Characterization of low-acetic-acid-producing yeast isolated from 2-deoxyglucose-resistant mutants and its application to high-gravity brewing

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We isolated a mutant with low acetic acid and high ethanol productivities from 2-deoxyglucose-resistant mutants of brewers’ yeast NCYC1245 (Saccharomyces cerevisiae). To determine the mechanism for these properties in the mutant (2DGR19) during fermentation, gene expression and enzyme activity related to acetic acid and ethanol production were investigated. DNA microarray analysis revealed that the transcriptional levels of many genes involved in glycolysis were higher in 2DGR19 than in NCYC1245. Among these transcriptional levels of 2DGR19 relative to NCYC1245, the expression level of ADH4 encoding alcohol dehydrogenase (ADH) was highest, which corresponded to the high ADH activity in 2DGR19. Quantitative PCR analysis also revealed that the transcriptional level of ADH4 was the highest among ADH1 to ADH4. Although no significant differences in the transcriptional levels of ALD2 to ALD6 encoding acetaldehyde dehydrogenase (ALD) between 2DGR19 and NCYC1245 were observed, ALD activity in 2DGR19 was lower. Using quantitative PCR analysis, ALD6 was found to be the most highly expressed among the ALD2 to ALD6 genes. These results indicate that ALD6 contributes to a low ALD activity, depending on post-transcriptional regulation. A high ADH activity appeared to be the major reason for the high ethanol productivity of 2DGR19. A low ALD activity was considered to be principally responsible for a low acetic acid productivity, although a high ADH activity also might have played a role. Beer brewed using 2DGR19 in pilot-scale high-gravity brewing contained about half as much acetic acid and 1.1% more ethanol compared with that brewed using NCYC1245. The use of 2DGR19 may overcome difficulties associated with high-gravity brewing.

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Yeast strain and culture conditions

A top-fermenting brewers’ yeast, S. cerevisiae NCYC1245 (National Collection of Yeast Cultures, Norwich, UK) was used. Yeast cells were cultivated at 25°C with YPD (1% Bacto yeast extract [Becton-Dickinson, Sparks, MD, USA], 2% Bacto peptone [Becton-Dickinson], 8% glucose) medium. The glucose concentration in YPD medium was increased to a level higher than usual to ensure glucose repression during cultivation. To prepare an agar medium, 2% agar was added to YPD medium.

Isolation of a low-acetic-acid-producing strain from 2-DG-resistant mutants

The yeast cultured in YPD

Isolation of low acetic-acid-producing strain from 2-DG-resistant mutants

Thirty colonies were isolated as 2-DG-resistant mutants, with an isolation rate of about 1×10−5. These mutants were used to select a mutant that produces less acetic acid and more ethanol than the parental strain NCYC1245 (hereafter represented as NCYC1245). Although ethanol productivities were almost the same among the mutants, which might be due to the normal original gravity of wort (13%, w/w), 24 mutants produced less acetic acid than NCYC1245. Moreover, we performed the tall-tube

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