is a weak
Organic Acid found in
Citrus Fruit s. It is a good, natural
Preservative and is also used to add an acidic (sour) taste to foods and
Soft Drink s. In
Biochemistry , it is important as an intermediate in the
Citric Acid Cycle and therefore occurs in the
Metabolism of almost all living things. It also serves as an environmentally benign cleaning agent and acts as an
Antioxidant .
Citric acid exists in a variety of fruits and vegetables, but it is most concentrated in
Lemon s and
Lime s, where it can comprise as much as 8% of the dry weight of the fruit.
Citric acid's
Chemical Formula is
C6 H8 O7 (structure shown at right). Its structure is reflected in its
IUPAC name .
The physical properties of citric acid are summarized in the table at right. The acidity of citric acid results from the three
Carboxyl Group s COOH each of which can lose a
Proton in solution. If this happens, the resulting ion is the . Citrates make excellent
Buffer s for controlling the
PH of acidic solutions.
Citrate ions form salts called with many metal ions. An important one is
Calcium Citrate or "sour salt", which is commonly used in the preservation and flavoring of food. Additionally, citrates can
Chelate metal ions, which gives them use as preservatives and water softeners.
At room temperature, citric acid is a white crystalline powder. It can exist either in an ''anhydrous'' (water-free) form, or as a
Monohydrate that contains one water molecule for every molecule of citric acid. The anhydrous form crystallizes from hot water, while the monohydrate forms when citric acid is crystallized from cold water. The monohydrate can be converted to the anhydrous form by heating it above 74 °C.
Chemically, citric acid shares the properties of other
Carboxylic Acid s. When heated above 175°C, it decomposes through the loss of
Carbon Dioxide and
Water .
The discovery of citric acid has been credited to the
8th Century Muslim Alchemist Jabir Ibn Hayyan (Geber). Medieval scholars in Europe were aware of the acidic nature of lemon and lime juices; such knowledge is recorded in the 13th century
Encyclopedia ''
Speculum Majus '' (''The Great Mirror''), compiled by
Vincent Of Beauvais . Citric acid was first isolated in
1784 by the
Swedish chemist
Carl Wilhelm Scheele , who crystallized it from lemon juice. Industrial-scale citric acid production began in 1860, based on the
Italian citrus fruit industry.
In
1893 , C. Wehmer discovered that ''
Penicillium ''
Mold could produce citric acid from
Sugar . However, microbial production of citric acid did not become industrially important until
World War I disrupted Italian citrus exports. In
1917 , the
American food chemist James Currie discovered that certain strains of the mold ''
Aspergillus Niger '' could be efficient citric acid producers, and
Pfizer began industrial-level production using this technique two years later.
In this production technique, which is still the major industrial route to citric acid used today, cultures of ''Aspergillus niger'' are fed on
Sucrose to produce citric acid. After the mold is filtered out of the resulting solution, citric acid is isolated by precipitating it with lime (
Calcium Hydroxide ) to yield calcium citrate salt, from which citric acid is regenerated by treatment with
Sulfuric Acid .
Alternatively, citric acid is sometimes isolated from the fermentation broth by
Extraction with a
Hydrocarbon solution of the
Organic Base Trilaurylamine , followed by re-extraction from the organic solution by water.
Most citric acid is used as a
Flavouring and
Preservative in
Food and
Beverage s, especially
Soft Drink s; it is denoted by
E Number E330. Citrate salts of various
Metal s are used to deliver those minerals in a biologically available form in many
Dietary Supplement s. The buffering properties of citrates are used to control
PH in household cleaners and
Pharmaceutical s.
Citric acid's ability to
Chelate metals makes it useful in
Soap s and laundry
Detergent s. By
Chelating the metals in
Hard Water , it lets these cleaners produce foam and work better without need for water softening. Similarly, citric acid is used to regenerate the
Ion Exchange materials used in
Water Softener s by stripping off the accumulated metal ions as citrate complexes.
It is used in the biotechnology and pharmaceutical industry to
Passivate high purity process piping in lieu of using
Nitric Acid , since nitric acid is a hazardous disposal issue once it is used for this purpose, while citric acid is not.
In the .
Citric acid is one of the chemicals required for the synthesis of
HMTD ; a highly heat, friction, and shock sensitive explosive similar to
Acetone Peroxide (also known as "Mother of Satan"). Due to this the purchase of large quantities of citric acid may be seen by some governments as an indicator of potential terrorist activity.
Citric acid can also be added to ice cream to keep fat globules separate.
Citric acid can be added to recipes in place of fresh lemon juice.
Citric acid is recognized as safe for use in food by all major national and international food regulatory agencies. It is naturally present in almost all forms of life, and excess citric acid is readily metabolized and eliminated from the body.
Contact with dry citric acid or with concentrated solutions can result in skin and eye irritation, so protective clothing should be worn when handling these materials.
