| Highland barley liquor is produced with unique raw materials, and is the special grainliquor in Qinghai Tibet plateau. The technique and style of it is similar to Fen flavor liquor,which is described as pure light-fragrance, nature and refreshing. The characteristic ofhighland barley liquor was determined by the microbial community involved in thefermentation, and yeasts represented the most important group. Therefore, studying the in situmetabolic characteristic of yeast can help to find the key yeast that contribute to flavor, andunderstand the mechanism involved in liquor fermentation.This thesis systematically studied the in situ metabolic characteristic of yeasts inhighland barley liquor fermentation process. The covariation between metabolic profiles andyeast communities has been modeled using Partial Least Square (PLS) regression method.The comparison of the brewing characteristics of the same yeasts isolated from highlandbarley liquor and fen flavor liquor respectively, highlighted the effects of differentgeographical strains on liquor flavor. In addition, the mixed culture fermentation of functionalyeasts carried out in laboratory-scale revealed the interactions between different yeasts andthe effects of these yeasts on the generation of volatile compounds.The result indicated that:(1) The yeast community structures and dynamics in four fermentation batches ofhighland barley liquor were studied by the spread plate method on WL medium and thefollowing sequencing of the26S rDNA D1/D2domains of cultivable yeasts. The yeastsisolated in Dacha and Ercha were Saccharomyces cerevisiae, Pichia anomala, Pichiamembranifaciens, Saccharomyces uvarum, Saccharomyces servazzii and Torulasporadelbrueckii. Two new yeast species, including Issatchenkia orientalis and Pichia scaptomyzaewere detected in Sancha and Sicha, but P. anomala was not detected. The changes in yeastcommunity were visualized by Principal Component Analysis (PCA). The results showed thatS. uvarum was dominated in the early stage of Dacha fermentation. The yeast community inErcha was stable. P. membranifaciens increased sharply in the late stage of Sancha and Sichafermentations.(2) The comparison of the brewing characteristic of the common yeasts (S. cerevisiae, P.anomala, I. orientalis and P. membranifaciens) isolated from highland barley liquor and fenflavor liquor was carried out. It was found that the production of ethanol from highland barleystrain S. cerevisiae was6.1g·L-1more than that of Fenjiu strain when they cultured withbarley juice, while the production of ethanol from Fenjiu strain S. cerevisiae was6.2g·L-1more than that of barley strain when they cultured with sorghum juice. In addition, the estersproduction of highland barley strain P. anomala was3times more than that of Fenjiu strain.(3) Fifty-eight volatile compounds were detected in fermented grains of highland barleyliquor. The covariation between metabolic profiles and yeast communities was modeled usingPLS regression method. The modulation of sixteen compounds, of these volatiles, wereassociated with variations in the yeast population (Q2>20%). The results showed that P.anomala was responsible for the characteristic aroma of Chinese liquor, such as ethyl lactate, octanoic acid, and ethyl tetradecanoate. Correspondingly, almost all of the compoundsassociated with P. anomala were detected in a pure culture of this yeast. Compared with thePLS regression results, however, ethyl lactate and ethyl isobutyrate were not detected in thesame pure culture, which indicated that some metabolites could only be generated by P.anomala when it existed in a community with other yeast species.(4) Interdelta fingerprinting has been carried out for S. cerevisiae strain typing by using72S. cerevisiae isolates from spontaneous fermentation of highland barley liquor. The resultsrevealed that12genetic patterns were distinguished by interdelta fingerprinting among thetotal S. cerevisiae isolates. ANOVA analysis showed a significant difference between S.cerevisiae strains with total acid, urea, and ethanol content in fermentation broth. Thegenotype XII produced the most ethanol,45.20g·L-1and less urea,20.19mg·L-1. In addition,the genotype XII proeuced more ethyl esters which had fruit aroma and the characteristicaroma compound β-damascenone.(5) The volatile compounds generated in different mixed cultures of functional yeastswere detected. The results revealed that in the mixed culture fermentation of S. cerevisiae, P.anomala, and I. orientalis, S. cerevisiae inhibit the growth of P. anomala and I. orientalis.While the inhibition was diminished when S. uvarum and S. servazzii were added. In addition,mixed culture fermentations can significantly reduce the generation of higher alcohols, suchas phenethyl alcohol and2-methylpropyl. Esters detected in mixed cultures ofS.c-P.a-I.o-S.u-S.s (S. crevisiae-P. anomala-I.orientalis-S. uvarum-S. servazzii) was the mostwith64.5μg·kg-1. |
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