Aeration Prior to Pitching Increases Intracellular Enzymatic and Transcriptional Responses under Nonnutritional Conditions

Abstract

Appropriate aeration of bottom-fermenting yeast suspended in water increased subsequent yeast growth under anaerobic conditions and reduced the synthesis of volatile esters during the fermentation of wort, whereas the absence of, or excess, aeration had no effect on cell growth and ester synthesis. In an effort to elucidate some of the underlying cellular mechanisms triggered by aeration under nutrient-lacking conditions, we analyzed alterations in the expression of the hypoxic repressor gene (ROX1), the expression of the Δ-9 fatty acid desaturase gene (OLE1), and the composition of cellular fatty acids. Our results show that higher ROX1 transcription levels resulted with increased aeration. However, OLE1 transcription levels initially were depressed but then increased with increased aeration. The ratio of unsaturated fatty acids to total fatty acid composition increased with increased aeration. Our results suggest that aeration of the yeast-containing medium, even under nutrient-lacking conditions, stimulated oxygen-signaling pathways. We also examined intracellular sugar consumption in response to aeration in the absence of nutrients in the medium by measuring intracellular trehalose levels. Trehalose levels decreased with increased aeration; however, excess aeration resulted in trehalose exhaustion but did not stimulate anaerobic growth or decrease the synthesis of volatile esters. Intracellular trehalose levels seem be a suitable index reflecting adequate levels of aeration for fermentation.