OVERVIEW
Citric acid (SIT-rik AS-id) is also known as 2-hydroxy-1,2,3-propanetricarboxylic acid and β-hydroxytricarballylic acid. It is a common constituent of plant and animal tissues. Its presence is especially noticeable in citrus fruits, such as lemons, limes, oranges, tangerines, grapefruits, and kumquat, which get their name from the acid. In pure form, citric acid is a colorless, translucent, odorless crystalline or powdery material with a pleasantly acidic taste. It frequently occurs as the monohydrate, with a single molecule of water associated with each citric acid molecule. The formula for the monohydrate is HOOCCH2C(OH)(COOH)CH2COOH·H2O. The monohydrate is efflorescent, meaning that it tends to lose its water of hydration when exposed to the air.
KEY FACTS
OTHER NAMES:
See Overview.
FORMULA:
HOOCCH2C(OH) (COOH)CH2COOH
ELEMENTS:
Carbon, hydrogen, oxygen
COMPOUND TYPE:
Carboxylic acid (organic)
STATE:
Solid
MOLECULAR WEIGHT:
192.12 g/mol
MELTING POINT:
153°C (307°F)
BOILING POINT:
Not applicable; decomposes above 175°C (347°F)
SOLUBILITY:
Soluble in water, alcohol, ether, and other organic solvents
Citric acid plays an important role in metabolism, the set of chemical reactions that occur when cells break down fats, carbohydrates, and other compounds to produce energy and compounds needed to build new cells and tissues. In fact, the series of reactions by which carbohydrates are converted to energy is generally known as the citric acid cycle because of the fundamental role played by the compound in those reactions. The citric acid cycle is also known as the Krebs cycle, after the German-British biochemist Sir Hans Adolf Krebs (1900–1981), who discovered the series of reactions in 1937. Citric acid also acts as an antioxidant, a substance that rid the body of molecules called free radicals that can damage healthy cells, promote cancer, and bring about ageing.
The discovery of citric acid is often credited to the Arab alchemist Jabir Ibn Hayyan (721-815), also known by his Latin name of Geber. Geber described a substance with all the properties that we equate with citric acid today, but he knew nothing about its chemical structure. The first person to isolate the compound as a pure substance was the Swedish chemist Karl Wilhelm Scheele (1742–1786), who obtained citric acid from the juice of lemons. By the mid-nineteenth century, citric acid was being produced commercially in Italy from lemons and other citrus fruits.
An important step in the commercial manufacture of citric acid occurred in 1892 when the German microbiologist Carl Wehmer (dates not available) found that citric acid could be produced by the penicillin mold. Wehmer's discovery paved the way for large-scale industrial production of citric acid. In 1917, the American chemist James Currie (dates not available) made another important breakthrough in the synthesis of citric acid. While studying the process of fermentation in cheese making, he discovered that the mold Aspergillus niger is able to convert sugar to citric acid. After Currie joined the pharmaceutical company Pfizerin 1917, he developed a process called SUCIAC—sugar under conversion into citric acid—by which the compound could be made in mass quantities. That process eventually became the primary method by which citric acid is produced today.
Interesting Facts
- The addition of citric acid to candy gives the product a "super sour" taste.
HOW IT IS MADE
At one time, citric acid was obtained primarily from citrus fruits, such as lemons and limes. Today, it is produced synthetically using the Aspergillus niger mold, as described above. The citric acid produced in this reaction is purified by crystallization. The anhydrous form crystallizes from hot water, and the monohydrate form from cold water.
COMMON USES AND POTENTIAL HAZARDS
Citric acid is added to foods, drinks, and medicines to make them more acidic. Increasing the acidity of these products not only gives them a tart taste, but also prevents the growth of bacteria. Citric acid is also used to preserve the flavors of canned fruits and vegetables and to maintain the proper acidic level of jams and jellies that will help them gel.
Words to Know
- ALCHEMY
- An ancient field of study from which the modern science of chemistry evolved.
- FERMENTATION
- Chemical process by which yeasts or molds break down sugar into alcohol and carbon dioxide.
- METABOLISM
- Process including all of the chemical reactions that occur in cells by which fats, carbohydrates, and other compounds are broken down to produce energy and the compounds needed to build new cells and tissues.
- MORDANT
- Substance used in dyeing and printing that reacts chemically with both a dye and the material being dyed to help hold the dye permanently to the material.
- POLYMER
- Compound consisting of very large molecules made of one or two small repeated units called monomers.
In addition to its use as a food additive, citric acid has a number of other commercial and industrial applications, including the following:
- As a sequestering agent to remove small amounts of metals in a solution. A sequestering agent is a substance that surrounds and captures some other substance (such as metals) and removes them from a solution;
- In the cleaning and polishing of stainless steel and other metals;
- As a mordant in the dyeing of cloth;
- In the production of certain kinds of polymers;
- For the removal of sulfur dioxide from the waste gases produced at smelters; and
- As a builder in detergents, a substance that increases the detergent's cleaning efficiency, usually by maintaining the proper acidity or softening the water in which the detergent acts.
FOR FURTHER INFORMATION
"Citric Acid." Jungbunzlauer. http://www.jungbunzlauer.com/products/product_1.html (accessed on October 3, 2005).
"Citric Acid, Anhydrous." J. T. Baker. http://www.jtbaker.com/msds/englishhtml/c4735.htm (accessed on October 3, 2005).
Kuntz, Lynn A. "Acid Basics." Food Product Design (May 1993). Also available online at http://www.foodproductdesign.com/toolbar_library.html (accessed on October 3, 2005).
