Citric acid was first produced on commercial scale in England around 1826 from imported Italian lemons (lemons contain 7–9% citric acid). Lemon juice remained the commercial source of citric acid till the first industrial process using Aspergillus Niger started in Belgium. It is a weak organic acid with a chemical formula C6H8O7. Citric acid is one of the most widely used organic acids, and its annual worldwide output has reached 2.1 million tons in 2016. It is used as a preservative in the food industries, and also in the pharmaceutical and cosmetic industries. Citric acid is also used as a complexing and chelating agent in metal treatment. It may also be used as a water softener for detergents. Citric acid is exclusively produced by fermentation by various sources like fruits, molasses, fungi etc. The industrial production is performed using carbohydrates or agro-industrial residues as substrates by three different type processes: submerged, surface, and solid fermentations.