| Koumiss is a kind of traditionally functional fermented dairy products,which has been produced by the traditional fermentation process by now.It is of great significance to develop the traditional koumiss for the industrial production through screening LAB and yeast strains with potential probiotic characteristics.The strains of LAB and yeast were isolated from koumiss samples by using pure culture method in this study.Three strains of LAB with potential probiotic characteristics and one strain of yeast with good fermentation characteristics were screened out.Fresh mare milk were fermented using these four strains.The p H value,titrated acidity value of koumiss and viable count of lactic acid bacteria and yeast were evaluated during the period fermentation,ripening and storage.The sensory evaluation was conducted.Finally,the content of volatile flavor compounds of koumiss during fermentation were determined by solid phase microextraction(SPME)coupled with gas chromatography-tandem mass spectrometry(GC-MS)to evaluate the industrial application of screened starter culture comprehensively.The main results were as follows:(1)The strains of LAB and yeast were isolated and identified through the pure culture method from koumiss samples collected from Mongolia,Central Asia,Xinjiang and Xilingol.A total of 264 strains LAB and 103 strains of yeast were isolated from 44koumiss samples.All the isolates were identified by 16S r RNA,26S r RNA gene sequence analysis and homology comparison.A total of 264 strains of LAB belonged to 5 genera including Lactobacillus,Leuconostoc,Lactococcus,Streptococcus and Pediococcus.The predominant species was L.helveticus.A total of 103 strains of yeast were isolated and belonged to 6 genera.The predominant species was Kazachstania unispora.(2)Three strains including L.plantarum Q18-3、L.paracasei Q18-5 and L.helveticus Q27-6 with potential probiotic characteristics were screened out from 264 strains of LAB by the tests of milk curd property,acid production capacity,tolerance of artificial stomach and intestinal juices and growth temperature.One strain of yeast S.cerevisiae Z7-10 with good fermentation characteristics was screened out from 103 strains of yeast,through the tests of gas production,ethanol production,acid tolerance and growth temperature.(3)The screened strains were used as the starter to ferment fresh mare milk concerning the ratio of L.plantarum Q18-3:L.paracasei Q18-5:L.helveticus Q27-6:S.cerevisiae Z7-10(1:1:1:1).The fermentation temperature was 30℃and the fermentation time was 12 h.The p H value was 4.27 and the titrated acidity value was 81.73 ~0T and the viable count of LAB was 7.75×10~8CFU/m L and that of yeast was 2.21×10~8CFU/m L at the end of fermentation.The koumiss contained protein 1.73 g/100 g,fat 1.25 g/100 g and carbohydrate 6.1 g/100 g.The fermentation temperature was suitabble,the fermentation time was appropriate,the acidity was moderate and the taste was good.(4)Volatile flavor compounds of koumiss during fermentation were determined by SPME-GC-MS.One hundred and twenty one kinds of volatile flavor compounds were detected in koumiss during its fermentation,including 22 kinds of acids,13 kinds of aldehydes,9 kinds of ketones,24 kinds of alcohols,20 kinds of esters,16 kinds of alkanes,10 kinds of alkenes and alkynes and 7 kinds of nitrogen compounds.Octanoic acid,n-Decanoic acid,2-methyl-butanol,3-methyl-butanol,ethyl ester octanoic acid were detected with higher content during fermentation.Acetic acid,hexanoic acid,nonanoic acid and 2-heptanone were arisen newly compounds during fermentation.In summary,the microbial diversity of koumiss was better understood through the pure culture method.Starter culture of koumiss with good fermentation characteristics and potential probiotic were screened,which provided theoretical support for the functional research and industrial production of koumiss. |
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