Synthetic Mechanism Of Characteristic Esters Produced By Yeast In Hongqu Glutinous Rice Wine

Fujian Hong Qu glutinous rice wine is a popular Chinese rice wine because of its unique flavor. During the brewing, yeast is a major determinant in formation of the flavor compounds. In the present study, the brewing characteristics and flavor characteristics of 12 yeast strains isolated from starters of Fujian Hong Qu glutinous rice wine were investigated. And the yeast strain with the highest ester-producing ability was selected for further study. Then the influence of fermentation temperature on the characteristic flavor compounds produced by the selected yeast strain was also investigated. Subsequently, the synthesis pathway of esters in yeast was studied with the combination of fermentation tracking method, reaction-system method and mutation method. This study provide enzymology basic for modifying the flavor-producing characteristics of yeast through molecular biological technique, and theoretical basis for directional improving the flavor and quality of glutinous rice wine. The major conclusion of this study was as follows.The results of the investigation of the brewing characteristics of the 12 yeast indicated that, except for NY01(Pichia guilliermondii sp.) and NY03 (Saccharomycopsis fibuligera sp.), the other eight non-saccharomyces yeast strains had equal fermenting power with saccharomyces cerevisiae.The volatile flavor compounds of the fermented broth of yeast were extracted with headspace solid-phase microextraction (HS-SPME), and quantitatively analyzed with gas chromatograph with an internal standard. The results indicated that alcohols and esters were dominant compounds, and isoamyl alcohol, isobutanol, β-benzyl ethanol, ethyl acetate and ethyl lactate were the highest five compounds. At the end point of fermentation, NY12 (Wickerhamomyces anomalus) had the highest amount of total flavor compouds and total esters and the lowest alcohols/esters ratio of 0.76, which showed that NY12 had the highest ester-producing ability among the 12 yeast strains. The ester compounds were mainly synthesized in the first six days then tended to decline. Similar to esters, a large number of alcohols were synthesized in the first 6 days, however alcohols didn’t decline dramatically.The efficiency of yeast and overall fermentation performance at two fermentation temperature (16℃ and 30℃) were compared. When at 16℃, the efficiency of yeast was lower, while the overall fermentation performance was similar. The alcohols productions were similar at the two temperatures, while esters production was significantly higher at 16℃, which indicated that low-temperature fermentation might slow down the decomposition of esters and benefit for the accumulation of esters, resulted in a better flavor of glutinous rice wine.With the combination of tracking method, reaction-system method and mutation method, the synthesis pathway of phenethyl acetate in NY12 was ingestigated. The results indicated that acyltransferase and esterase bothexist in the fermentation. And the amount of acetic acid (as the substrate) had higher correlation with the amount of phenethyl acetate (as the product), which indicated that esterase was the key enzyme in synthesizing phenethyl acetate. The mutation method investigation showed that the synthesis pathway of phenethyl acetate was mainly through esterase route, and esterase acted as a synthesizing enzyme rather than a hydrolyzing enzyme.