Natural occurrence and industrial production of citric acid
- Citric acid is found in high concentrations in citrus fruits such as lemons, oranges, and limes.
- Lemons and limes can contain as much as 8% of citric acid in their juices.
- The concentration of citric acid varies within species and depends on the cultivar and growing conditions.
- Citric acid was first isolated from lemon juice in 1784 by Carl Wilhelm Scheele.
- Industrial-scale production of citric acid began in 1890, using the Italian citrus fruit industry as a base.
- Microbial production of citric acid became important during World War I when Italian citrus exports were disrupted.
- Certain strains of the mold Aspergillus niger were found to be efficient citric acid producers.
- The major industrial route for citric acid production involves feeding A. niger cultures with a sugar-containing medium.
- The source of sugar can be corn steep liquor, molasses, or hydrolyzed corn starch.
- After filtering out the mold, citric acid is isolated by precipitating it with calcium hydroxide and then regenerating it with sulfuric acid.

Chemical characteristics of citric acid
- Citric acid can exist in an anhydrous form or as a monohydrate.
- The anhydrous form crystallizes from hot water, while the monohydrate forms when citric acid crystallizes from cold water.
- Citric acid dissolves in absolute ethanol at a ratio of 76 parts per 100 parts of ethanol.
- It decomposes with the loss of carbon dioxide above approximately 175°C.
- Citric acid is a tribasic acid with pK values of 3.128, 4.761, and 6.396 at 25°C.

Global production and usage of citric acid
- Global production of citric acid exceeded 2,000,000 tons in 2018.
- More than 50% of this volume was produced in China.
- Over 50% of citric acid is used as an acidity regulator in beverages.
- Approximately 20% is used in other food applications.
- Around 20% is used for detergent applications, while the remaining 10% is used in cosmetics, pharmaceuticals, and the chemical industry.

Biochemistry - Citric acid cycle
- Citrate is an intermediate in the citric acid cycle, also known as the TCA cycle or Krebs cycle.
- The cycle is a central metabolic pathway for animals, plants, and bacteria.
- Citrate is formed by the condensation of oxaloacetate with acetyl CoA, catalyzed by citrate synthase.
- The cycle ends with the regeneration of oxaloacetate.
- The citric acid cycle is responsible for two-thirds of the food-derived energy in higher organisms.

Applications of citric acid
- Citric acid is commonly used as a flavoring and preservative in food and beverages.
- It is denoted by E number E330 within the European Union.
- Citrate salts of various metals are used in dietary supplements to deliver minerals.
- Citric acid is used in ice cream as an emulsifying agent and in caramel to prevent sucrose crystallization.
- It is used in effervescent formulae and food coloring to balance pH levels.
- Citric acid is an excellent chelating agent, used to remove limescale and treat water.
- It improves the effectiveness of soaps and laundry detergents by chelating metals in hard water.
- Citric acid is used in bathroom and kitchen cleaning solutions and can remove hard water stains from glass.
- It can be used in shampoo to wash out wax and coloring from the hair.
- Citric acid is used to dissolve rust from steel and passivate stainless steels.
- Citric acid is used as an acidulant in creams, gels, and liquids.
- It is an active ingredient in chemical skin peels and facial tissues with antiviral properties.
- Citric acid is used as a buffer to increase the solubility of brown heroin.
- It is used in the production of facial tissues with antiviral properties.
- Citric acid is used in the synthesis of other organic compounds, such as itaconic acid and aconitic acid.
- Citric acid is used to control pH in household cleaners and pharmaceuticals.
- It is used as an odorless alternative to white vinegar for fabric dyeing.
- Sodium citrate, derived from citric acid, is used in Benedicts reagent for identifying reducing sugars.
- Citric acid can be used as an alternative to nitric acid in passivation of stainless steel.
- It can be used as a lower-odor stop bath in photographic film development and as a blood acid regulator.

Citric acid (Wikipedia)

"E330" redirects here. For the locomotive, see FS Class E330.

Citric acid is an organic compound with the chemical formula HOC(CO₂H)(CH₂CO₂H)₂. It is a colorless weak organic acid. It occurs naturally in citrus fruits. In biochemistry, it is an intermediate in the citric acid cycle, which occurs in the metabolism of all aerobic organisms.

