Screening Of Brewer's Yeast With Low Acetaldehyde

Acetaldehyde was the highest concentrations substance of carbonyl compounds in beer, accounted for 60% of the total aldehydes, its threshold was 10mg/L in beer. Bad grassy was brought by high levels of acetaldehyde and the shelf life of beer flavor was shortened, even the side effects of cancer after drinking. Alcohol dehydrogenase2 was a key enzyme in acetaldehyde metabolism, which catalyzed the conversion of ethanol to acetaldehyde; lots of studies have shown that bringing down the activity of alcohol dehydrogenase2 can reduce the amount of acetaldehyde.A yeast strain MI4 preserved by our laboratory was mutagenized by UV in this study. Two different screening methods were compared, which were used by only Schiff's reagent and 4-methylpyrazole resistant plate combining with Schiff reagent method for screening, as the results was proved by secondary screening that using only Schiff reagent efficiency was 44.8%, while 4-methylpyrazole resistant plate combining with Schiff reagent method efficiency was 52.5%., A low-yield acetaldehyde strain was selected respectively by the two methods (each named LA35 and LA02). Through EBC tube and the flask fermentation experiments was carried out with the two mutant growth capability, genetic stability and fermentability and the practical application, and the investigation results showed that the mutant LA02 derived by 4-methylpyrazole resistance and Schiff's reagent screening was decreased acetaldehyde 82.0%, the acetaldehyde content of finished beer was reduced from original strain of 16.77mg/L to 3.02mg/L, and LA35 finished beer was 5.85mg/L, lower than the original strain 65.1%. The genetic stability of the two strains were confirmed well, and the TBA values of two strains were decreased in the fermentation, which helped to improve the anti-aging ability of beer. The flocculation of LA35was showed somewhat less than the original strain, but its fermentation rate was the highest, LA02 fermentation rate was the lowest. As generalized above-mentioned, the two mutants could basically sufficed the requirements of the fermentation with some industrial application.Moreover, considering both the two mutant strains reducing the amount of acetaldehyde, and two mutants'alcohol dehydrogenase2 enzyme activity and ADH2 genetic changes were examined in the study. The results showed that both the alcohol dehydrogenase2 enzyme activity of LA02 and LA35 were lower than original strain. And then LA35's ADH2p enzyme had three amino acid mutations, respectively, F12S, L153S, V174A; the mutant LA02's ADH2p enzyme occurred five mutations, respectively, N108V, N167D, P205T, N298D, G312I. Likewise, the alcohol dehydrogenase2 of LA02 was displayed more amino acid mutations, leading to more structural changes in the enzyme relatively, as the ultimate expression of the enzyme activity was lower and the acetaldehyde yield was reduced.