Esters represent the largest group of flavor compounds in alcoholic beverages, generating the “fruity” aromas in beer (not including, of course, the direct addition of fruit and fruit flavors in certain beers). The esters are formed by the reactions of organic acids and alcohols created during fermentation. The most significant esters found in beer are isoamyl acetate (banana, peardrop), ethyl acetate (light fruity, solvent-like), ethyl caprylate (apple-like), ethyl caproate (apple-like with a note of aniseed), and phenylethyl acetate (roses, honey).
Esters are produced primarily through the action of yeasts during fermentation and are influenced by three features of the fermentation process: yeast characteristics, wort composition, and fermentation conditions. Brewers look to control all three to produce exactly the flavor and aroma they wish to create in their beers. The selection of yeast strain is very important in determining the type and level of ester found in beer. Some strains are characterized by the production of high levels of isoamyl acetate, most notably in the brewing of traditional Bavarian wheat (weizen;
The composition of the wort can also influence ester formation in beer. High dissolved oxygen levels in the wort tend to inhibit ester formation, whereas high sugar concentrations increase ester levels. Higher alcohol beers tend to be marked by high ester levels and the disproportionately high levels need to be accommodated in the application of high gravity brewing to the production of beers of lower alcohol content.
The shape of the fermentation vessels can also impact the production of esters—tall, narrow fermenters tend to produce lower levels of esters than does fermentation in shallow, open fermentation vessels. The effect is attributable to a combination of high hydrostatic pressure and high CO2 levels in taller vessels. The stirring of fermenters increases ester levels, which is an issue in continuous fermentation systems where excessive amounts of esters are produced.