Study On Screening And Fermentation Properties Of Priority Lactic Acid Bacteria In Traditional Dairy Fermented Products Of Tibet

The traditional fermented dairy products in the Tibetan pastoral area contained rich and different microbial resources.Lactic acid bacteria(LAB)were a kind of beneficial bacteria which could be used for industrial use and had excellent fermentation performance such as high acid production and aroma production.113 strains of LAB isolated and identified from traditional fermented dairy products were studied.The effects of different fermentation characteristics on acid production and extracellular polysaccharide were discussed.And the optimal LAB were mixed and fermented to optimize the optimal strain combination.At the same time,the volatile flavor components were explored in fermented milk of cattle and sheep.The main conclusions were as follows:(1)First screening: A total of 113 strains of LAB were isolated from the traditional fermented dairy products(milk residue,milk pimple and milk skin)in Tibet.7 LAB strains showing better fermentation performance were obtained through preliminary selection by genetic stability,coagulation time,titratable acidity and sensory evaluation,including A(GY-L109)? B(GY-L003)? C(GY-L004)? D(GY-L055)? E(GY-L112)? F(GY-L111)?G(GY-L104).The curd time and titratable acidity of the 7 strains were 3 to 6 h and 60.46 to 70.40 oT;after refrigeration 24 h,titratable acidity was 93.76 to 109.20 oT.(2)The results of fermentation of 7 strains of LAB showed that:(1)LAB reached the maximum level in the liquid medium for about 10 h.(2)The optimal performance of EPS was B,the yield of EPS was 379.94 mg/L,followed by C,the yield was 333.46 mg/L;The yields of F and G were 94.13 mg/L and 78.37 mg/L and the ability of producing extracellular polysaccharide was weak.(3)During the 21 d storage period at 4 ?,F and G strains of anti-acidification ability was weak,the acidity increased 17.76 oT and 16.01 oT.The other 5 strains during storage and acidity variation between 7.5~10 o T,indicated a strong anti-acidification ability.(3)The fermentation characteristics of priority LAB were studied.Results were as follows:(1)During the 21 d storage period at 4 ?,the viable number of LAB in all yoghurt were kept more than 106 CFU/mL.(2)Strains of LAB showed different degrees of fermentation tolerance characteristics.A,B and C at the limit of pH 1.5,bile acid concentration reached 0.4% and NaCl concentration of up to 9% the number of viable bacteria were more than 103 CFU/mL,105 CFU/mL,105 CFU/mL,showed good tolerance.(3)The antimicrobial activity of strains was different.C had the strongest effect of inhibition from Staphylococcus aureus,with antibacterial circle diameter(15.94 ± 0.31)mm.B had the strongest effect of inhibition from E.coli and Salmonella,with antibacterial circle(18.21 ± 0.65),(16.27 ± 0.50)mm.For Listeria monocytogenes,the strongest inhibitory effect was E and the antibacterial circle diameter was(15.24 ± 0.35)mm.(4)Study on optimization of combined fermentation of LAB.(1)Optimization of the number of live bacteria enrichment performance and acid production performance showed that AB and AC have a good symbiotic relationship.After fermentation of 8 h in skim milk,AB of the highest number of viable cells was 6.46×109 CFU/mL,AC combination of curd acidity was highest.The performance of AC and AB combinations were superior to those of single strains,and the difference was significant(P < 0.05).While there was no significant difference in the fermentation performance between AD and AE(P > 0.05);(2)the optimal strain combination was AB(1:1).The number of live bacteria was up to 4.57×109 CFU/mL,curd acidity and after fermentation acidity were 71.54 oT?115.30 oT.The acetaldehyde content of fermented milk products was 36.91 ?g/mL and the content of butanedione was 13.82 ?g/mL.(5)The volatile aroma components in cattle and sheep yoghurt were analyzed by HS-SPME and GC-MS.The result showed that AB(1:1)fermented Cattle and sheep yoghurt after 24 h,many kinds of volatile compounds generated and the species composition is basically the same,including the alcohols,aldehydes,ketones,esters,carboxylic acids,hydrocarbons and other categories.The compound fermentation agent AB(1:1)had good performance,which provides the basis for industrialization.