WHY do yeast produce esters?

The synthesis of esters requires the expenditure of metabolic energy suggesting that ester formation must fulfil an important metabolic role. It may be a mechanism for regulating the ratio of acyl-CoA to free CoA (Thurston et al., 1981). Peak ester concentrations are reached after the formation of higher alcohols has ceased (Fig. 12.1). Rates of ester synthesis are maximal at the mid-point of fermentation coinciding with the cessation of lipid synthesis. Thus, when acetyl-CoA cannot be utilized by lipid synthesis, the formation of esters provides an alternative use for this substrate. Intermediates of lipid biosynthesis influence ester formation. Supplementation of worts with the unsaturated fatty acid, linoleic acid (50 mg/l) causes a dramatic decrease in ester formation (Thurston et al., 1982). It was suggested that this effect was due to inhibition of alcohol acyl- transferase by unsaturated fatty acids. This effect has been confirmed by others (Yoshioka and Hashimoto, 1982a, b, 1984) and led to the proposal that ester and lipid syntheses are inversely correlated. This is supported by the observation that increasing oxygen supply to wort tends to decrease ester synthesis. In this case, oxygen promotes the synthesis of unsaturated fatty acids, which in turn reduces the activity of alcohol acyltransferase.

It now appears that this effect is exerted at a more fundamental level (Malcorps et al., 1991; Fuji et al., 1997). These reports provided evidence that oxygen and linoleic acid caused repression of alcohol acyltransferase. In later studies (Dufour and Malcorps, 1994) the same groups demonstrated the existence of multiple isozymes of alcohol acyltransferase. These have different substrate specificity and not all are subject to repression by oxygen and unsaturated fatty acids.

The spectrum of esters produced during fermentation is controlled by the range and substrate specificity of the alcohol acyltransferases possessed by individual yeast strains. The concentrations of esters produced by given yeast strains can be modulated by factors that influence the availability of acyl-CoA esters and lipid biosynthesis. In particular, the provision of oxygen is crucial.

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tackett said:

So the short answer is:

No one knows for sure.

There's a lot of words like, "suggested", "proposed", "appears" etc. in that article.

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tackett said:

Edit:
Oh but thanks for taking the time to post that. My comments made it sound like I didn't appreciate the effort you took to answer my question.

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MalFet said:

Nobody ever knows anything for sure in science; rather, there are just varying degrees of not-sureness. On the other hand...as far as scientific arguments go, this is a fairly robust one. We've got strong correlative trends, clear and plausible mechanisms, and evidence from both the lab and the field.

Beyond that, there's never going to be a simple "why" in things as complicated as metabolism. Barring major new findings, this is a reasonably solid explanation.



No problem

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