Study On Lactic Acid Bacteria Strains From "Douchi" With High Yield Of Exopolysaccharides, And Its Fermentation Charateristics Of Fermented Milk

"Douchi" is one of China’s many traditional fermented soybean foods, which contains abundant microbial flora, serves as a natural and secure microbial research food material. Research has found that the bacteria-fermented Douchi could use soybean protein to produce viscous material with its unique protease system. In this study, lactic acid bacteria were isolated and screened for high-yield exopolysaccharides (EPS) from "Douchi" in order to identify bacterial strains that can be used in dairy products as functional lactic acid bacteria. Also, the optimization of fermentation conditions for producing high-yield EPS and a preliminary study have been done for its application in fermented milk. With comprehensive utilization of characteristics of the conventional yogurt starter culture and Leuconostoc.mesenteroides subsp. mesenteroides (Got from Douchi), the two stage fermented milk has been tried and studied, and the process also have been optimized. The main research contents of this paper are as follows:Direct culture method and dissolved calcium circle test were used in the screen, and phenol-sulfuric acid method was used to determine EPS production. The screened strains with high-yield EPS were analyzed for their morphology, along with physiological and biochemical tests and 16S rRNA sequence analysis. The eight strains of three species of lactic acid bacteria were obtained, and three lactic acid bacteria strains (Q17, Q22, and Q23) with high-yield EPS were identified as Enterococcus durans, L.mesenteroides subsp. mesenteroides, and Streptococcus thermophilus. Further studies on its ability of producing EPS had been done, and their yields of EPS were as high as 1230.00±13.35 mg/L, 954.90±31.37 mg/L, and 759.20±8.98 mg/L, respectively.By single factor experiment of fermentation conditions on four factors (fermentation temperature, initial fermentation pH, fermentation inoculum and fermentation time) were analyzed to study their impact on the EPS synthesis of L.mesenteroides subsp. mesenteroides Q22 from Douchi, and the response surface experimental were designed to improve the production of EPS by optimizing fermentation conditions. Finally, combined with the actual, The experiment results demonstrated that the optimum conditions for increasing EPS yield were:fermentation temperature 31.5℃, initial fermentation pH 8.8,3.2% inoculum fermentation, fermentation time 26.4h.In order to explore the application of L.mesenteroides subsp. mesenteroides Q22 in dairy products, the strain of genetic stability and the conventional yogurt starter culture were to compose the complex microbial agents, and get it apply to the production of fermented milk with studying its fermentation characteristics. To make full use of the characteristics of producing EPS of L.mesenteroides subsp. mesenteroides, the two stage fermentation were tried to make fermented milk. Through the research by the relevant single factor (the strain’s ratio, fermentation temperature mode, temperature change time and temperature) on the impact of fermented milk’s physical and chemical properties, the optimal two-stage fermentation process were got:(1)the proportion of the fermentation strains:S. thermophiles:L. bulgaricus:L.mesenteroides subsp. mesenteroides= 1:1:2; (2) the fermentation process:a high temperature (41℃) 2.0h, and then low temperature(31 ℃) fermentation. Finally, the experiment results of the fermented milk’s smell analysis by electronic nose and sensory evaluation demonstrated the superiority of the optimized two-stage fermentation process.