Isolation Of Commercial Starter Strains And The Effects Of Combined Bacteria On The Fermentation Characteristics And Quality Of Soy Yoghurt

Fermented soy milk products can combine the nutrition and physiological functions of soybean and lactic acid bacteria fermentation,and has a broad market prospect.Lactic acid bacteria is one of the key factors affecting the quality of fermented soymilk.The existing commercial starters are basically developed for the production of milk yogurt,and the best results cannot be obtained when they are directly applied to the fermentation of plant-based yogurt.Single strain of lactic acid bacteria from the yogurt starters was separated,and recombined on the basis of investigating the fermentation characteristics of single strain to further study its fermentation characteristics and product properties in soymilk-base.This will provide a basis for how to choose and combine starters when fermented soymilk is produced.The properties related to the fermentation of lactic acid bacteria in soymilk and milk were compared.The results showed that the buffering capacity of milk was significantly higher than that of soymilk(P<0.05).When 0.2% sodium citrate and 0.2% dipotassium hydrogen phosphate were added to soymilk,there was no significant difference in buffering capacity between soymilk and milk(P>0.05).The free amino acid content of soymilk was higher(36.00 mg/100 m L).The carbon source content in soymilk was low,and the total content of sucrose and glucose was 15.30 mg/g,which was much lower than the content of fermentable sugar in milk.When 1.5% glucose was added to the soymilk,the soymilk system could reach the maximum acid production rate.In the thesis,25 strains of lactic acid bacteria belonging to 3 genera was isolated from commercial starters and identified.The count results of different lactic acid bacteria on M17,MRS-5.4 and MRS-6.2 medium were investigated,and the results showed that the determination of the number of Streptococcus thermophilus in the two-strains mixed fermentation system of Streptococcus thermophilus with Lactobacillus or Bifidobacterium,M17 medium was the best;MRS-5.4 medium was more suitable for the determination of the number of Lactobacilli or Bifidobacteria in the two-strains mixed fermentation system.For multi-strains mixed fermentation system with 3 strains or more,the plate counting method was no longer applicable,and real-time fluorescent quantitative PCR method was needed for counting.For this reason,specific primers were designed with 6 representative strains of lactic acid bacteria as examples,and the DNA extraction method of fermented soymilk was optimized.The results showed that for Streptococcus thermophilus and Lactobacillus paracasei,the real-time fluorescent quantitative PCR counting results were higher than the plate counting results,while for Lactobacillus bulgaricus,Lactobacillus plantarum,Lactobacillus rhamnosus and Lactobacillus fermentum,their results were consistent.On this basis,the fermentation characteristics of the isolated lactic acid bacteria in soymilk and milk were studied.The end point of the fermentation was 6 h,and the end point p H,acidity and free amino groups content were determined.The results showed that many strains of Streptococcus thermophilus produced acid faster in soymilk,with acidity reaching60.68 °T to 82.52 °T,which was generally higher than the acidity in milk.The acid production of Streptococcus thermophilus CG45-1 was the fastest,which was significantly higher than other Streptococcus thermophilus(P<0.05).Subsequently,the Streptococcus thermophilus CG45-1 was used as the basic strain to be combined with other Lactobacilli and Bifidobacteria for two-strains fermentation.The results showed that the combination with Lactobacillus bulgaricus CG90-2 had high proteolytic activity;The combination with Lactobacillus fermentum CGF or Lactobacillus paracasei CG3K-2,CG31 respectively had higher consistency;The combination with Lactobacillus plantarum CG47-2 or Lactobacillus rhamnosus CGS was conducive to the production of flavor components.Then,the dominant strains were selected from the above-mentioned researches for multi-strains mixed fermentation,and their fermentation characteristics were compared with commercial starters.The results showed that the free amino groups content in soymilk fermented by the optimized multi-strains combination reached 0.64 mg/g?0.74 mg/g(calculated as leucine),which was higher than the soymilk fermented by commercial starters;The fermented soymilk gel was softer and the hardness value was lower than the soymilk fermented by commercial starters;The content of off-flavor components like hexanal and1-octen-3-ol were lower than the soymilk fermented by commercial starters,the final sensory evaluation results showed that the overall acceptance was higher;The lactic acid content was6.56 mg/g?7.77 mg/g,which was higher than the lactic acid content in soymilk fermented by commercial starters(6.24 mg/g?7.00 mg/g);The number of Streptococcus thermophilus and Lactobacillus fermentum at the end of fermentation were as high as 8 lg cfu/g,Lactobacillus bulgaricus and Lactobacillus rhamnosus were 6.8 lg cfu/g?7.2 lg cfu/g,and Lactobacillus plantarum and Lactobacillus paracasei were 6.0 lg cfu/g?6.9 lg cfu/g.More than 90% of the lactic acid bacteria in the soymilk fermented by commercial starters were Streptococcus thermophilus,and in most cases,the number of Lactobacilli was below the minimum limit.The commercial starter was combined and fermented on the basis of the above-mentioned strain combinations.The results showed that when 3 kinds of starters were added with Lactobacillus rhamnosus and Lactobacillus bulgaricus,the sensory score of the soy milk fermentation product reached 82 points,which was higher than that of the single starter.