Lactose is a disaccharide composed of galactose and glucose subunits and has the molecular formula C 12 H 22 O 11 . Lactose makes up around 2–8% of milk (by weight). It is derived from the condensation of galactose and glucose, which form a β(1→4) glycosidic linkage. Its systematic name is β-D-galactopyranosyl-(1→4)-D-glucose. The glucose can be in either the α-pyranose form or the β-pyranose form, whereas the galactose can only have the β-pyranose form: hence α-lactose and β-lactose refer to the anomeric form of the glucopyranose ring alone.