3-Methylbutanoic acid, also known as β-methylbutyric acid or more commonly isovaleric acid, is a branched-chain
alkylcarboxylic acid with the
chemical formula (CH3)2CHCH2CO2H. It is classified as a
short-chain fatty acid. Like other low-molecular-weight carboxylic acids, it has an
unpleasant odor. The compound occurs naturally and can be found in many foods, such as cheese, soy milk, and apple juice.
History
3-Methylbutanoic acid is a minor constituent of the perennial flowering plant
valerian (Valeriana officinalis), from which it got its
trivial name isovaleric acid: an
isomer of
valeric acid which shares its unpleasant odor.[2] The dried root of this plant has been used medicinally since antiquity.[3][4] Their chemical identity was first investigated in the 19th century by oxidation of the components of
fusel alcohol, which includes the five-carbon
amyl alcohols.[5]
Alternative syntheses of β-hydroxy β-methylbutyric acid
Uses
Isovaleric acid has a strong pungent cheesy or sweaty smell,[11] but its volatile
esters such as
ethyl isovalerate[12] have pleasant odors and are widely used in perfumery. It is also the primary flavor added to wine when made using Brettanomyces yeasts.[13] Other compounds produced by Brettanomycesyeasts include
4-ethylphenol,
4-vinylphenol, and
4-ethylguaiacol.[14] An excess of isovaleric acid in wine is generally seen as a defect,[14] as it can smell sweaty, leathery, or "like a barnyard", but in small amounts it can smell smokey, spicy, or medicinal.[13] These phenomena may be prevented by killing any Brettanomyces yeasts, such as by
sterile filtration, by the addition of relatively large quantities of
sulfur dioxide and sometimes
sorbic acid, by mixing in alcoholic spirit to give a fortified wine of sufficient strength to kill all yeast and bacteria, or by
pasteurization. Isovaleric acid can also be found in beer, and, excepting some English–style ales, is usually considered a flaw.[15] It can be produced by the oxidation of
hop resins, or by Brettanomyces yeasts present.[15]
The compound's safety as a food additive was reviewed by an
FAO and
WHO panel, who concluded that there were no concerns at the likely levels of intake.[16]
Isovaleric acid is a major component of the cause of intense
foot odor, as it is produced by skin bacteria metabolizing
leucine and in rare cases a condition called
isovaleric acidemia can lead to heightened levels of this metabolite.[19]
Salts and esters
An isovalerate or 3-methylbutanoateion is (CH3)2CHCH2COO−, the
conjugate base of the acid. It is the form found in biological systems at
physiological pH. An isovalerate or 3-methylbutanoate compound is a
salt or
ester of the acid.