CITRIC ACID: STRUCTURE, PROPERTIES, PRODUCTION AND USES - CHEMISTRY - 2024

The citric acid is an organic compound consisting of a weak acid whose chemical formula is C ₆ H ₈ O ₇. As its name indicates, one of its main natural sources is citrus fruits, and it is also derived from the Latin word 'citrus', which means bitter.

Not only is it a weak acid, but it is also polyprotic; that is, it can release more than one hydrogen ion, H ⁺. It is precisely a tricarboxylic acid, so it has three –COOH groups that donate H ⁺ ions. Each of them has their own tendency to release themselves into their environment.

Source: Maxpixel

Therefore, its structural formula is better defined as C ₃ H ₅ O (COOH) ₃. This is the chemical reason for its contribution to the characteristic flavor of, for example, orange segments. Although it comes from the fruits, its crystals were not isolated until 1784 from a lemon juice in England.

It makes up about 8% by mass of some citrus fruits, such as lemons and grapefruits. It can also be found in peppers, tomatoes, artichokes, and other foods.

Where is citric acid found?

It is found in low proportions in all plants and animals, and is a metabolite of living beings. It is an intermediate compound of aerobic metabolism present in the tricarboxylic acid cycle or the citric acid cycle. In biology or biochemistry this cycle is also known as the Krebs cycle, an amphibolic pathway of metabolism.

In addition to being naturally found in plants and animals, this acid is synthetically obtained on a large scale through fermentation.

It is widely used in the food industry, in pharmaceuticals and chemicals, and it behaves as a natural preservative. It and its derivatives are massively manufactured at an industrial level to flavor solid and liquid foods.

Finds use as an additive in varieties of skin beauty products; it is also used as a chelating, acidifying and antioxidant agent. However, its use in high or pure concentrations is not recommended; as it can cause irritation, allergies and even cancer.

Citric acid structure

Source: Benjah-bmm27, from Wikimedia Commons

In the upper image the structure of citric acid is represented with a model of spheres and bars. If you look closely, you can find the skeleton of just three carbons: propane.

The carbon atom in the center is linked to a group –OH, which in the presence of carboxyl groups, –COOH, adopts the terminology 'hydroxy'. The three –COOH groups are easily recognizable at the left and right ends, and at the top of the structure; it is from these where the H ⁺ are released.

On the other hand, the –OH group is also capable of losing an acidic proton, so that in total there would not be three H ⁺, but four. However, the latter requires a considerably strong base, and consequently, its contribution to the acidity characteristic of citric acid is much lower compared to that of the -COOH groups.

From all the above it follows that citric acid can also be called: 2-hydroxy-1,2,3-tricarboxylic propane.

There is a –OH group in C-2, which is adjacent to the –COOH group (look at the upper center of the structure). Because of this, citric acid also falls under the classification of alpha-hydroxy acids; where alpha means 'adjacent', that is, there is only one carbon atom separating –COOH and –OH.

Intermolecular interactions

As can be appreciated, the citric acid structure has a high capacity to donate and accept hydrogen bonds. This makes it very related to water, and also the reason why it forms a monohydrated solid, rhombohedral crystals, very easily.

These hydrogen bonds are also responsible for erecting the colorless monoclinic crystals of citric acid. Anhydrous crystals (without water) can be obtained after forming in hot water, followed by complete evaporation.

Physical and chemical properties

Molecular weight

210.14 g / mol.

Physical appearance

Colorless and odorless acid crystals.

Taste

Acid and bitter.

Melting point

153 ° C.

Boiling point

175 ° C.

Density

1.66 g / mL.

Solubility

It is a highly soluble compound in water. It is also very soluble in other polar solvents such as ethanol and ethyl acetate. In apolar and aromatic solvents such as benzene, toluene, chloroform, and xylene, it is insoluble.

pKa

-3.1

-4.7

-6.4

These are the pKa values ​​for each of the three –COOH groups. Note that the third pKa (6,4) is barely slightly acidic, so it dissociates little.

Decomposition

At extreme temperatures or above 175 ° C it decomposes releasing CO ₂ and water. Therefore, the liquid does not reach a significant boil as it decomposes first.

Derivatives

As it loses H ⁺, other cations take its place but in an ionic way; that is, the negative charges of the –COO ^- groups attract other species of positive charges, such as Na ⁺. The more deprotonated citric acid is, the more cations its derivatives called citrates will have.

An example is sodium citrate, which has a very useful chelating effect as a coagulant. These citrates can therefore complex with metals in solution.

On the other hand, the H ⁺ of the -COOH groups can even be substituted by other covalently linked species, such as R side chains, giving rise to the citrate esters: C ₃ H ₅ O (COOR) ₃.

The diversity is very great, since not all H must necessarily be replaced by R, but also by cations.

Production

Citric acid can be produced naturally and commercially obtained by the fermentation of carbohydrates. Its production has also been carried out synthetically using chemical processes that are not very current today.

Several biotechnological processes have been used for its production, since this compound is in high demand worldwide.

Chemical or synthetic synthesis

-One of these chemical synthesis processes is carried out under high pressure conditions from calcium salts of isocitrate. The juice extracted from citrus fruits is treated with calcium hydroxide, and calcium citrate is obtained.

This salt is then extracted and reacted with a dilute sulfuric acid solution, the function of which is to protonate the citrate to its original acid form.

-Also citric acid has been synthesized from glycerin by replacing its components with a carboxyl group. As just mentioned, these processes are not optimal for large-scale citric acid production.

natural

In the body, citric acid is produced naturally in aerobic metabolism: the tricarboxylic acid cycle. When Acetyl coenzyme A (acetyl-CoA) enters the cycle, it binds with oxaloacetic acid, forming citric acid.

