Citric acid is a weak organic acid. It is a natural preservative and is also used to add an acidic, or sour, taste to foods and soft drinks. 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, most notably citrus fruits. Lemons and limes have particularly high concentrations of the acid; it can constitute as much as 8% of the dry weight of these fruits (1.44 and 1.38 grams per ounce of the juices, respectively^[1]). The concentrations of citric acid in citrus fruits range from .005 mol/L for oranges and grapefruits to .030 mol/L in lemons and limes. These values will vary depending on the circumstances in which the fruit was grown.

Properties

Citric acid crystal under polarized light, enlarged 200x

In chemical structure, citric acid shares the properties of other carboxylic acids. When heated above 175 °C, it decomposes through the loss of carbon dioxide and water.

Measurement

Citric acid has been used as an additive to soft drinks, beer, and seltzer, and occurs naturally in many juices. This causes a problem in measurement because the standard measuring technique for sugar is refractive index. The refractive index of sugar and citric acid is almost identical. For soft drinks and orange juice the best measure of sweetness is the sugar/acid ratio. Recently, the use of infrared sensors has allowed measurement of both Brix (sugar content) and acidity by detecting sugars and citric acid through their characteristic molecular vibrations; this gives an accurate assessment of a drink's sweetness.

History

Lemons, oranges, and other citrus fruits contain high concentrations of citric acid

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, followed by Citrique Belge in 1929.

Production

Citric acid monohydrate

Krebs cycle

Citric acid is one of a series of compounds involved in the physiological oxidation of fats, proteins, and carbohydrates to carbon dioxide and water.

This series of chemical reactions is central to nearly all metabolic reactions, and is the source of two-thirds of the food-derived energy in higher organisms. It was discovered by Sir Hans Adolf Krebs. Krebs received the 1953 Nobel Prize in Physiology or Medicine for the discovery. The series of reactions is known by various names, including the citric acid cycle, the Krebs cycle, and the tricarboxylic acid cycle (or TCA cycle).

Uses

In 2007 world wide annual production stands at approximately 1700.000 MT/year. More than 50% of this volume is being produced in China. More than 50% is being used as acidulent in beverages and some 20% in other food applications. 20% is being used for detergent applications and 10% for other non-food related applications like cosmetics and in the chemical industry.

Food additive

As a food additive, citric acid is used as a flavoring and preservative in food and beverages, especially soft drinks. It is denoted by E number E330. Citrate salts of various metals are used to deliver those minerals in a biologically available form in many dietary supplements. The buffering properties of citrates are used to control pH in household cleaners and pharmaceuticals. In the United States the purity requirements for citric acid as a food additive is defined by the Food Chemical Codex (FCC), which is published by the United States Pharmacopoeia (USP).

Water softening

Citric acid's ability to chelate metals makes it useful in soaps and laundry detergents. By chelating the metals in hard water, it lets these cleaners produce foam and work better without need for water softening. In a similar manner, citric acid is used to regenerate the ion exchange materials used in water softeners by stripping off the accumulated metal ions as citrate complexes.

Others

Citric acid is used in biotechnology and the pharmaceutical industry to passivate high-purity process piping (in lieu of using nitric acid). Nitric acid is considered hazardous to dispose once used for this purpose, while citric acid is not.

Citric acid is the active ingredient in some bathroom and kitchen cleaning solutions. A solution with a 6% concentration of citric acid will remove hard water stains from glass without scrubbing. In industry it is used to dissolve rust from steel.^[9]

Citric acid is commonly used as a buffer to increase the solubility of brown heroin. Single-use citric acid sachets have been used as an inducement to get heroin users to exchange their dirty needles for clean needles in an attempt to decrease the spread of AIDS and hepatitis^[10]. Other acidifiers used for brown heroin are ascorbic acid, acetic acid, and lactic acid; in their absence, a drug user will often substitute lemon juice or vinegar.

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. For this reason, purchases of large quantities of citric acid may rouse suspicion of potential terrorist activity.

Citric acid can be added to ice cream to keep fat globules separate, and can be added to recipes in place of fresh lemon juice as well. Citric acid is used along with sodium bicarbonate in a wide range of effervescent formulae, both for ingestion (e.g., powders and tablets) and for personal care (e.g., bath salts, bath bombs, and cleaning of grease).

Citric acid is commonly employed in wine production as a substitute or improver where fruits containing little or no natural acidity are used. It is mostly used for inexpensive wines due to its low cost of production.^[11]

When applied to hair, citric acid opens up the outer layer, also known as the cuticle. While the cuticle is open, it allows for a deeper penetration into the hair shaft. It can be used in shampoo to wash out wax and coloring from the hair. It is notably used in the product "Sun-in" for bleaching, but is generally not recommended due to the amount of damage it causes.

Citric acid is also used as a stop bath in photography. The developer is normally alkaline, so a mild acid will neutralize it, increasing the effectiveness of the stop bath when compared to plain water.^[12]

Citric acid is used as one of the active ingredients in the production of anti-viral tissues.^[13]

Safety

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.

Excessive consumption is capable of eroding the tooth enamel.^[14]

Close contact to the eyes can cause a burning sensation, and may cause loss of sight.

Sometimes a high concentration of citric acid can damage hair, since it opens hair cuticles. It can strip the hair of needed minerals and bleach it.

The leaflet of Villejuif

The leaflet of Villejuif (also known as the flyer of Villejuif or the list of Villejuif) was a scientifically inaccurate rumour, passed via a leaflet or flyer, that caused mass panic in Europe in the 1980s as it included common unharmful chemical substances such as citric acid (E330) in a list of 10 dangerous carcinogens.

See also

• Citric acid intolerance
• Citric acid cycle
• The closely related acids isocitric acid, aconitic acid, and propane-1,2,3-tricarboxylic acid (tricarballylic acid, carballylic acid)
• Acids in wine

References

• Citric Acid MSDS Sheet

External links

• NextBio Citric Acid Entry
• Citric acid analysis - free spreadsheet for titration of acids and pH calculation
• Applications of Citric Acid
• MSDS sheet for Citric Acid

ar:??? ????? bg:???????? ???????? cs:Kyselina citronová da:Citronsyre de:Citronensäure et:Sidrunhape el:??????? ??? es:Ácido cítrico fa:???? ?????? fr:Acide citrique gl:Ácido cítrico ko:???? hr:Limunska kiselina id:Asam sitrat it:Acido citrico he:????? ?????? lv:Citronsk?be lb:Zitrounesaier hu:Citromsav nl:Citroenzuur ja:???? no:Sitronsyre nn:Sitronsyre pl:Kwas cytrynowy pt:Ácido cítrico ro:Acid citric ru:???????? ??????? simple:Citric acid sl:Citronska kislina fi:Sitruunahappo sv:Citronsyra tr:Sitrik asit uk:??????? ??????? zh:???