Butyric acid is an organic compound that is responsible for the smell of butter that has gone sour. It is also known as butanoic acid or butyrate. The butyric acid formula is CH3CH2CH2-COOH. It is a carboxylic acid, as defined by the COOH group. Carboxylic acids are important in their own right, but they also function as the ingredients for making larger compounds.

The hydrogen group of the COOH is readily replaced with an organic compound to make an ester, known as a butanoate. Such compounds are found in plant oils and animal fats. The esters of low molecular weights, such as methyl butanoate, have different properties: they generally smell and taste pleasant. This has made them useful for the food and perfume industry. Methyl butyric acid is a synonym for this ester.

Butyric acid is prepared on a large scale by the fermentation of starch or sugar. Fermentation uses microorganisms and takes place under conditions where there is no oxygen. It is the transformation of the glucose in these compounds to alcohol or butyric acid, in this case. The microbes gain energy from this process. Less energy is produced than if the compounds were transformed in the presence of oxygen.

Various species of bacteria produce butyric acid as a product of fermentation. They live in places where there is no oxygen, such as the rumen of cows and goats. This is a special digestive organ for degrading plant compounds that are extremely difficult to digest. Bacteria generate butyrate from the plant fibers consumed. Butyrate is also produced as a side product in marine sediments.

Bacteria that live in the human colon transform various fibers such as oat bran to a series of compounds, including butyric acid. This contributes to the foul smell of flatulence. People who consume a low carbohydrate diet have lower amounts of butyric acid in their colon. Some researchers think that butanoate protects against colon cancer; however, the evidence is mixed.

Butyric acid is a component of vomit. Its unpleasant smell has made it a useful stink bomb for non-lethal political attacks. It has been used to attack whaling boats and taint the whale meat, so that it cannot be consumed. It has also been used in a large number of attacks on abortion clinics.

Gamma amino butyric acid (GABA) is a major neurotransmitter in mammals. This is a chemical that relays signals throughout the central nervous system. Many anti-anxiety and anti-convulsive drugs act by increasing the amount of GABA that is available. Despite its name, GABA is not an amino acid.

Gamma hydroxy butyric acid (GHB) is found naturally in the brain. It is also abused as an illegal drug that gives some people a feeling of euphoria, although it is also used as a "date-rape" drug. The first use of GHB was as an anaesthetic. For awhile, it had a legal following with body builders, but that ended with an association with a number of side effects. GHB has been linked with a number of deaths, but its use as an illegal drug continues.

Another chemical derivative of butyric acid of interest is phenyl butyric acid. This compound is used as an intermediate in a number of chemical processes. Indole butyric acid is a plant growth hormone and causes shoots to grow roots.