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.