Isovaleric Acid, also known as 3-methylbutanoic acid, 1-pentanoic acid, or delphinic acid, is a natural fatty acid found in many plants, essential oils, old hops, foot sweat, and some cheese. It is characterized by a pungent aroma often described as cheesy (especially referring to aged hard cheeses) or “gym socks.” In beer, the flavor threshold of isovaleric acid is in the wide range of 0.1–1.5 mg/l, varying with the taster’s sensitivity. Isovaleric acid is a product of the oxidation of hop resins and is often quite pronounced in hops that have been exposed to oxygen over a long period of time. It is also a by-product of contamination by, or fermentation with, Brettanomyces yeasts. See brettanomyces. The presence of isovaleric acid in beer is usually considered a flaw, but it is considered appropriate as a background note in some English-style ales and can be stronger in beers purposely fermented with Brettanomyces. Excessive isovaleric acid in beer is best prevented by appropriate hop storage and clean brewing practices to prevent wild yeast infections. Because oxygen is the catalyst that forms isovaleric acid in hops, all hops should be stored cold and tightly packed to limit their exposure to the air.

See hops.