Lactose is the main carbohydrate component of milk. It is a diholoside or disaccharide comprising two molecules, two simple sugars : one molecule of galactose (sugar with 6 atoms of carbon) and a molecule of glucose (or dextrose whenever D-glucose is indicated). Lactose is the milk sugar, it is naturally present in mammal milk.

Lactose is the main component of whey, which is the liquid obtained from milk coagulation. It is extracted by crystallisation.

After separation of fat and caseine precipitate, whey may be obtained. This whey has a very high lactose content. Lactose is then evaporated and transferred to crystallisation tanks. Hard and sandy crystals of lactose hydrate are then obtained. These crystals are then separated, decanted and washed to purify lactose.