Bacterial Phase Analysis Of Tibetan Kefir Grains And Research On Fermented Kefir Milk With Compound Strains

Tibetan Kefir grain is a natural fermention starter whose complex bacterial phase structure gives Tibetan kefir milk its rich nutritional value and probiotic effect.However,the complex and variable composition of the Tibetan kefir grain is not easy to clean and is not conducive to the industrial production of Tibetan kefir milk.In response to the above issues,this research firstly studied the proliferation conditions of Tibetan kefir grain.Further this study investigated the change of bacterial phase evolution of Tibetan kefir grain and kefir milk in the fermentation process.Lactic acid bacteria with excellent fermentation performance are isolated from Tibetan kefir grains and Tibetan kefir milk.Finally,the experiment was carried out on the compound strain starter.The main findings of this paper are as follows:(1)Optimal conditions for the proliferation of Tibetan kefir grain by single-factor and orthogonal experiments:the incubation temperature was 25℃;the inoculum amount was5%;and the incubation time was 48 h.Under the culture condition,the total proliferation rate of Tibetan kefir grains at passage 9 reached 177.67%.(2)Illumina Novaseq sequencing technology was used to investigate the bacterial phase composition and dynamics of Tibetan kefir grain and Tibetan kefir milk during the fermentation process.The results showed that during the fermentation process of Tibetan kefir grain,the bacterial flora structure changed little and was in a relatively stable state.Firmicutes and Lactobacillus were the absolute dominant phyla and genera.The relative abundance of these species were above 97%.The bacterial flora structure of Tibetan kefir milk changed significantly during the fermentation process.Proteobacteria,Firmicutes,Acetobacter and Lactobacillus were the main dominant bacteria.phyla and genera.The relative abundance of Lactobacillus in Tibetan kefir milk was the highest at 56.40%,followed by Acetobacter at 23.82%.With the prolongation of fermentation time,the relative abundance of Acetobacter gradually increased to 54.16%,while the relative abundance of Lactobacillus species gradually decreased to 19.74%.At the same time,the relative abundance of other genera such as uncultured＿bacterium＿f＿Enterobacteriaceae and Lactococcus also increased.(3)12 strains of lactic acid bacteria with excellent fermentation performance,stable genetics,suitable acidity,high water holding capacity and good sensory evaluation were selected from Tibetan kefir grain and Tibetan kefir milk.There were 10 strains of Lactobacillus plantarum and 2 strains of Lactococcus lactis,respectively.Lactococcus lactis M41,Lactobacillus plantarum MA2,and Lactobacillus Kefiranofaciens ZW3 were selected as compound starter strains.(4)The optimal conditions for the combined fermentation of Lactococcus lactis M41,Lactobacillus Kefiranofaciens ZW3 and Lactobacillus plantarum MA2 by single-factor and orthogonal experiments:the inoculation ratio of bacteria was cocci:bacilli=1:1.5;the fermentation temperature was 36℃;the addition of sucrose was 4%,and the inoculation amount was 7×10~7CFU/m L.Under these conditions,the sensory score reached 86.63.(5)By comparing the relevant indicators of compound fermented milk and Tibetan kefir milk,it can be seen that in terms of fermentation characteristics,The fermentation time of compound fermented milk was shortened by nearly 19 hours;The water-holding capacity of the compounded fermented milk has been increased by 2.02%;The overly acidic taste has also been improved,raising the sensory score to 87.67.In terms of texture,compound fermented milk increased the hardness,consistency,cohesion and viscosity index of fermented milk;In terms of rheological properties,compound fermented milk increased the viscosity of fermented milk,showed more solid-phase behavior,and improved the rheological properties of fermented milk;The volatile flavor compounds of the two fermented milks were analyzed by GC-MS technology,and it was concluded that the compound fermented milk could enrich the flavors of acids,alcohols,esters,and ketones.At the same time,the bad flavor was removed,so that the taste of fermented milk was better and the acceptance was higher.