Propionic acid (, from the Greek words πρῶτος : prōtos, meaning "first", and πίων : píōn, meaning "fat"; also known as propanoic acid) is a naturally occurring carboxylic acid with chemical formula CH3CH2CO2H. It is an oily liquid with a pungent and unpleasant smell. The anion CH3CH2CO−2 as well as the salts and esters of propionic acid are known as propionates or propanoates. Propionic acid is the third simplest carboxylic acid, with the first two being formic acid and acetic acid. About half of the world production of propionic acid is consumed as a preservative for both animal feed and food for human consumption. It is also useful as an intermediate in the production of other chemicals, especially polymers.
History Propionic acid was first described in 1844 by Johann Gottlieb, who found it among the degradation products of sugar. Over the next few years, other chemists produced propionic acid by different means, none of them realizing they were producing the same substance. In 1847, French chemist Jean-Baptiste Dumas established all the acids to be the same compound, which he called propionic acid, from the Greek words πρῶτος (prōtos), meaning first, and πίων (piōn), meaning fat, because it is the smallest H(CH2)nCOOH acid that exhibits the properties of the other fatty acids, such as producing an oily layer when salted out of water and having a soapy potassium salt.
Properties Propionic acid has physical properties intermediate between those of the smaller carboxylic acids, formic and acetic acids, and the larger fatty acids. It is miscible with water, but can be removed from water by adding salt. As with acetic and formic acids, it consists of hydrogen bonded pairs of molecules in both the liquid and the vapor. Propionic acid displays the general properties of carboxylic acids: it can form amide, ester, anhydride, and chloride derivatives. It undergoes the Hell–Volhard–Zelinsky reaction that involves α-halogenation of a carboxylic acid with bromine, catalysed by phosphorus tribromide, in this case to form 2-bromopropanoic acid, CH3CHBrCOOH. This product has been used to prepare a racemic mixture of alanine by ammonolysis.
Manufacture
Chemical In industry, propionic acid is mainly produced by the hydrocarboxylation of ethylene using nickel carbonyl as the catalyst:
It is also produced by the aerobic oxidation of propionaldehyde. In the presence of cobalt or manganese salts (manganese propionate is most commonly used), this reaction proceeds rapidly at temperatures as mild as 40–50 °C:
Large amounts of propionic acid were once produced as a byproduct of acetic acid manufacture. At the current time, the world's largest producer of propionic acid is BASF, with approximately 150 kt/a production capacity.
Biotechnological Biotechnological production of propionic acid mainly uses Propionibacterium strains. However, large scale production of propionic acid by Propionibacteria faces challenges such as severe inhibition of end-products during cell growth and the formation of by-products (acetic acid and succinic acid). One approach to improve productivity and yield during fermentation is through the use of cell immobilization techniques, which also promotes easy recovery, reuse of the cell biomass and enhances microorganisms' stress tolerance. In 2018, 3D printing technology was used for the first time to create a matrix for cell immobilization in fermentation. Propionic acid production by Propionibacterium acidipropionici immobilized on 3D-printed nylon beads was chosen as a model study. It was shown that those 3D-printed beads were able to promote high density cell attachment and propionic acid production, which could be adapted to other fermentation bioprocesses. Other cell immobilization matrices have been tested, such as recycled-glass Poraver and fibrous-bed bioreactor. Alternative methods of production have been trialled, by genetically engineering strains of Escherichia coli to incorporate the necessary pathway, the Wood-Werkman cycle.
Industrial uses Propionic acid inhibits the growth of mold and some bacteria at levels between 0.1 and 1% by weight. As a result, some propionic acid produced is consumed 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. This application accounts for about half of the world production of propionic acid. The antibiotic monensin is added to cattle feed to favor propionibacteria over acetic acid producers in the rumen; this produces less carbon dioxide and feed conversion is better. Another major application is as a preservative in baked goods, which use the sodium and calcium salts. As a food additive, it is approved for use in the EU, US, Australia and New Zealand. Propionic acid is also useful as an intermediate in the production of other chemicals, especially polymers. Cellulose-acetate-propionate is a useful thermoplastic. Vinyl propionate is also used. In more specialized applications, it is also used to make pesticides and pharmaceuticals. The esters of propionic acid have fruit-like odors and are sometimes used as solvents or artificial flavorings. In biogas plants, propionic acid is a common intermediate product, which is formed by fermentation with propionic acid bacteria. Its degradation in anaerobic environments (e.g. biogas plants) requires the activity of complex microbial communities. In production of Jarlsberg cheese, a propionic acid bacterium is used to give both taste and holes.
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