And where does acetyl-CoA come from?

In the catabolism reactions of fatty acids, carbohydrates, among other substrates, acetyl-CoA is produced in the presence of O ₂. This is formed as a product of the beta-oxidation of fatty acids, of the transformation of pyruvate generated in glycolysis.

Citric acid formed in the Krebs cycle or citric acid cycle will be oxidized to alpha-ketoglutaric acid. This process represents an amphibolic oxidation-reduction pathway, from which equivalents are generated that will then produce energy or ATP.

However, the commercial production of citric acid as an intermediate for aerobic metabolism has not been profitable or satisfactory either. Only under conditions of organic imbalance can the concentration of this metabolite be raised, which is not viable for microorganisms.

By fermentation

Microorganisms, such as fungi and bacteria, produce citric acid by fermenting sugars.

The production of citric acid from microbial fermentation has yielded better results than obtaining it by chemical synthesis. Research lines have been developed related to this massive commercial production method, which has offered great economic advantages.

Cultivation techniques at an industrial level have varied over time. Cultures for surface and submerged fermentation have been used. Submerged cultures are those in which microorganisms produce fermentation from substrates contained in liquid media.

The citric acid production processes by submerged fermentation, which occurs under anaerobic conditions, have been optimal.

Some fungi such as Aspergillus niger, Saccahromicopsis sp, and bacteria such as Bacillus licheniformis, have allowed to obtain a high yield with this type of fermentation.

Fungi such as Aspergillus niger or candida sp, produce citric acid as a result of the fermentation of molasses and starch. Cane, corn, and beet sugar, among others, are also used as fermentation substrates.

Applications

Citric acid is widely used in the food industry, in the manufacture of pharmaceutical products. It is also used in countless chemical and biotechnological processes.

In the food industry

-Citric acid is used mainly in the food industry as it gives them a pleasant acid taste. It is very soluble in water, so it is added to drinks, sweets, candies, jellies, and frozen fruits. Likewise, it is used in the preparation of wines, beers, among other beverages.

-In addition to adding an acid flavor, it inactivates trace elements giving protection to ascorbic acid or vitamin C. It also acts as an emulsifier in ice cream and cheeses. It contributes to the inactivation of oxidative enzymes by lowering the pH of food.

-Increases the effectiveness of preservatives added to food. By providing a relatively low pH, it decreases the likelihood of microorganisms surviving in processed foods, thereby increasing their shelf life.

-In fats and oils, citric acid is used to reinforce the synergistic antioxidant effect (of all fatty components) that this type of nutrients may have.

In the pharmaceutical industry

-Also citric acid is widely used as an excipient in the pharmaceutical industry to improve the taste and dissolution of medicines.

-In combination with bicarbonate, citric acid is added to powdered and tablet products so that it acts as an effervescent.

-The salts of citric acid allow its use as an anticoagulant, since it has the ability to chelate calcium. Citric acid is administered in mineral supplements such as citrate salts.

-Citric acid by acidifying the medium of the absorption process at the intestinal level optimizes the uptake of vitamins and some medicines. Its anhydrous form is administered as an adjunct to other drugs in the dissolution of stones.

-It is also used as an acidifier, astringent, as an agent that facilitates the dissolution of the active ingredients of various pharmaceutical products.

In the cosmetic industry and in general

-In toiletries and cosmetics, citric acid is used as a chelating agent for metal ions.

-It is used for cleaning and polishing metals in general, removing the oxide that covers them.

-At low concentrations it serves as an additive in ecological cleaning products, which are benign for the environment and nature.

-It has a wide variety of uses: it is used in photographic reagents, textiles, in leather tanning.

-Added to printing inks.

Toxicity

Reports of its toxicity are associated with a high concentration of citric acid, exposure time, impurities, among other factors.

Citric acid solutions that are diluted do not pose any risk or danger to health. However, pure or concentrated citric acid does pose a safety hazard, and therefore should not be consumed.

Pure or concentrated, it is corrosive and irritant on contact with the skin and mucous membranes of the eyes, nose and throat. May cause allergic skin reactions, and acute toxicity if ingested.

Inhalation of pure citric acid dust can also affect the mucosa of the respiratory tract. Inhalation can cause shortness of breath, allergies, sensitization of the respiratory mucosa, and can even trigger asthma.

Reproductive toxic effects are reported. Citric acid can cause genetic defects, causing mutation in germ cells.

And finally, it is considered dangerous or toxic to the aquatic habitat, and in general concentrated citric acid is corrosive to metals.

References

1. BellChem (April 21, 2015). The Uses of Citric Acid in the Food Industry. Recovered from: bellchem.com
2. Vandenberghe, Luciana P. S, Soccol, Carlos R, Pandey, Ashok, & Lebeault, Jean-Michel. (1999). Microbial production of citric acid. Brazilian Archives of Biology and Technology, 42 (3), 263-276. dx.doi.org/10.1590/S1516-89131999000300001
3. PubChem. (2018). Citric Acid. Recovered from: pubchem.ncbi.nlm.nih.gov
4. Wikipedia. (2018). Citric Acid. Recovered from: en.wikipedia.org
5. Whitten, K., Davis, R., Peck M. and Stanley, G. (2008). Chemistry. (8 ^ava. Ed). CENGAGE Learning: Mexico.
6. Berovic, M. and Legisa, M. (2007). Citric Acid Production. Biotechology annual review. Recovered from: researchgate.net