Citric acid is an organic compound with the formula C6H8O7.[10] It is a colorless weak organic acid.[10] 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.[10] More than two million tons of citric acid are manufactured every year. It is used widely as acidifier, flavoring, preservative, and chelating agent.[11] A citrate is a derivative of citric acid; that is, the salts, esters, and the polyatomic anion found in solutions and salts of citric acid. An example of the former, a salt is trisodium citrate; an ester is triethyl citrate. When citrate trianion is part of a salt, the formula of the citrate trianion is written as C 6H 5O3− 7 or C 3H 5O(COO)3− 3. Natural occurrence and industrial production Lemons, oranges, limes, and other citrus fruits contain high concentrations of citric acid. Citric acid occurs 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 (about 47 g/L in the juices[12]).[a] The concentrations of citric acid in citrus fruits range from 0.005 mol/L for oranges and grapefruits to 0.30 mol/L in lemons and limes; these values vary within species depending upon the cultivar and the circumstances under which the fruit was grown. Citric acid was first isolated in 1784 by the chemist Carl Wilhelm Scheele, who crystallized it from lemon juice.[13][14] Industrial-scale citric acid production first began in 1890 based on the Italian citrus fruit industry, where the juice was treated with hydrated lime (calcium hydroxide) to precipitate calcium citrate, which was isolated and converted back to the acid using diluted sulfuric acid.[15] In 1893, C. Wehmer discovered Penicillium mold could produce citric acid from sugar.[16] However, microbial production of citric acid did not become industrially important until World War I disrupted Italian Citrus exports