Study On The Selenium-Rich Properties Of Lactic Acid Bacteria And Their Use In The Preparation Of Yogurt

Selenium(Se)is one of the essential trace elements for the human body,with antioxidant,anti-cancer,antibacterial and immunity-boosting functions.As a recognised safe probiotic,Lactobacillus can accumulate inorganic selenium and convert it to organic state selenium or monomerselenium,and is also an excellent fermenting agent for fermented products.In this study,lactic acid bacteria were isolated from commercially available yogurt,identified and investigated for their selenium-enrichment characteristics,so that the selenium-enriched strains screened for the fermentation of yogurt can provide theoretical and practical basis for the selection and breeding of selenium-enriched lactic acid bacterial resources and the optimisation of process conditions for the fermentation of selenium-enriched yogurt.The main results of the research are as follows:1.Six different strains of lactic acid bacteria were isolated from commercially available yoghurt using MRS plates for physiological and biochemical assays and 16S r DNA identification,and the strains were Lacticaseibacillus rhamnosus LLR-L2(ST),Lactobacillus sp.JCM7736(LB),L.rhamnosus PS5(WX),L.rhamnosus RT-B(RS),L.reuteri 1663(FG),and Limosilactobacillus 3630(SS),respectively.2.The MRS medium with different concentrations of sodium selenite of(0,25,50,100,200,400μg/m L)was used to culture six strains of lactic acid bacteria.The results showed that the colour of the bacteria gradually became redder with increasing selenium concentration,and the strains ST,RS and FG had pink colour at a selenium concentration of 50μg/m L.Strain ST had the highest number and weight of bacteria with 3.48±0.06×10~8cfu/m L and 0.96±0.02 mg/m L,respectively.Strain RS had the highest selenocysteine and selenomethionine content with 90.04±0.57 mg/g and2.72±0.17 mg/g respectively,while the FG bacterium had the highest selenocysteine content of 3.75±0.21 mg/g.The selenium content and selenium accumulation rate of these three strains were 11.85,13.27 and 13.83 times higher than those of the control,and the selenium accumulation rates were 94.01±1.20%,93.02±2.37%and91.03±1.55%respectively.3.The changes in selenocysteine synthase(Sel A)gene expression in six strains of Lactobacillus lactis at a selenium concentration of 50μg/m L were analysed by fluorescence quantitative PCR,and the results showed that sodium selenite significantly increased the expression level of Sel A gene in strains ST,LB,WX and SS,and significantly decreased the expression level of Sel A gene in strains RS and FG compared with no selenium supplementation.The strains ST,RS and FG were screened for selenium enrichment in combination with the selenium content and enrichment rate of the bacterium.4.Different combinations of selenium-rich strains ST,RS and FG were designed as single factors for yoghurt fermentation and the results showed that ST+RS was the best strain combination.Sensory evaluation,protein content,fat content,acidity and water holding capacity of selenium-rich yoghurt were used as indicators,and inoculation ratio,fermentor inoculation and time were used as a three-factor response surface to optimise the fermentation conditions of selenium-rich yoghurt.The results showed that the optimal fermentation conditions were 300:300(μL:μL volume ratio)for the selenium-enriched strain,6.1%inoculum of fermenting agent and the fermentation time 22.8 h.The yoghurt enriched with selenium has an organoleptic score of 90.80,a protein of 35.11 mg/g,a fat of 45.50 mg/g,an acidity of 91.60 ~oT and a water retention capacity of 93.35%.