The citric acid is an organic compound that consists of a weak acid whose chemical formula is C6H8OR7. As its name indicates, one of its main natural sources are citrus fruits, and it also derives from the Latin word 'citrus', which means bitter..
Not only is it a weak acid, it is also polyrotic; 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 free themselves towards their environment..
Therefore, its structural formula is better defined as C3H5O (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..
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It is found in low proportions in all plants and animals, and it is a metabolite of living beings. It is an intermediate compound of aerobic metabolism present in the tricarboxylic acid cycle or 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 obtained synthetically 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.
In the upper image, the structure of citric acid is represented with a spheres and bars model. If you look closely, you can find the skeleton of just three carbons: propane.
The carbon atom in the center is linked to a -OH group, 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 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 the -COOH and -OH.
As can be seen, 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 monohydrate 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 formation in hot water, followed by complete evaporation..
210.14 g / mol.
Colorless and odorless acid crystals.
Sour and bitter.
153 ° C.
175 ° C.
1.66 g / mL.
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.
-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 hardly slightly acidic, so it dissociates little.
At extreme temperatures or above 175 ° C it decomposes releasing COtwo and water. Therefore, the liquid does not reach a significant boil as it decomposes first..
As you lose H+, other cations take their 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 the citric acid, 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 form complexes 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 citrate esters: C3H5OR (COOR)3.
The diversity is very great, since not all H must necessarily be replaced by R, but also by cations.
Citric acid can be produced naturally and commercially obtained by the fermentation of carbohydrates. Its production has also been carried out synthetically through chemical processes that are not kept very much in force today..
Several biotechnological processes have been used for its production, since this compound is in high demand worldwide.
-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, whose function is to protonate the citrate to its original acid form..
-Citric acid has also 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..
In the body, citric acid occurs 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 reactions of the catabolism of fatty acids, carbohydrates, among other substrates, in the presence of Otwo acetyl-CoA is produced. 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 of aerobic metabolism has not been profitable or satisfactory either. Only under conditions of organic imbalance can the concentration of this metabolite be increased, which is not viable for microorganisms.
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 mass 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 production processes of citric acid by submerged fermentation, which occurs under anaerobic conditions, have been optimal..
Some mushrooms like Aspergillus niger, Saccahromicopsis sp, and bacteria like Bacillus licheniformis, have allowed to obtain a high yield with this type of fermentation.
Mushrooms like Aspergillus niger or candida sp, They 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..
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..
-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. It is also used in the preparation of wines, beers, among other beverages..
-In addition to adding an acid taste, it inactivates trace elements giving protection to ascorbic acid or vitamin C. It also acts as an emulsifier in ice cream and cheeses. 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 the fatty components) that this type of nutrients may have..
-Citric acid is also 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 in a way that 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.
-By acidifying the medium of the absorption process at the intestinal level, citric acid 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..
-Citric acid is used as a chelating agent for metal ions in toiletries and cosmetics..
-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.
-Adds to printing inks.
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 swallowed..
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..
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