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; see kristtallweizen) beers, where the distinctive banana (and clove-like) character of these beers is determined by the specific yeast strains used. It has been suggested that the ale strains of yeast (“top fermenters”; see top fermentation) are more liable to produce esters than are lager strains (“bottom fermenters”; see bottom fermentation), but it is probable that the higher levels of esters normally found in ales are attributable to the higher fermentation temperatures used in ale brewing. Additionally, the physiological state of the yeast can also influence ester production.

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. See high gravity brewing. Other wort components such as the levels of free amino nitrogen (FAN), zinc, and lipids also influence ester formation. See free amino nitrogen (fan). High FAN and zinc levels in wort tend to increase ester formation, whereas high lipid content (particularly through a high trub carryover) tends to lower ester levels. See trub.

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. See continuous fermentation. Brewers must take these factors into account when brewing specific styles and brands of beer in different brewing locations to achieve consistent ester profiles.

See also flavor.