The chemical compound propionic acid (systematically named propanoic acid) is a naturally occurring carboxylic acid with chemical formula CH3CH2COOH. In the pure state, it is a colorless, corrosive liquid with a sharp, somewhat unpleasant odor.
It is produced biologically from the metabolic breakdown of fatty acids containing odd numbers of carbon atoms, and also it the breakdown of some amino acids. Bacteria of the genus Propionibacterium produce propionic acid as the end product of their anaerobic metabolism. These bacteria are commonly found in the stomachs of ruminants, and their activity is partially responsible for the odor of both Swiss cheese and sweat.
Priopionic acid was first described in 1844 by Gottlieb, who found it among the degradation products of sugar. Over the next few years, other chemists produced propionic acid in various other ways, none of them realizing they were producing the same substance. In 1847, the French chemist Jean-Baptiste Dumas established that all the acids were the same compound, which he called propionic acid, from the Greek words protos and pion, meaning "first" and "fat" respectively, because it was the smallest acid that exhibited the properties of the other fatty acids, such as producing an oily layer when salted out of water and having a soapy potassium salt.
Priopionic acid has physical properties intermediate between those of the smaller carboxylic acids, formic and acetic acid, and the larger fatty acids. It is miscible with water, but it can be removed from water by adding salt. Like acetic and formic acids, its gas grossly violates the ideal gas law because it does not consist of individual propionic acid molecules, but instead of hydrogen bonded pairs of molecules. It also undergoes this pairing in solution.
Chemically, propionic acid displays the general properties of carboxylic acids, and like most other carboxylic acids, it can form amide, ester, anhydride, and chloride derivatives. It can react with chlorine or other halogens to replace one of the hydrogen atoms attached to its carbon atoms with a halogen atom.
Industrially, propionic acid is usually made from the air oxidation of propionaldehyde. In the presence of cobalt or manganese ions, this reaction proceeds rapidly even at mild termperatures. Usually, the industrial process is carried out at 40-50°C, and is represented by the chemical equation
CH3CH2CHO + ½ O2 → CH3CH2COOH
Large amounts of propionic acid were once produced as a byproduct of acetic acid manufacture, but changes in the way acetic acid is made have made this a very minor source of propionic acid today.
Propionic acid inhibits the growth of mold and some bacteria. Accordingly, most propionic acid produced is used as a preservative for both animal feed and food for human consumption. For animal feed, it is used either directly or as its ammonium salt. In human foods, especially bread and other baked goods, it is used as its sodium or calcium salt.
Propionic acid is also useful as a chemical intermediate. It can be used to modify synthetic cellulose fibers. It is also used to make pesticides and pharmaceuticals. The esters of propionic acid are sometimes used as solvents or artificial flavorings.
The chief danger from propionic acid is chemical burns that can result from contact with the concentrated liquid. In studies on laboratory animals, the only adverse health effect associated with long-term exposure to small amounts of propionic acid has been ulceration of the esophagus and stomach from consuming a corrosive substance. No toxic, mutagenic, carcinogenic, or reproductive effects have ever been observed. In the body, propionic acid is readily oxidized, metabolized, and eliminated from the body as carbon dioxide in the Krebs cycle, and so it does not bioaccumulate.