Organic Acid

Organic acids are a diverse group of carbon-based compounds that possess acidic properties due to the presence of proton-donating functional groups. They occur widely in nature, are essential to biochemical pathways, and have numerous industrial, agricultural, and food-related applications. Their acidity varies considerably according to their molecular structure, the stability of their conjugate bases, and the nature of substituent groups attached to the carbon framework.

Chemical nature and functional groups

The acidity of most organic acids is associated with specific functional groups capable of releasing hydrogen ions. Carboxylic acids, characterised by the presence of the –COOH group, are the most common category. Their acidic strength is determined by the relative stability of their conjugate bases and by the electron-withdrawing ability of substituents positioned on the molecule.
Sulfonic acids, containing the –SO₂OH group, exhibit much stronger acidity than carboxylic acids because their conjugate bases are highly stabilised through resonance and inductive effects. Alcohols, which contain the hydroxyl group –OH, can act as acids but do so only weakly because their conjugate bases (alkoxides) are relatively unstable. Other groups that grant weak acidity include thiol groups –SH, enol hydroxyl groups, and phenolic groups, each contributing acidity through resonance and polarisation effects.
In biological contexts, any organic compound containing carboxyl, hydroxyl, thiol, enol, or phenolic groups may be categorised as an organic acid. Many examples exist in metabolic pathways, including intermediates and end products of carbohydrate, amino acid, and lipid metabolism.

Physical characteristics and solubility

Most organic acids are weak acids, meaning that they dissociate only partially in aqueous solution. Their dissociation behaviour contrasts with the full ionisation exhibited by strong mineral acids such as hydrochloric or nitric acid. Molecular size strongly influences solubility: low molecular-mass acids such as formic and lactic acids dissolve readily in water due to their polarity, while higher molecular-mass acids such as benzoic acid are generally insoluble in their neutral form because of increased hydrophobic character.
Organic acids are typically more soluble in organic solvents than in water. An important example is p-toluenesulfonic acid, which combines high acidity with effective solubility in non-aqueous media, making it valuable in synthetic organic chemistry. Certain structural substituents, including halogens or nitro groups, can modify solubility by influencing molecular polarity, and exceptions to general solubility trends occur where substituent effects are particularly strong.

Biological distribution and metabolic roles

Organic acids are widespread in living organisms as natural constituents of plant and animal tissues. Many arise endogenously during physiological processes or are formed through microbial fermentation in the gastrointestinal tract, particularly in the large intestine. Short-chain organic acids (C₁–C₇) occur frequently as metabolic intermediates or end products of biochemical reactions.
Human blood and urine contain a variety of organic acids, including lactic acid, citric acid, and D-glucuronic acid, as well as degradation products from amino acids and neurotransmitters. Examples include:

• Alpha-ketoisocaproic acid, formed in the catabolism of leucine.
• Vanillylmandelic acid, produced during the breakdown of adrenaline.
• D-lactic acid, derived from microbial fermentation of carbohydrates in the intestine.

Industrial and chemical applications

Organic acids play significant roles in industrial chemistry and engineering. Formic acid and acetic acid are widely used in oil and gas well stimulation, where their relatively mild reactivity compared with strong mineral acids prevents excessive corrosion of metal equipment. Their suitability for use at high temperatures or for prolonged contact times makes them particularly advantageous in drilling and maintenance operations.
The conjugate bases of organic acids, such as lactate and citrate ions, are important in buffering systems used in biological and medical settings, where maintaining stable pH is essential. Additionally, citric and oxalic acids are commonly employed in rust removal because they dissolve iron oxides effectively without damaging the base metal. Their ability to chelate metal ions helps accelerate rust removal and enhances cleaning performance.

Food preservation and antimicrobial effects

Organic acids are widely used in food preservation due to their ability to inhibit microbial growth. Their antimicrobial mechanism involves the penetration of undissociated acid molecules through bacterial cell walls. Inside the cell, where the pH is closer to neutrality, the acids dissociate, raising the internal pH and disrupting essential physiological processes. The resulting accumulation of anions interferes with metabolic pathways and increases osmotic pressure, ultimately inhibiting or eliminating pH-sensitive bacteria such as Escherichia coli, Salmonella spp., Listeria monocytogenes, Clostridium perfringens, and Campylobacter species.
Lactic acid and its salts, including sodium lactate and potassium lactate, are widely used in dairy, meat, and poultry products. They function as antimicrobials in products such as sausages and ham, extending shelf life while maintaining sensory qualities.

Applications in animal nutrition

Organic acids have been used effectively in animal feeds, particularly in intensive pig production for over two decades. They support enhanced growth and performance similar to, or better than, antibiotic growth promoters, without associated public health concerns. Their use has also expanded into poultry production, where they contribute to improved gut health and reduced incidence of diseases such as necrotic enteritis.
For maximum effectiveness, these acids often require protection to prevent premature dissociation in high-pH regions of the digestive tract. This allows them to reach the lower intestine, where most bacterial populations reside. Benefits observed in livestock include:

• Improved feed conversion and growth efficiency.
• Reduced intestinal disorders and lower pathogen load.
• Diminished carriage of Salmonella and Campylobacter species in poultry and pigs.

Emerging research and environmental implications

Scientific interest continues to grow in expanding the use of organic acids in environmentally benign applications. Research conducted at the University of Eastern Finland examined acetic, formic, and propionic acids as potential wood preservatives. While laboratory tests showed strong antifungal effects against brown-rot and white-rot fungi, practical application revealed limitations: the acids leached out of treated wood and caused partial chemical degradation, reducing their protective efficacy.