The effects of ethanol, hexan-1-ol, and 2-phenylethanol on cider yeast growth, viability, and energy status; Synergistic inhibition

An ethanol-tolerant strain of Saccharomyces cerevisiae, used for cider manufacture, when grown aerobically in the presence of either 1.7 M ethanol (10%, v/v), 3.35 mM hexan-1-ol (0.05%), or 25 mM 2-phenylethanol (0.25%), showed decreased growth rates (about 60%, 60% and 75%) and diminished viability. The normal changes in medium pH (increased acidity over the first 24 h from pH 6.0 to 5.4 followed by a decreased acidity to pH 7.8 by 70 h) were altered in the presence of an added alcohol. ''Acidification power'' (glucose-driven pH change) was also markedly reduced by growth with the alcohols from a value of 1.77 for control cultures, to 0.5, 1.35 or 0.6 respectively after 48 h. Adenylate energy charge values were decreased from 0.8 to 0, 0.3 and 0.2 respectively after 115 h; control cultures without an added alcohol showed a final value of 0.4. Ethanol plus either hexan-1-ol or 2-phenylethanol gave much more pronounced decreases in viability, medium pH changes during growth, acidification power, and adenylate energy change values than predicted by summation of individual effects. It is concluded that synergy between ethanol and the higher alkanol or aryl-alcohol occurs to produce membrane-associated lesions with deleterious effects during cider fermentation.