Esterase activity and release of ethyl esters of medium-chain fatty acids by Saccharomyces cerevisiae during anaerobic growth

During anaerobic fermentation, Saccharomyces cerevisiae releases large amou nts of medium-chain fatty acids (MCFAs) and related ethyl esters which are very important for aromatic quality of fermented beverages. The physiologic al function of ester synthesis is not yet understood. As MCFAs are toxic, t heir conversion to esters has been proposed to be a detoxification mechanis m. Esterases possess ester synthesizing ability. Throughout an anaerobic fe rmentation of a lipid-free synthetic medium carried out with a S. cerevisia e strain selected for wine making, we have monitored MCFA and ethyl ester p roduction and, at the same time, measured growth and esterasic activity of intact cells. Because no correlation was found between the concentration of each fatty acid and its ethyl ester, there is no evidence that ester synth esis reduces the toxicity of MCFAs. Esterasic activity did not show any cor relation with eater synthesis, but it was related to the release of MCFAs. A model is proposed in which ester synthesis is a consequence of the arrest of lipid biosynthesis resulting from a lack of oxygen. Under these conditi ons, an excess of acyl coenzyme A is produced, and acyl esters are formed a s secondary products of reactions aimed at recovering free coenzyme A.