Organic acid

An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids whose acidity is associated with their carboxyl group -COOH. Sulfonic acids, containing the group -SO₂OH, are relatively stronger acids. The relative stability of the conjugate base of the acid determines its acidity. Other groups can also confer acidity, usually weakly: -OH, -SH, the enol group, and the phenol group. In biological systems organic compounds containing only these groups are not generally referred to as organic acids.

A few common examples include:

• Lactic acid
• Acetic acid
• Formic acid
• Citric acid
• Oxalic acid
• Uric acid

Characteristics

Generally, organic acids are weak acids and do not dissociate completely in water, whereas the strong mineral acids do. Lower molecular weight organic acids such as formic and lactic acids are miscible in water, but higher molecular weight organic acids such as benzoic acid are insoluble in molecular (neutral) form.

On the other hand, most organic acids are very soluble in organic solvents. p-Toluenesulfonic acid is a comparatively strong acid used in organic chemistry often because it is able to dissolve in the organic reaction solvent.

Exceptions to these solubility characteristics exist in the presence of other substituents which affect the polarity of the compound.

Applications

Simple organic acids like formic or acetic acids, are used for oil and gas well stimulation treatments. These organic acids are much less reactive with metals than are strong mineral acids like hydrochloric acid (HCl) or mixtures of HCl and hydrofluoric acid (HF). For this reason, organic acids are used at high temperatures or when long contact times between acid and pipe are needed.

The conjugate bases of organic acids such as citrate and lactate are often used in biologically-compatible buffer solutions.

Citric and oxalic acids are used as rust removal. As acids, they can dissolve the iron oxides, but without damaging the base metal like stronger mineral acids. In the dissociated form, they may be able to chelate the metal ions, helping to speed removal.

Biological systems create many and more complex organic acids such as L-lactic, citric and D-glucuronic acids that contain hydroxyl or carboxyl groups. Human blood and urine contain these plus organic acid degradation products of amino acids, neurotransmitters and intestinal bacterial action on food components. Examples of these categories are alpha-ketoisocaproic, vanilmandelic and D-lactic acids, derived from catabolism of L-leucine and epinephrine (adrenaline) by human tissues and catabolism of dietary carbohydrate by intestinal bacteria, respectively.

The general structure of a few weak organic acids. From left to right: phenol, enol, alcohol, thiol. The acidic hydrogen in each molecule is colored red.

The general structure of a few strong organic acids. From left to right: carboxylic acid, sulfonic acid. The acidic hydrogen in each molecule is colored red.

Application in food

Organic acids are used in food preservation because of their effects bacteria. The key basic 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 spp.

Upon passive diffusion of organic acids into the bacteria, where the pH is near of above neutrality, the acids will dissociate and lower the bacteria internal pH, leading to situations that will impair or stop the growth of bacteria. On the other hand, the anionic part of the organic acids that cannot escape the bacteria in its dissociated form will accumulate within the bacteria and disrupt many metabolic functions and lead to osmotic pressure increase, incompatible with the survival of the bacteria.

It has been well demonstrated that the state of the organic acids (undissociated or dissociated) is extremely important to define their capacity to inhibit the growth of bacteria, compared to undissociated acids.

Lactic acid and its salts sodium lactate and potassium lactate are widely used as antimicrobials in food products, particularly meat and poultry such as ham and sausages.^[1]

Application in nutrition and animal feeds

Organic acids have been used successfully in pig production for more than 25 years. Although less research has been done in poultry, organic acids have also been found to be effective in poultry production.

Organic acids (C1-C7) are widely distributed in nature as normal constituents of plants or animal tissues. They are also formed through microbial fermentation of carbohydrates mainly in the large intestine. They are sometimes found in their sodium, potassium or calcium salts.

Logically, organic acids added to feeds should be protected to avoid their dissociation in the crop and in the intestine (high pH segments) and reach far into the GIT, where the bulk of the bacteria population is located.

From the use of organic acids in poultry and pigs one can expect an improvement in performance similar or better than the antibiotic growth promoters, without the public health concern, a preventive effect on the intestinal problems like necrotic enteritis in chickens and Escherichia coli infection in young pigs. Also one can expect a reduction of the carrier state for Salmonella spp. and Campylobacter spp.

Sources

• Dibner J.J., P. Butin. 2002 - Use of organic acids as a model to study the impact of gut microflora on nutrition and metabolism. J. Appl. Poultry Res. 11:453-463
• Patanen K.H., Z. Mroz. 1999 – Organic acids for preservation. Disinfection, sterilization & preservation (S.S. Block editor) Lea Febiger, Philadelphia PA.
• Brul S., P. Coote. 1999 – Preservative agents in foods, mode of action and microbial resistance mechnismes. Intl. J. Food Microbiology 50:1-17

See also

• Acid-base extraction

References