Citric acid

What is citric acid?
The citric acid belongs to the fruit acids as so-called carboxylic acid. According to chemical nomenclature, the scientific name includes 2-hydroxypropane-1,2,3-tricarboxylic acid. In the INCI database for cosmetic products, citric acid is also listed as CITRIC ACID INCI. It is a colorless crystalline substance which is readily soluble in water and which can be both anhydrous and hydrous (monohydrate). The anhydrous molecule has the molecular formula C6H8O7, wherein the monohydrate (C6H8O7 · H2O) still contains a water molecule. As acid, the citric acid can also form salts. These are called citrates.

The citric acid was first isolated in 1784 by the German Swedish chemist and pharmacist Carl Wilhelm Scheele from the juice of the lemon, which contains between five and seven percent citric acid. However, citric acid occurs more or less in all organisms. There it is an important intermediate of the so-called citric acid cycle. The citric acid cycle is a key component of the energy metabolism of all oxygen-breathing organisms. However, in addition to lemons, citric acid is also found in high concentrations in other fruits such as apples, pears or berries.

Originally, citric acid was extracted mainly from citrus fruits using ammonia and calcium chloride. First, the slightly soluble ammonium citrate was added with a calcium chloride solution, whereby calcium citrate precipitated. By adding sulfuric acid, citric acid could be liberated under the simultaneous precipitation of calcium sulfate (gypsum).

Although citric acid is now mostly produced industrially, it is still a bio-based chemical. Their industrial production is based on the fermentation of renewable raw materials such as corn or molasses. For fermentation Aspergillus Niger strains are used. Since there is no synthetic process for making the citric acid, it is always nature-based.

In the biotechnological manufacturing process, three conditions must be fulfilled:

• High glucose and oxygen levels are required.
• The pH must be very low to prevent further metabolism of the citric acid.
• Divalent iron ions must be trapped by complexing agents such as potassium hexacyanidoferrate (III) as these promote citric acid degrading enzymes.

Physical and chemical properties
The use of citric acid is determined by its physical and chemical properties.

The citric acid consists of white rhombic crystals that are very soluble in water. Furthermore, it has a slightly sour taste.

Due to its chemical structure, citric acid belongs to the tricarboxylic acids. That is, it has three carboxy groups (-COOH). At each carboxy group the typical reactions of organic acids can take place. These include salt formation, substitution reactions, esterification, amide formation, anhydride formation or halogenation.

Uses of citric acid
The use of citric acid is determined by its physical and chemical properties.

For example, citric acid is used as an acidulant in food. Furthermore, it is also used for preservation. Because it can reverse the coagulation of proteins, it also finds application for the preparation of stable suspensions. In blood preserves it prevents the blood clotting, so that it is also used there.

Another large field of application of citric acid are detergents. There it mainly acts as a lime remover or plasticizer. Lime consists of calcium carbonate. As a decalcifier, the citric acid converts carbonic acid into carbonic acid, which in turn decomposes into carbon dioxide and water. As citric acid is also a complexing agent for calcium, it can also act as softener or alternative fabric softener in certain detergents and cleaners.