Citric Acid
Citric acid is an organic acid that is found naturally in plants and animal tissues and fluids. It is a white, crystalline material that is soluble in water. It is a carboxylic acid known by the chemical name beta-hydroxytricarballylic acid. It has a melting point of 307 °F (153 °C), a density of 1.542, and a slightly sour taste.
Citric acid has been known since the 1200's. At this time Vincentius Bellovacensis, an alchemist, identified it as an acid component of lemon and lime juices. In 1784, Carl Wilhelm Scheele was able to isolate this acid component and create crystalline citric acid. Later in 1834, Justus von Liebig found that citric acid is a hydroxy tribasic acid. The basic process for producing citric acid on an industrial scale was developed in 1893. At this time, scientists discovered that citric acid was one of the byproducts fungi growing on a sugar solution.
In nature, citric acid is found as an intermediate product of metabolism. It is in the citric acid cycle, or Krebs cycle, where food molecules are broken down to carbon dioxide, water and energy by enzymes. In the citric acid cycle, a food molecule is first broken down into acetyl groups which contain two carbon atoms, an oxygen atom and three hydrogen atoms. The molecule then combines with oxaloacetate to form citric acid. Over the next steps in the cycle, the citric acid molecule is chemically modified. These modifications results in the release of energy which is transferred to adenosine triphosphate (ATP), the main energy storage molecule in the body.
Citric acid is typically produced on an industrial scale by fermentation of crude sugar solutions of molasses with Aspergillus niger mold. It may also be extracted from citrus fruits such as lemons, oranges and grapefruits. This is done by the addition of lime to fruit juices. This results in a precipitate of calcium citrate that is reacted with sulfuric acid to produce citric acid.
Citric acid has a variety of uses. Since it is a flavor enhancer, it is added to foods, confectionaries, and beverages to provide a tart taste. For a similar reason, it is used in medicines. It can also be used to adjust the pH in different systems such as cosmetic formulations and foods, and it is an antioxidant used in processing of cheese. Citric acid is also used in the production of other compounds. For example, it is used to manufacture alkyd resins. Additionally, it is included as a plasticizer and a foam inhibitor in other polymeric systems. It is also used as a chelating agent which can be used to remove metal contaminants from a variety of systems. Finally, citric acid is used in textile printing, papermaking, electroplating and in special inks.
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