Parameters Affecting Ethyl Ester Production bySaccharomyces cerevisiaeduring Fermentation

Abstract

Volatile esters are responsible for the fruity character of fermented beverages and thus constitute a vital group of aromatic compounds in beer and wine. Many fermentation parameters are known to affect volatile ester production. In order to obtain insight into the production of ethyl esters during fermentation, we investigated the influence of several fermentation variables. A higher level of unsaturated fatty acids in the fermentation medium resulted in a general decrease in ethyl ester production. On the other hand, a higher fermentation temperature resulted in greater ethyl octanoate and decanoate production, while a higher carbon or nitrogen content of the fermentation medium resulted in only moderate changes in ethyl ester production. Analysis of the expression of the ethyl ester biosynthesis genes EEB1 and EHT1 after addition of medium-chain fatty acid precursors suggested that the expression level is not the limiting factor for ethyl ester production, as opposed to acetate ester production. Together with the previous demonstration that provision of mediumchain fatty acids, which are the substrates for ethyl ester formation, to the fermentation medium causes a strong increase in the formation of the corresponding ethyl esters, this result further supports the hypothesis that precursor availability has an important role in ethyl ester production. We concluded that, at least in our fermentation conditions and with our yeast strain, the fatty acid precursor level rather than the activity of the biosynthetic enzymes is the major limiting factor for ethyl ester production. The expression level and activity of the fatty acid biosynthetic enzymes therefore appear to be prime targets for flavor modification by alteration of process parameters or through strain selection.During fermentation the yeast Saccharomyces cerevisiae produces a broad range of aroma-active substances, which are vital for the complex flavor of fermented beverages, such as beer and wine. In particular, volatile esters are of major industrial interest because the presence of these compounds determines the fruity aroma of beer and wine (6,7,9,20,24,26,27,30,31,36). Even small changes in the concentrations of these secondary metabolites can have large effects on the final sensorial quality of fermented beverages. There are two main groups of flavor-active esters in fermented beverages. The first group contains the acetate esters (in which the acid group is acetate and the alcohol group is ethanol or a complex alcohol derived from amino acid metabolism), such as ethyl acetate (solventlike aroma), isoamyl acetate (banana aroma), and phenyl ethyl acetate (roses, honey). The second group is the ethyl esters (in which the alcohol group is ethanol and the acid group is a medium-chain fatty acid [MCFA]) and includes ethyl hexanoate (anise seed, applelike aroma), ethyl octanoate (sour apple aroma), and ethyl decanoate (floral odor). Of these two groups, the acetate esters have received the most attention, not because they are more important but because ...

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