Dynamic Changes Of Lactic Acid Bacteria Microflora During Continuous Fermentation Of Naturally Fermented Milk From Xinjiang

Naturally fermented milk is sweet and sour,rich in nutrition,it is able to regulate gastrointestinal tract,improve lactose intolerance and lower cholesterol et al,it contains abundant resources of lactic acid bacteria(LAB).However,the naturally fermented dairy products are produced in open environment,their quality are unstable and hard to ensure the security.Currently,it is greatly significant to unify and standardize the production process to achieve industrial production,however,when the naturally fermented dairy products are out of the original fermentation environment to be produced industrially,whether the unique microbial composition and quality of the naturally fermented dairy products can still be retained,which requires sufficient scientific data support.Therefore,it is of great significance for the industrialized production to investigate the dynamic changes in the community structure of LAB during continuous fermentation of spontaneously fermented milk.In this thesis,3 samples of spontaneously fermented milk from Xinjiang were fermented continuously for 9 months in the laboratory.Then,the changes of pH value and LAB viable count were studied,at the same time,the LAB diversity in fermented milk samples from ten different periods were analyzed by conventional culture methods and PacBio SMRT sequencing,Lactobacillus helveticus in WQ8 original samples and WQ8samples fermented continuously for 9 months were used as the research objects,cell concentration under different pH condition(pH3.0,pH3.5,pH4.0),artificial gastric juice tolerance and milk fermentation time were measured.The main results were as follows:(1)During the continuous fermentation,the pH value of fermented milk samples showed a general downward trend,which indicated the acid production capacity of microbiota was enhanced.In addition,the LAB viable count decreased,but still remained above 10~7 CFU/m L.(2)A total of 758 LAB strains were isolated from 12 samples of fermented milk at different periods by conventional culture methods.The dominant species in the 0th month samples were Lactobacillus helveticus,Lactobacillus delbrueckii and Lactobacillus delbrueckii subsp.bulgaricus,the dominant species in the 9th month samples were Lactobacillus fermentum,Lactobacillus helveticus,Enterococcus durans and Lactobacillus casei.(3)A total of 2 phyla,19 genera and 41 species were detected from 30 fermented milk samples from different periods by PacBio SMRT sequencing technology.During the continuous fermentation,Lactobacillus was the dominant genus in fermented milk samples,and Lactobacillus helveticus has remained dominant,the previously dominant Lactobacillus delbrueckii and Streptococcus thermophilus were greatly reduced,at the same time,low abundance species were not detected(e.g.,Lactococcus lactis,Streptococcus salivarius,Lactobacillus reuteri,Lactobacillus leichmannii).However,the relative contents of Enterococcus durans,Lactobacillus helveticus,Lactobacillus fermentum,Lactobacillus otakiensis increased.(4)After continuous fermentation of WQ8 fermented milk sample for 9 months,the cell biomass of Lactobacillus helveticus in it significantly increased at low pH(P<0.05);The survival rates in artificial gastric juice of 43.75%Lactobacillus helveticus at the 9th month were>1%,only 1 strain of the original Lactobacillus helveticus had a survival rate of>1%in artificial gastric juice;The average fermentation time of Lactobacillus helveticus in the original and the 9th month samples was 9.23 h±1.23 h and 7.08 h±0.79 h respectively,the fermentation time of Lactobacillus helveticus at the 9th month was significantly lower than that of original Lactobacillus helveticus(P<0.05).The results showed that the ability to grow in acidic environment,artificial gastric juice tolerance and acid production capacity of Lactobacillus helveticus in the original WQ8 sample were significantly enhanced after continuous fermentation for 9 months.