Isolation And Identification Of High Quality Lactic Acid Bacteria And Yeast In Kefir Grains And The Preparation Of Kefir Compound Fermentation Starter

Kefir,a low-alcohol fermented milk with the color of milky-white,smelt fragrance and slightly carbonated.Due to rich white foam,delicate and smooth texture,sweet and sour taste and rich nutrition,Kefir is regarded as?champagne in fermented milk?.Because of its functions such as lowering blood pressure and cholesterol,improving constipation,enhancing immunity,and alleviating lactose intolerance,kefir is also known as the?healthy drink?.At present,a lot of researches had carried out on kefir.Kefir grain,a natural fermentation starter of Kefir,has complex and changeable composition of bacteria phase and is easy to be unbalanced,which were unfavourable for large-scale industrial production of Kefir.In order to solve this problem,the paper mainly researched perform from three aspects.First,the main forming bacteria of Kefir grain were isolated and purified.Second,the isolated bacterial were identified and screened,meanwhile,fermentation performance was analysed.At last,the strains with excellent characteristics were subjected to prepare compound fermentation starter of Kefir,which was expected to develop a compound fermentation starter formulation with excellent flavor and stable physical and chemical properties.These studies can lay a foundation for further production of direct vat set powder starter.The results of research are as follows:?1?Fifty-one strains of lactic acid bacteria and twenty-two strains of yeasts were isolated from ten kinds of Kefir grains collected from the herdsman's family in Xinjiang.They were identified by morphological observation and 16S rDNA/5.8S rDNA sequence analysis.Fifty-one strains of lactic acid bacteria including twenty-one strains of Lactobacillus Kefir,fifteen strains of Lactobacillus casei,five stains of Lactobacillus Parolens,four strains of Lactobacillus hilgardii,two strains of Enterococcus faecalis,one strain of Leuconostoc mesenteroides,one stain of Lactobacillus diolivorans,one stain of Lactobacillus naglli,one stain of Lactobacillus harbinensis;twenty-two strains of yeasts include nine strains of Kluyveromyces marxianus,six stains of Kazachstania unispora,six strains of Pichia kudriavzevii,one stain of Candida stellimalicola,and one stain of Pichia occidentalis strains.The main composition of the dominant bacteria of Kefir grains in Xinjiang was identified.The lactic acid bacteria are mainly Lactobacillus kefir and Lactobacillus casei;the yeasts are mainly Kluyveromyces marxianus,Kazachstania unispora,and Pichia kudriavzevii.?2?Three strains of lactic acid bacteria with excellent fermentation performance were obtained.The fifty-one strains of lactic acid bacteria were screened and analyzed by fermentation performance.Three strains of lactic acid bacteria had excellent fermentation performance.They were Lactobacillus kefir MLK5,Leuconostoc mesenteroides NLM2,and Lactobacillus casei SLC1.The ability of three strains of lactic acid bacteria to produce acid,acetaldehyde,diacetyl and amino nitrogen is better.After fermentation,the number of living bacteria,water holding capacity,texture and volatile components are best,and sensory evaluation is also best.?3?One strain of yeast with excellent fermentation performance was obtained.twenty-two strains of yeasts were screened and analyzed by fermentation performance.Among them,eight strains with stable heredity,better abilities of producing alcohol and scent were screened out.Through the principal component analysis of volatile aromacomponents,asensoryevaluationmodel F=0.524190F₁+0.220025F₂+0.136850F₃+0.069364F₄+0.04958084F₅ was established,and the FY1 synthesis index of Kluyveromyces marxianus was optimal.This is in accordance with the results of sensory evaluation,which can be used for the preparation of Kefir complex starter.?4?A good compound fermenting starter formula of pure strain was obtained,and the fermentation conditions of the compound fermentation starter were optimized.The best fermentation conditions were:temperature 34oC,inoculation volume 3×10⁶cfu/m L,lactic acid bacteria:yeast=5:1?lactic acid bacteria inoculation proportion:Leuconostoc mesenteroides:Lactobacillus kefir:Lactobacillus casei=1:2:1?.Analyzing and comparing the compound fermented milk and the Kefir grain primary fermentation product,there was no significant difference in physical and chemical indicators such as the acidity,water holding capacity,ethanol content,viable count and so on.But the former were superior to the latter in volatile aroma components and sensory evaluation indexes.