Production and biological function of volatile esters in Saccharomyces cerevisiae

Abstract

The need to understand and control ester synthesis is driven by the fact that esters play a key role in the sensorial quality of fermented alcoholic beverages like beer, wine and sake. As esters are synthesized in yeast via several complex metabolic pathways, there is a need to gain a clear understanding of ester metabolism and its regulation. The individual genes involved, their functions and regulatory mechanisms have to be identified. In alcoholic beverages, there are two important groups of esters: the acetate esters and the medium-chain fatty acid (MCFA) ethyl esters. For acetate ester synthesis, the genes involved have already been cloned and characterized. Also the biochemical pathways and the regulation of acetate ester synthesis are well defined. With respect to the molecular basis of MCFA ethyl ester synthesis, however, significant progress has only recently been made. Next to the characterization of the biochemical pathways and regulation of ester synthesis, a new and more important question arises: what is the advantage for yeast to produce these esters? Several hypotheses have been proposed in the past, but none was satisfactorily. This paper reviews the current hypotheses of ester synthesis in yeast in relation to the complex regulation of the alcohol acetyl transferases and the different factors that allow ester formation to be controlled during fermentation.

Figure 1

Flavour‐active esters produced by Saccharomyces yeast in wine, beer and sake. Ethyl caproate = ethyl hexanoate, ethyl caprylate = ethyl octanoate.

Figure 2

Biochemical synthesis of esters (A) and the activation of the acyl moiety (B).

Figure 3

Biosynthesis of fatty acids and its relationship with medium‐chain fatty acid ester formation as proposed by Dufour and colleagues (2003). Acetyl‐CoA carboxylase initiates fatty acid synthesis and is inhibited by long‐chain saturated acyl‐CoAs. As a result, medium‐chain fatty acid CoAs are released from the fatty acid synthase complex, which can then be converted to the corresponding esters. In the presence of oxygen, long‐chain saturated acyl‐CoAs are converted to unsaturated acyl‐CoAs, which do not inhibit acetyl‐CoA carboxylase, and thus no longer cause release of medium‐chain fatty acid CoAs from the fatty acid synthase complex. Saturated and unsaturated fatty acids are used for the synthesis of phospholipids which are then incorporated into cellular membranes.