Citric acid

Names
IUPAC name Citric acid
Preferred IUPAC name 2-Hydroxypropane-1,2,3-tricarboxylic acid
Identifiers
CAS Number	77-92-9 Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:30769 Y
ChEMBL	ChEMBL1261 Y
ChemSpider	305 Y
DrugBank	DB04272 Y
ECHA InfoCard	100.000.973
EC Number	201-069-1
E number	E330 (antioxidants, ...)
IUPHAR/BPS	2478
KEGG	D00037 Y
PubChem CID	311 22230 (monohydrate)
RTECS number	GE7350000
UNII	XF417D3PSL Y
CompTox Dashboard (EPA)	DTXSID3020332
InChI InChI=1S/C6H8O7/c7-3(8)1-6(13,5(11)12)2-4(9)10/h13H,1-2H2,(H,7,8)(H,9,10)(H,11,12) Y Key: KRKNYBCHXYNGOX-UHFFFAOYSA-N Y InChI=1/C6H8O7/c7-3(8)1-6(13,5(11)12)2-4(9)10/h13H,1-2H2,(H,7,8)(H,9,10)(H,11,12) Key: KRKNYBCHXYNGOX-UHFFFAOYAM
SMILES OC(=O)CC(O)(C(=O)O)CC(=O)O
Properties
Chemical formula	C6H8O7
Molar mass	192.123 g/mol (anhydrous), 210.14 g/mol (monohydrate)
Appearance	white solid
Odor	Odorless
Density	1.665 g/cm3 (anhydrous) 1.542 g/cm3 (18 °C, monohydrate)
Melting point	156 °C (313 °F; 429 K)
Boiling point	310 °C (590 °F; 583 K) decomposes from 175 °C
Solubility in water	54% w/w (10 °C) 59.2% w/w (20 °C) 64.3% w/w (30 °C) 68.6% w/w (40 °C) 70.9% w/w (50 °C) 73.5% w/w (60 °C) 76.2% w/w (70 °C) 78.8% w/w (80 °C) 81.4% w/w (90 °C) 84% w/w (100 °C)
Solubility	Soluble in acetone, alcohol, ether, ethyl acetate, DMSO Insoluble in C 6H 6, CHCl3, CS2, toluene
Solubility in ethanol	62 g/100 g (25 °C)
Solubility in amyl acetate	4.41 g/100 g (25 °C)
Solubility in diethyl ether	1.05 g/100 g (25 °C)
Solubility in 1,4-dioxane	35.9 g/100 g (25 °C)
log P	−1.64
Acidity (pKa)	pKa1 = 3.13 pKa2 = 4.76 pKa3 = 6.39, 6.40
Refractive index (nD)	1.493–1.509 (20 °C) 1.46 (150 °C)
Viscosity	6.5 cP (50% aq. sol.)
Structure
Crystal structure	Monoclinic
Thermochemistry
Heat capacity (C)	226.51 J/(mol·K) (26.85 °C)
Std molar entropy (S⦵298)	252.1 J/(mol·K)
Std enthalpy of formation (ΔfH⦵298)	−1543.8 kJ/mol
Heat of combustion, higher value (HHV)	1985.3 kJ/mol (474.5 kcal/mol, 2.47 kcal/g), 1960.6 kJ/mol 1972.34 kJ/mol (471.4 kcal/mol, 2.24 kcal/g) (monohydrate)
Pharmacology
ATC code	A09AB04 (WHO)
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	Skin and eye irritant
GHS labelling:
Pictograms
Signal word	Warning
Hazard statements	H290, H319, H315
Precautionary statements	P305+P351+P338
NFPA 704 (fire diamond)	2 1 0
Flash point	155 °C (311 °F; 428 K)
Autoignition temperature	345 °C (653 °F; 618 K)
Explosive limits	8%
Lethal dose or concentration (LD, LC):
LD50 (median dose)	3000 mg/kg (rats, oral)
Safety data sheet (SDS)	HMDB
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Y verify (what is YN ?) Infobox references

More than two million tons of citric acid are manufactured every year. It is used widely as an acidifier, as a flavoring, and a chelating agent.

A citrate is a derivative of citric acid; that is, the salts, esters, and the polyatomic anion found in solution. An example of the former, a salt is trisodium citrate; an ester is triethyl citrate. When part of a salt, the formula of the citrate anion is written as C
₆H
₅O³⁻
₇ or C
₃H
₅O(COO)³⁻
₃.