Fusel Alcohols are by-products of ethanol fermentation.
The individual alcohols playing the most important role in beer flavor include propanols and butanols conveying alcoholic, wine-like, and ripe fruity notes; active amyl alcohol (a methyl butanol) presenting the characteristic “fusely” or boozy pungent note; phenyl ethanol, producing a pleasant rose or sweetish note; and tryptophol and tyrosol, exhibiting bitter, almond-like, or solvent-like taste characteristics. The yeast strain, fermentation conditions, and wort composition all have significant effects on the combination and levels of higher alcohols formed. Fusel alcohols will be present at higher concentrations in more robust and alcoholic beers such as barley wines and imperial stouts, partly because of stresses inflicted upon the yeast by high ethanol concentrations and more strenuous fermentations. Otherwise, in most beer styles they should not be perceptible or just barely so. As a group, fusel alcohols are a major part of the “alcoholic” perception of beer and overall core beer flavor.