Lactose

Lactose, sugar with formula C₁₂H₂₂O₁₁, present in milk. Lactose is obtained from milk in the form of hard, gritty crystals of composition C₁₂H₂₂O₁₁·H₂O, by evaporation of the whey remaining after removal of the fat and precipitation of the casein. The crystals lose their water when heated to 140°C (284°F) and melt and decompose at 202°C (396°F). Upon hydrolysis, lactose gives glucose and galactose. In the presence of the proper enzymes it ferments into lactic acid and butyric acid. Lactose is less sweet than sucrose, rotates the plane of polarized light to the right (dextro-rotatory), and is less soluble in water than either sucrose or glucose. It is important in the diet of young mammals and is often added to baby foods; it is also used in confections and pharmaceutical tablets.

Most humans can digest lactose during infancy. A small percentage of people are lactose intolerant from birth and suffer digestive problems from consuming fresh milk or lactose-containing milk products (Food Allergy and Intolerance). European peoples with a long history of eating dairy products are usually able to digest lactose as adults. However, about 75 percent of the world’s population, mainly in Asia and Africa, lose the ability to digest lactose after weaning. Lactose breaks down when milk is fermented (see Fermentation), so most adults can consume fermented milk products such as yogurt and cheese. Many commercially made yogurts and cheeses now have added powdered milk and so contain lactose, making them unsuitable for lactose-intolerant individuals.