Esters are a fruity flavor produced during fermentation that can vary in taste and aroma between pears, roses, bananas or other light fruits. In very high concentration, it can create a solvent-like flavor.
Esters are formed in beer by the “esterification” of ethanol which is the primary alcohol in beer. Ethanol combines with fatty acids and a molecule called acetyl coenzyme (ACOA) forming ethyl acetate. Ethyl acetate’s flavor varies from a light, pear-like character to solvent-like in high concentrations.
Other alcohols present in the beer may also combine to produce additional esters. For example, isoamyl alcohol will combine and produce isoamyl acetate, which tastes like bananas in low concentration. This ester is the distinct banana flavor that is the defining characteristic of a hefeweizen.