An organic acid is an organic compound with acidic properties. The relative stability of the conjugate base of the acid determines its acidity. In biological systems, organic compounds containing these groups are generally referred to as organic acids.
Common organic acids are the carboxylic acids, whose acidity is associated with their carboxyl group â€"COOH. Sulfonic acids, containing the group â€"SO2OH, are stronger acids. Phenols, with aromatic â€"OH, are usually much weaker. Phosphoric acid derivatives are also moderately strong. Other groups, such as the thiol group â€"SH and the enol group, can also confer acidity, usually weakly.
Characteristics
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In general, organic acids are weak acids, i.e. they do not fully dissociate in water. Lower molecular mass organic acids such as formic and lactic acids are miscible in water, but higher molecular mass organic acids, such as benzoic acid and oleic acid, are less soluble. Organic acids are very soluble in organic solvents. p-Toluenesulfonic acid is a comparatively strong acid that dissolves in many organic solvents.
Examples
The pKa, a logarithmic measure of the acid dissociation constant, categorizes the strength of an acid: the lower, or more negative, the number, the stronger the acid. A few examples include: (COOH is the carboxyl group)
Applications
In terms of production, the dominant organic acid is acetic acid. It is heavily used for the synthesis of polymers, e.g. polyvinyl acetate. It is also a precursor to myriad compounds such as pharmaceuticals, agrichemicals and fragrances.
The global organic acid market was $6.94 billion in 2016, and is projected to grow at a CAGR of 6.11% in value terms to reach $12.54 billion by the end of 2026, on the back of rising demand for natural and environmentally acceptable additives and rapid growth in food & beverage processing industry across the globe.
Application in food
Organic acids are used in food preservation because they have bacteriostatic properties. The key principle on the mode of action of organic acids on bacteria is that non-dissociated (non-ionized) organic acids can penetrate the bacteria cell wall and disrupt the normal physiology of certain types of bacteria that we call pH-sensitive, meaning that they cannot tolerate a wide internal and external pH gradient. Among those bacteria are Escherichia coli, Salmonella spp., C. perfringens, Listeria monocytogenes, and Campylobacter species.
Upon passive diffusion of organic acids into the bacteria, where the pH is near or above neutrality, the acids will dissociate and lower the bacteria internal pH, leading to conditions that will impair or stop the growth of bacteria. Thereafter, the anionic part of the organic acids, which cannot escape the bacteria in its dissociated form, will accumulate within the bacteria and disrupt many metabolic functions, leading to osmotic pressure increase, incompatible with the survival of the bacteria.
Lactic acid and its salts sodium lactate and potassium lactate are widely used as antimicrobials in food products, in particular, meat and poultry such as ham and sausages.
Biochemistry
Carboxylic acids are pervasive in nature. Major classes of compounds include the amino acids and fatty acids, which comprise building blocks of life. Citric, lactic, pyruvic, oxaloacetic and other acids are continuously recycled in energy producing systems such as the citric acid cycle.
Biological information is stored and transmitted DNA and RNA, which are composed of nucleic acids, which feature the conjugate bases of phosphoric acid esters.
Production
For commercial applications, almost all organic acids are produced industrially. Illustrative of the scale of the industrial production, 5.5 billion kilograms of acetic acid were produced in 1996. Dominant acids include acetic acid produced by carbonylation]] of methanol, propionic acid produced by hydrocarboxylation of ethylene, and terephthalic acid produced by aerobic oxidation of para-xylene. Sulfonic acids are produced by treatment of organic compounds with sulfur trioxide or sulfuric acid.
See also
- Acid-base extraction
- Organic base
References
Further reading
- Dibner, J. J.; Buttin, P. (2002). "Use of Organic Acids as a Model to Study the Impact of Gut Microflora on Nutrition and Metabolism". The Journal of Applied Poultry Research. 11 (4): 453â€"463. doi:10.1093/japr/11.4.453.Â
- Patanen, K. H.; Mroz, Z. (1999). "Organic acids for preservation". In Block, S. S. Disinfection, sterilization & preservation (5th ed.). Philadelphia: Lea Febiger. ISBNÂ 0-683-30740-1.Â
- Brul, S; Coote, P (1999). "Preservative agents in foods. Mode of action and microbial resistance mechanisms". International Journal of Food Microbiology. 50 (1â€"2): 1â€"17. doi:10.1016/s0168-1605(99)00072-0. PMID 10488839.Â