Alcohol is an organic compound in which the hydroxyl group (-OH) is attached to an alkyl or substituted alkyl group. The most notable alcohol in a brewing context is ethanol (ethyl alcohol) and of course the words “ethanol” and “alcohol” are frequently used synonymously in this context. See ethanol. However, beer contains a diversity of alcohols, including traces of methanol, propanol, iso-butanol, 2-methylpropan- 1-ol, iso-amylalcohol, tyrosol, and phenylethanol.

Higher alcohols (containing more carbon atoms than ethanol) are sometimes known as fusel alcohols. See fusel alcohols. These higher alcohols are important flavor contributors and their levels are influenced by the strain of yeast and fermentation conditions, notably temperature. Beers containing notable levels of higher alcohols are often described as tasting “hot.” Ales tend to contain greater levels of these alcohols than do lagers. For example, propanol levels are typically about four times higher in ales than lagers and iso-butanol levels are three times higher. It has been suggested that higher concentrations of fusel alcohols in beer can be associated with hangovers. If this is true, it is most likely that the aldehydes produced from the alcohols in the body are the culprit.

The alcohols serve as the precursors of even more flavor-potent esters, produced by enzyme- catalyzed reactions in yeast through the coupling of the alcohols with acids. Oxidation of the alcohols leads to aldehydes and ketones and then to acids.

Quantitatively the most important alcohol in beer is ethanol. In addition to its key role in alcoholic beverages where it provides the warming alcoholic note but also impacts the distribution of other flavor-active molecules into the headspace of the beverage, it is used as a solvent, as a gasoline replacement in cars, and as a reactant in diverse industrial chemical reactions. Apart from via fermentation, alcohols can be made by hydrating ethylene derived from the cracking of distilled crude oil, but such “industrial” alcohol is not potable.

See also alcohol strength and measurement.