Study Of Influencing Factors Of The Metabolic Byproducts Of Beer Yeast

In this paper, the contents of higher alcohols and diacetyl of the main byproducts of fermentation could be controlled in optimal range by screening beer yeast species and controlling the ingredient of wort and fermentation process. We could acquire good flavor and quality beer.First, The main studies of beer yeast species were as follow: The quantity relations of fermentation byproducts higher alcohols and diacetyl were contrasted between S.cerevisiae APV and APK. It was found that the contents of diacetyl and higher alcohols were alternatively up to 0.20 mg·L-1 and 76.4 mg·L-1 after 12 days' fermentation with S.cerevisiae APV, while the contents of those were only respectively 0.07 mg·L-1 and 53.4 mg·L-1 with S.cerevisiae APK; S.cerevisiae APK has better flocculation which indicated the specialty of fermentation of S.cerevisiae was an important factor in affecting quality of beer. The excellent beer yeast species should have lower maximum content of diacetyl and higher reducing power of diacetyl during the course of fermentation. Thus, we have done experiment to ameliorate S.cerevisiae APV in order to reduce the diacetyl contents. In this experiment, two ways were used for screening low-diacetyl beer yeast species.1. Placed APV strains in different culture medium of diacetyl contents and cultivated it, selected anti-diacetyl mutant strains, acquired a No.7 strain by separation and screening and fermentation and diacetyl-domestication. When it was fermented at 11α, the maximum content of diacetyl was 0.36 mg·L-1, the diacetyl content of beer was 0.06 mg·L-1 and the fermentation degree was 66.1%.2. Mutated S.cerevisiae APV by UV and selected anti-tribenuron-metyl (TM) strains, acquired a No.2 strain by species separation and screening and fermentation and diacetyl-domestication; Its maximum content of diacetyl was 0.36 mg·L-1, the diacetyl content of beer was 0.07 mg·L-1 and the fermentation0.36 mg·L-1 , the diacetyl content of beer was 0.07 mg·L-1 and the fermentation degree was 65.8%. Thus, we acquired two mutant strains which maintained the flavor of beer and improved reducing power of diacetyl.Second, Ingredients of wort were important nutrition supplying yeast metabolism. During the course of fermentation, we analyzed the effects of different wort concentration and different a-AN content wort, wort added Fe, wort added Zn, wort added Se on growth of yeast, pH value, a-AN content, apparent sugar degree, the content of diacetyl and higher alcohols of fermented broth and several characteristics of beer. The results were as follows:(1) It was best to control the contents of byproducts of beer yeast species and the flavor of beer when the content of α-AN in wort was 167 mg·L-1 and the concentration of wort was 11 P.(2) When the concentrations of FeSO4·7H2O and ZnSO4·7H2O in wort were respectively 0.70 mg·L-1 and 0.75 mg·L-1, the diacetyl of beer were respectively 0.07 mg·L-1 and 0.05 mg·L-1, the higher alcohols contents were 56.2 mg·L-1 and 59.1 mg·L-1, alcohol contents were 4.70%(W/W) and 4.67%(W/W) , fermentation degrees were 68.3% and 67.3%, beer flavor was maintained and the fermentation time was less than half of ordinary fermentation time.(3) When the wort which was saccharified the malt added 200 mg·L-1 of Na2SeO3 was used for fermentation, higher alcohols and diacetyl contents were respectively 63.2 mg·L-1 and 0.07 mg·L-1 and the flavor of beer was fresh and mellow.Third, Excellent fermentation process could ensure good flavor of beer. Controlled the inoculative use level of S.cerevisiae APK was 2.0 X 10⁷ unit·mL-1, the main fermentation temperature was 13℃, we could acquire flavor-due beer and the metabolic byproducts contents of beer were optimal.and reduced diacetyl content with immobilized yeast in late fermentation. The experiment showed: immobilized yeast could halve the beer's diacetyl reduction time. In result, it could shorten beer's process period and saved production cost.