The major sugar in human and bovine milk. Conversion of lactose to lactic acid by Lactobacilli etc. Is important in the production of yoghurt and cheese. A sugar comprising one glucose molecule linked to a galactose molecule; occurs only in milk; cow's milk contains about 4.7% lactose.A disaccharide sugar which is commonly found in milk. It consists of galactose and glucose, where an enzyme called lactase can break lactose into these two smaller components. Microorganisms convert lactose to pyruvic acid, which results in the souring of the milk for example.