Citric Acid

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).

Citric Acid

Citric acid is a weak organic (carbon-based) acid found in nearly all citrus fruits, particularly lemons, limes, and grapefruits. It is widely used as a flavoring agent, natural preservative, and cleaning agent. It is also added to make foods and soft drinks taste acidic, or sour. The structure of citric acid is shown below. The COOH group (consisting of carbon, oxygen, and hydrogen) is a carboxylic acid group, so citric acid is a tricarboxylic acid, possessing three of these groups. Its density is about 1, 665 kg/m³ and is soluble in water at around 133 g/100 ml (at 68°F [20°C]). Citric acid has a melting point of about 307°F (153°C) and a boiling point of approximately 347°F (175°C).

It is produced commercially by the fermentation of sugar by several species of mold. As a flavoring agent, it can help produce both a tartness [caused by the production of hydrogen ions (H⁺)] and sweetness (the result of the manner in which citric acid molecules are structured (fit) into sweet receptors on tongues). Receptors are protein molecules that recognize specific other molecules.

Citric acid helps to provide the fizz in remedies such as Alka-Seltzer^®. The fizz comes from the production of carbon dioxide gas, which is created when sodium bicarbonate (baking soda) reacts with acids. The source of the acid in this case is citric acid, which also helps to provide a more pleasant taste.

In addition, citric acid is used in the production of hair rinses and low pH (highly acidic) or slightly acidic shampoos and toothpaste. As a preservative, citric acid helps to bind (or sequester) metal ions that may get into food via machinery used in processing. Many metals ions speed up the degradation of fats. Citric acid prevents the metal ions from being involved in a reaction with fats in foods and allows other preservatives to function much more effectively.

Citric acid is also an intermediate in metabolic processes in all mammalian cells. One of the most important of these metabolic pathways is called the citric acid cycle (it is also called the Krebs cycle, after the man who first determined the role of this series of reactions). Some variants of citric acid containing fluorine have been used as rodent poisons.

See also Metabolism.

Citric acid

Citric acid is an organic (carbon based) acid found in nearly all citrus fruits , particularly lemons, limes, and grapefruits. It is widely used as a flavoring agent, preservative, and cleaning agent. The structure of citric acid is shown below. The COOH group is a carboxylic acid group, so citric acid is a tricarboxylic acid, possessing three of these groups.

Citric acid is produced commercially by the fermentation of sugar by several species of mold . As a flavoring agent, it can help produce both a tartness [caused by the production of hydrogen ions (H+)] and sweetness (the result of the manner in which citric acid molecules "fit" into "sweet" receptors on our tongues). Receptors are protein molecules that recognize specific other molecules.

Citric acid helps to provide the "fizz" in remedies such as Alka-Seltzer trademark. The fizz comes from the production of carbon dioxide gas which is created when sodium bicarbonate (baking soda) reacts with acids. The source of the acid in this case is citric acid, which also helps to provide a more pleasant taste .

Citric acid is also used in the production of hair rinses and low pH (highly acidic) or slightly acidic shampoos and toothpaste. As a preservative, citric acid helps to bind (or sequester) metal ions that may get into food via machinery used in processing. Many metals ions speed up the degradation of fats. Citric acid prevents the metal ions from being involved in a reaction with fats in foods and allows other preservatives to function much more effectively. Citric acid is also an intermediate in metabolic processes in all mammalian cells. One of the most important of these metabolic pathways is called the citric acid cycle (it is also called the Krebs cycle , after the man who first determined the role of this series of reactions). Some variants of citric acid containing fluorine have been used as rodent poisons.

See also Metabolism.

citric acid An organic acid (chemically a tricarboxylic acid) which is widely distributed in plant and animal tissues; it is an important metabolic intermediate, and yields 2.47 kcal (10.9 kJ)/g. It is used as a flavouring and acidifying agent, and its salts (citrates) are used as acidity regulators. Commercially it is either prepared by the fermentation of sugars by the mould Aspergillus niger or extracted from citrus fruits (lemon juice contains 5–8% citric acid).

citric acid A white crystalline hydroxy carboxylic acid, HOOCCH₂C(OH)(COOH)CH₂COOH. It is present in citrus fruits and is an intermediate in the Krebs cycle in plant and animal cells.