Citric acid occurs widely in nature. It derives its name from the citrus fruits where its presence imparts a refreshing, tart taste. It also occurs in many other fruits and plants and in milk.
Citric acid plays a vital role in the metabolism of humans and animals during the production of energy from food. In humans, up to 1.5-2.0 kg of citric acid is produced as citrate and metabolised per day. The citric acid cycle (see illustration) was first described by Sir Hans Krebs, a discovery for which the German born British biochemist was awarded the Nobel prize in 1953.