Ethyl Acetate is a compound produced by yeast that is quantitatively the major ester found in both beer and wine. Esters are aromatic compounds formed by the reaction between alcohols and acids. They are widely found in nature and contribute to the aromas of many varieties of fruit. Over 90 esters can occur in beer, and the ethyl esters predominate.
These esters contribute to the overall flavor and aroma of beer, giving a “fruity” quality to a drink that rarely contains any fruit. As ethyl acetate intensifies, however, the aromatic perception can skew from pleasant and “fruity” to “solventy” and “perfumy;” abnormally high levels are therefore regarded as off-flavors. In addition to being produced by brewer’s yeast strains (Saccharomyces cerevisiae and Saccharomyces pastorianus), ethyl acetate is also produced in large quantities by the wild yeasts Brettanomyces, Hansenula, and Pichia via aerobic fermentation.
Because ethyl acetate is such an influential part of beer aromatics—for good or ill—brewers seek to control its levels in their beers. Many factors, in addition to the yeast strain employed, have been found to influence the concentration of ethyl acetate formed during fermentation. These include fermentation temperature, where an increase from 50°F to 77°F (10°C–25°C) has been found to increase the concentration of ethyl acetate from 12.5 to 21.5 mg/L. Continuous fermentation results in higher levels of esters as compared to conventional batch fermentation.
Ethyl acetate arises as a result of the reaction between ethanol and acetyl CoA. Practical measures can be taken to lower ester levels (particularly in high gravity worts), including the production of wort with a suitably low carbon-to-nitrogen ratio and the supply of adequate oxygen at the outset of fermentation, both of which promote yeast growth. The application of pressure during fermentation reduces both yeast growth and ester synthesis.