Lactose is a disaccharide that consists of β-D-galactose and β-D-glucose molecules bonded through a β1-4 glycosidic linkage. Lactose makes up around 2-8% of the solids in milk. The name comes from the Latin word for milk, plus the -ose ending used to name sugars.
 Digestion of lactose
Infant mammals are fed on milk by their mothers. To digest it an enzyme called lactase (β1-4 disaccharidase) is secreted by the intestinal villi, and this enzyme cleaves the molecule into its two subunits glucose and galactose for absorption.
Since lactose occurs mostly in milk, in most species the production of lactase gradually ceases with maturity, and they are then unable to metabolise lactose. This loss of lactase on maturation is also the default pattern in most adult humans. However, many people with ancestry in Europe, the Middle East, India, and the Maasai of East Africa, have a version of the gene for lactase that is not disabled after infancy, and in many of these cultures other mammals such as cattle, goats, and sheep are milked for food.
 Brewing with Lactose
Lactose is largely unfermentable and is often used with brewing for sweetness and body. It can be added at any stage of the brewing process since the sugars will not be converted by yeast. It is a characteristic ingredient in Milk Stout, also called Sweet Stout.