Lactose (also referred to as milk sugar) is a sugar that's found most notably in milk. Lactose makes up around 2–8% of milk (by weight), although the amount varies among species and individuals. It is extracted from sweet or sour whey. The name comes from ', the Latin word for milk, plus the -ose ending used to name sugars. Chemistry Lactose is a disaccharide that consists of galactose and glucose fragments bonded through a β-1→4 glycosidic linkage. Its systematic name is β… (More on Lactose)