Flavor-active Wine Yeasts: The Acetate and Ethyl Esters

If you look for it, you can find it. This morning I was interested in knowing whether the Internet had any information on the impact of yeast derived flavors during primary fermentation and there it was, a mini-review entitled "Flavour-active wine yeasts" authored by Antonio G. Cordente, Christopher D. Curtin, Cristian Varela, and Isak S. Pretorius in Applied Microbiology and Biotechnology in 2012.¹ Best of all, it was freely available! (Please see the References for a link to the article).
Reading this article is of interest to me because I was initially under the impression that a yeast is a yeast is a yeast, and it did not leave it's fermentation fingerprint on the wine. However, through discussions with winemakers and yeast purveyors, I did learn that the selection of yeast can impart a difference in the resultant wine.
This review updates another review published in 1986 by Nykanen where he mentions that there are more than 1300 volatile compounds that arise from fermentation by yeast.² It is also known that wines made through single-yeast inoculation differ in sensory properties to those made by spontaneous fermentations.
So, I'm combing through this new review to understand what kinds of aroma and flavor compounds are produced by the yeasts. The first group of compounds are the acetate esters. The acyl group denoted below in red is derived from acetate in the form of acetyl-CoA and the alcohol group is ethanol or a complex alcohol derived from amino acid metabolism to produce the following compounds with their distinctive aromas.

The esters formed during fermentation are dependent upon (1) the concentration of the substrates aceyl-CoA and the alcohol and (2) the activity of the enzymes such as acyltransferases and esterases that are involved in the synthesis of the acetate esters. The review mentions the experiments that were conducted on the genes encoding the following proteins Atf1p, Atf2p, Eht1p, Eeb1p and Iah1p can modulate the formation of acetate esters.
The ethyl esters are medium-chain fatty acids (MCFA), where the alcohol group is ethanol, (highlighted in red below) and the acyl group is derived from activated medium-chain fatty acids. These compounds contribute apple-like aromas.

The article also included a diagram of yeast flavor phenotypes, reproduced below:

This article presents experiments that show how certain genes can modulate the expression of desirable and undesirable characters in wine. Yeasts are not only capable of producing ethanol, they also produce other compounds that make a wine memorable, for better or worse. Selection of the appropriate yeast for the grape variety is another variable that the winemaker must be aware of, when making quality wine.
References:
1. Antonio G. Cordente, Christopher D. Curtin, Cristian Varela, Isak S. Pretorius, Flavor-active Wine Yeast , Applied Microbiology and Biotechnology, November 2012, Volume 96, Issue 3, pp 601-618.
2. Nykanen, L., Formation and occurrence of flavor compounds in wine and distilled alcoholic beverages, Am J Enol Vitic, (1986), 37:84–96.
All structures were drawn by the freely available drawing program from ACD Labs called ACD/ChemSketch Freeware.