Screening Of Lipase-producing Lactic Acid Bacteria And Its Effect On Cheese Flavor

In this study,traditional culture methods and 16S r DNA sequence analysis were used to isolate cultivable lactic acid bacteria from 23 handmade cheese samples from Yili,Tacheng and Hami regions in Xinjiang.In addition,the lactic acid bacteria with better lipase production were selected,and the enzyme production characteristics and enzymatic properties were studied.Then the enzyme-producing lactic acid bacteria is used as a starter in the production of yogurt cheese,through gas chromatography(GC),gas chromatography-Ion mobility spectroscopy(GC-IMS)to analyze the influence of different starters on cheese fatty acids and flavor substances.The experimental results are as follows:(1)A total of 282 pure cultures were isolated from 23 hand-made cheese samples.After Gram staining and catalase experiments,267 strains were tentatively designated as lactic acid bacteria.After preliminary screening of the selection medium,35 strains of lactic acid bacteria can produce obvious transparent circles.Through 16S r DNA gene detection and analysis,35 strains of lactic acid bacteria were sequenced,and 35 strains were determined to be lactic acid bacteria,11 of which were Lactobacillus plantarum,4 Lactobacillus brevis,and 8 Lactobacillus fermentum,7strains of Weissella confusa,1 strain of Weissella cibaria,1 strain of Lactobacillus paracasei,1strain of Pediococcus acidilactici,2 strains of Lactobacillus helveticus.The enzyme activity of 35strains of lipase-producing lactic acid bacteria was determined by the copper soap method,and it was determined that 4 strains of lactic acid bacteria had better enzyme-producing ability.Among them,the strains H1-6 Lactobacillus helveticus,B2-5 Lactobacillus plantarum,T1-3 and T1-5 are fused with Weissella confusa.(2)Study on the enzyme-producing characteristics and enzymatic properties of the 4 lactic acid bacteria with better lipase-producing ability.The results showed that:Lactobacillus helveticus B2-5 was inoculated at 2%,the initial p H was 7 and polyvinyl alcohol(PVA)was added.In the fermentation medium,fermented at 37?for 48 h,the enzyme production effect is the best,the enzyme activity can reach 5.29 U/m L;the enzyme activity of Lactobacillus helveticus H1-6 is3.46 U/m L;the strain Weissella confusa T1-5 and T1-3 Large enzyme production and good tolerance to the conditions of the fermentation environment.Adding K⁺,Na⁺,Fe^2+,Mn²⁺to the fermentation broth can significantly increase lipase activity(P<0.05),Cu²⁺,Al³⁺,Fe³⁺all have inhibitory effects on lipase activity,and Cu²⁺,Al³⁺have obvious inhibitory effects on lipase(P<0.05).The lipases produced by the 4 lactic acid bacteria are all acidic medium temperature lipases,which are better produced under acidic conditions.(3)Take the commercial starter selected in this experiment as a control,4 strains of lipase-producing lactic acid bacteria were used as starter to make yogurt cheese.After the cheese has matured for 50 days,the p H value,titer acidity,peroxide value(POV),thiobarbituric acid value(TBARS),fatty acids,and volatile flavor substances in the cheese made with different starters were measured,and the results showed:Lipase-producing lactic acid bacteria involved in the fermentation of cheeses have lower oxidation degrees than the control group,and the types and content of fatty acids in the experimental group cheeses are also more abundant.Using GC-IMS to qualitatively determine the volatile organic compounds in cheese,combined with sensory evaluation results,it is clear that the addition of lipase-producing lactic acid bacteria makes the texture,taste and flavor of the experimental group cheese better than the control group.