantarum B3,S.thermophilus F5,and L.lactis G13 were selected from67 strains of lactic acid bacteria.The bean yogur fermented by B3 had better curd and relatively strong acid production.There is better curd,a stringy phenomenon and soy milk flavor in F5 fermented bean yogurt.The bean yogurt fermented by G13 had better curd and suitable bean yoghurt odor.(3)Fermented soybean yoghurt at 25°C for 48h,Lb.plantarum,Lb.casei,Lb.paracasei,E.faecalis,Lb.helveticus,and Lb.rhamnosus have the function of converting isoflavones.The effect of isoflavone in different species was significantly different p<0.05,and there were also significant differences in different strains of the same lactic acid bacteria p<0.05.There are 14 strains that can convert soybean isoflavones at 25°C for 48 h,the content of daidzein in the fermentation broth was 0.0250~0.0334 mmol/L and the genistein content was 0.0589~0.0748mmol/L.(4)Compare the transformation of isoflavones by Lb.casei H4,Lb.paracasei D3,E.faecalis I1,and Lb.rhamnosus L2 at 25°C for different periods of time.I1 was first converted to isoflavones and started after 6 hours.D3,L2 and H4 started to transform isoflavones after 12 h,12 h and 24 h,respectively.L2 was the first to completely degrade soy isoflavones.The highest levels of daidzein and genistein were found at 24 h,which were(0.0306±0.0008)mmol/L and(0.0661±0.0014)mmol/L,respectively.(5)L.lactis G13,Lb.plantarum B3,Lb.paracasei D3,Lb.rhamnosus L2,and S.thermophilus F5 are used to ferment yoghurt with 5 strains of lactic acid bacteria,when the inoculum size of the starter is 1×10~8 cfu/L,fermented for 18h at 30°C,moderately sweet and sour yogurt,pH 4.34,water holding capacity 58.61%,genistein content increased to 0.0119mmol/L.During the 21d storage period,the pH of the bean yoghurt remained basically unchanged,the acidity did not change significantly p<0.05,and showed good weak acidification characteristics.This study screened the fermentable strains of soybean yoghurt and the strains that biotransformed soybean isoflavones,developed a suitable fermenting agent for soybean yoghurt,solved the stability problem of soybean yoghurt starter and the problem of acidification of soybean yoghurt.It provides the experimental basis for the industrialization of bean yoghurt starter.