FORMATION OF ETHYL ACETATE IN FERMENTATION WITH BREWER'S YEAST: V. EFFECT OF SOME VITAMINS AND MINERAL NUTRIENTS

Gradual enhancement of ethyl acetate production through promoter engineering in chinese liquor yeast strains

As content and proportion of ethyl acetate is critical to the flavor and quality of beverages, the concise regulation of the ethyl acetate metabolism is a major issue in beverage fermentations. In this study, for ethyl acetate yield regulation, we finely modulated the expression of ATF1 through precise and seamless insertion of serially truncated PGK1 promoter from the 3' end by 100bp steps in the Chinese liquor yeast, CLy12a. The three engineered promoters carrying 100-, 200-, and 300-bp truncations exhibited reduced promoter strength but unaffected growth. These three promoters were integrated into the CLy12a strain, generating strains CLy12a-P-100, CLy12a-P-200, and CLy12a-P-300, respectively. The transcription levels of CLy12a-P-100, CLy12a-P-200, and CLy12a-P-300 were 20%, 17%, and 10% of that of CLy12a-P, respectively. The AATase (alcohol acetyl transferases, encoded by the ATF1 gene) activity of three engineered strains were 36%, 56%, and 62% of that of CLy12a-P. In the liquid fermentation of corn hydrolysate at 30°C, the concentration of ethyl acetate in CLy12a-P-100, CLy12a-P-200, and CLy12a-P-300 were reduced by 28%, 30%, and 42%, respectively, compared to CLy12a-P. These results verifying that the ethyl acetate yield could be gradually enhanced by finely modulating the expression of ATF1. The engineered strain CLy12a-P-200 produced the ethyl acetate concentration with the best sensorial quality compared to the other engineered yeast strains. The method proposed in this work supplies a practical proposal for breeding Chinese liquor yeast strains with finely modulated ethyl acetate yield. © 2018 American Institute of Chemical Engineers Biotechnol. Prog., 34:328-336, 2018.

Influence of assimilable nitrogen on volatile acidity production by Saccharomyces cerevisiae during high sugar fermentation

We analyzed the variability of volatile acidity and glycerol production by Saccharomyces cerevisiae on a large sample of high sugar musts. The production of volatile acidity was inversely correlated with the maximum cell population and the assimilable nitrogen concentration. The higher the nitrogen concentration, the less volatile acidity was produced. An approach to minimize volatile acidity production during high sugar fermentations by adjustment of assimilable nitrogen in musts was investigated in terms of both quantity and addition time. It was found that the optimal nitrogen concentration in the must is 190 mgN.l(-1). The best moment for nitrogen addition was at the beginning of fermentation. Addition at the end of the growth phase had less effect on volatile acidity reduction. We suggest that by stimulating cell growth, nitrogen addition provides NADH in the redox-equilibrating process, which in turn reduces volatile acidity formation.