Isolation And Identification Of Lactic Acid Bacteria From Xinjiang Traditional Yogurt And Its Application In The Development Of Donkey Milk Yogurt

Xinjiang traditional fermented yoghurt contains rich microbial resources with different properties and is the main source for obtaining excellent fermented lactic acid bacteria.Xinjiang has a large population of donkey and abundant donkey milk resources.In recent years,donkey milk yoghurt has also become a hot spot for development and research.In this experiment,92 strains of lactic acid bacteria were isolated from traditional yoghurt made by farmers and herdsmen in Kashi region of Xinjiang.Through 16S rDNA molecular identification,single strain acid production capacity evaluation and curd capacity detection,the combination of dominant lactic acid bacteria was screened and applied to fermented donkey milk yoghurt.The optimal fermentation process conditions of donkey milk yoghurt were determined through single factor test,Plackett-Burman test,steepest ascent test and response surface optimization test.The fermented donkey milk yoghurt was subjected to a series of physical and chemical tests,microbiological tests,organic acid tests and volatile flavor components tests.Finally,the production process conditions of the donkey milk yoghurt were further optimized in combination with the pilot test.The main conclusions are as follows:(1)Isolation and identification of lactic acid bacteria.Ninety-two strains of lactic acid bacteria were isolated and purified from 41 traditional yogurt samples collected from kashgar city and other 13 surrounding counties.75 strains of lactic acid bacteria were classified by 16S rDNA amplification sequence comparison identification and construction phylogeny to obtain 10 strains of Lactobacillus delbrueckii,16 strains of Lactobacillus fermentum,7 strains of Pediococcus acidilactici,36 strains of Enterococcus faecium,14 strains of Enterococcus durans,1strains of Leuconostoc mesenteroides.Enterococcus faecium is the dominant strain in this area.(2)Screening of excellent strains.The acid-producing ability of 92 lactic acid bacteria was evaluated by detecting pH value and total acidity value.The results showed that 64 strains had pH?5.05,accounting for 69.57%of the total number of strains,mainly concentrated in the range of 4.26~4.45.There were 59 strains with acidity?100°T,accounting for 64.13%of the total number of strains,mainly between121 and 130°T.The strain SL 1-1(Lactobacillus delbrueckii)has the lowest pH value of 3.85 and the highest total acidity value of 183°T after 24 hours of culture,and can be preserved as an excellent strain.Further 20 lactic acid bacteria were screened for curdling test.After pH value and acidity value were detected and tissue status was observed,12 of them had good curdling effect.After observation in cold storage,7 of them had less acid production and weak acidizing ability after curdling.The yoghurt fermented by the combination of SL 1-1,WQ 1-3 and TX 2-3 strains according to the inoculation ratio of 1:1:1 was found to be in good condition and had short solidification time.(3)Optimization of technology for donkey milk yoghurt.Through single factor analysis test,the best fermentation conditions of donkey milk yoghurt are:inoculation amount 2%,whole milk powder amount 9%,fresh donkey milk amount 35%,sucrose amount 5%,galactooligosaccharide amount 2%,fermentation time 4.0 h and fermentation temperature 42?.According to Plackett-Burman test and steepest ascent test,four factors including fermentation time,inoculation amount,whole milk powder addition amount and fresh donkey milk addition amount were screened out,and response surface analysis was used to carry out four factors and three levels of optimization test.The final fermentation process of donkey milk yoghurt was determined as follows:fermentation time 4.50 h,inoculation amount 2.0%,whole milk powder addition amount 9.0%,fresh donkey milk amount 41.5%,fermentation temperature 42?,sucrose amount 5.0%,galactooligosaccharide amount 2.0%.At this time,the sensory score of donkey milk yoghurt was 91.0 points,and all physical and chemical indexes met the requirements.(4)Quality analysis of donkey milk yoghurt.Physical and chemical indexes,microorganisms,sensory evaluation,10 kinds of organic acids and flavor components of donkey milk yoghurt were tested during cold storage.According to the test results,the acidity value of donkey milk yoghurt showed an overall upward trend,while the pH value,viscosity value and water holding capacity showed a downward trend during 21 d of cold storage.The total viable count of lactic acid bacteria was higher than 1×10~6 cfu/mL.The donkey milk yoghurt can keep good taste and flavor for 15days before sensory evaluation.As time goes on,the sour taste is slightly heavier and the taste decreases.Ten kinds of organic acids(except propionic acid)increased or decreased during the whole refrigeration period,with lactic acid,acetic acid and citric acid having the highest content.A total of 69 flavor substances were detected from donkey milk yoghurt,including 17 acid compounds,15 alcohol compounds,12ketone compounds,3-hydroxy-2-butanone,caproic acid,acetic acid,butyric acid,caprylic acid,benzoic acid,etc.These substances constitute the main volatile flavor substances of donkey milk yoghurt.(5)Pilot test.Combined with the actual conditions of the factory,the production process of donkey milk yoghurt factory was adjusted,and finally the process route and process parameters for the production of donkey milk yoghurt in the factory were determined.The addition amount of purified water in the material-liquid ratio was reduced by 5%~10%from the original amount,the inoculation amount of fermentation agent for production was 2.5%,the inlet temperature of fermentation incubator was 45?±1?,and the fermentation time was 5.5~6.0 h.