Lactose is the principal carbohydrate found in milk and, after water, is the component of milk present in the greatest amount. Milk contains only trace amounts of other carbohydrates. Bovine milk contains ca. 4.6 % lactose but levels are affected by breed of cow, udder infection (mastitis) and stage of lactation. Unlike protein and fat, the levels of which decrease with advancing lactation, levels of lactose increase with stage of lactation. Lactose levels in the milks of other mammals varies widely.

Lactose in milk is important as

· It is essential for the production of fermented dairy products (cheese and yogurt) as it acts as a growth substrate for lactic acid bacteria.

· It contributes to the nutritive value of milk (although some people cannot metabolise lactose, a condition called lactose intolerance, see below).

· It influences the texture of frozen dairy foods (especially ice cream).

· It can become involved in the Maillard reaction leading to browning and the production of flavour compounds. (Browning of dairy products is often a defect.)

Lactose is a disaccharide of galactose and glucose linked by a b1-4 glycosidic bond. Its systematic name is b-O-D-galactopyranosyl-(1-4)-a-D-glucopyranose (a-latcose) or b-O-D-galactopyranosyl-(1-4)-b-D-glucopyranose (b-latcose). Lactose is a reducing sugar and may exist in two anomeric forms (a- or b-lactose).