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Lactose (LAK-tose) is a white, odorless, sweet-tasting solid commonly known as milk sugar because it occurs in the milk of many animals, primarily the mammals. The lactose content of milk ranges from about 2 to 8 percent in cows and 5 to 8 percent in human milk. Lactose occurs in two isomeric forms, α-lactose and β-lactose, with the latter somewhat sweeter than the former. The alpha form tends to occur as the monohydrate, C12H22O11·H2O.



D-lactose; milk sugar; many others




Carbon, hydrogen, oxygen


Disaccharide; carbohydrate (organic)




342.30 g/mol


222.8°C (433.0°F)


Not applicable; decomposes


Very soluble in water; slightly soluble in ethyl alcohol; insoluble in organic solvents


Lactose is synthesized in the mammary (milk-producing) glands of mammals. The milk of such animals contains an enzyme called lactose synthetase, which acts on the compound uridine diphosphate D-galactose to produce lactose. The compound is obtained commercially from whey, a byproduct of the cheese-making process. The solids in whey contain about 70 percent lactose by weight. These solids are filtered to remove the proteins they contain. After removal of minerals in the whey, the resulting solution consists of about 50 to 65 percent by weight, which is allowed to crystallize out of the resulting solutions. The lactose produced by this process is a racemic mixture of the D and L isomers. To obtain the alpha isomer, the lactose is dissolved with water and treated with activated carbon to remove any color. When the water evaporates from the solution, α-lactose monohydrate remains. The product is most commonly made available in this form. To obtain the beta isomer, α-lactose is heated with water in the presence of a base, which converts the alpha isomer to its beta form.


Both forms of lactose are used in the preparation of baby foods and food or infants and for convalescents. It is also used in the dairy industry to feed foals that have been orphaned. Lactose is also used as a food additive, primarily as a humectant and to increase the sweet flavor of a food. A humectant helps foods retain their moisture. It is used in products such as ice creams, baked goods, confectionary items, whipped toppings, and breakfast foods. Lactose may also be added to a number of foods with limited natural sweetness, such as margarine, butter, frozen vegetables, and processed meats. It is also used as a filler for many pharmaceutical products. A filler acts bulk to the product without significantly changing its nutritional or medical properties.

Many people are lactose intolerant. Lactose intolerance is a condition that develops when a person lacks enough (or any) of the enzyme lactase that is responsible for digestion of lactose in the body. People who are lactose intolerant and consume lactose experience a range of unpleasant symptoms, including fluid retention, gas, cramps, and diarrhea. By some estimates, up to 75 percent of the world's population may experience lactose intolerance to a greater or lesser degree.

Interesting Facts

  • Only one group of mammals does not produce lactose in their milk, the Pinnipedia, a group that includes seals, sea lions, and walruses.
  • Alpha lactose is found in cow's milk and beta lactose in human milk. During pasteurization of cow's milk, the alpha isomer is converted into the beta isomer.

One method for dealing with lactose intolerance is for a person to avoid consumption of any food or food product that contains lactose. The option has been made somewhat easier by the introduction of lactose-free products by a number of food companies. Another method to avoid the problems associated with lactose intolerance is to use a dietary supplement that contains lactase, restoring to the body the enzyme that it lacks naturally.

Words to Know

A chemical compound formed when one or more molecules of water is added physically to the molecule of some other substance.
A group of warm-blood animals whose females produce milk for their young. This group includes humans, cows, bears, and dogs, among others.
A mixture of equal amounts of two opposite isomers (the "right-handed," or "D" form and the "left-handed," or "L" form).


"Lactose." J. T. Baker. (accessed on October 14, 2005).

"Lactose Intolerance." Medline Plus. (accessed on October 14, 2005).

"Lactose Intolerance." National Digestive Diseases Information Clearinghouse. (accessed on October 14, 2005).

See AlsoSucrose

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lactose (lăk´tōs) or milk sugar, white crystalline disaccharide (see carbohydrate). It has the same empirical formula (C12H22O11) as sucrose (cane sugar) and maltose but differs from both in structure (see isomer). It yields the simple sugars D-glucose and D-galactose on hydrolysis, which is catalyzed by lactase, an enzyme found in gastric juice. People who lack this enzyme after childhood cannot digest milk and are said to be lactose intolerant. Lactose is formed in the mammary glands of all lactating animals and is present in their milk. It is produced commercially as a byproduct of milk processing. When milk sours, the lactose in it is converted by bacteria to lactic acid. Lactose is less sweet-tasting than sucrose and is not found in plants.

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lactose The carbohydrate of milk, sometimes called milk sugar. A disaccharide of glucose and galactose. Used pharmaceutically as a tablet filler and as a medium for growth of micro‐organisms. The fermentation of lactose to lactic acid by bacteria is responsible for the souring of milk. Ordinary lactose is α‐lactose, which is 16% as sweet as sucrose; if crystallized above 93 °C, it is converted to the β‐form which is more soluble and sweeter.

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lactose (milk sugar) Disaccharide present in milk, made up of a molecule of glucose linked to a molecule of galactose. It is important in cheesemaking, when lactic bacteria turn it into lactic acid. This sours the milk and results in the production of cheese curd.

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lac·tose / ˈlakˌtōs; -ˌtōz/ • n. Chem. a sugar present in milk. It is a disaccharide containing glucose and galactose units.

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lactoseadiós, chausses, Close, Davos, dose, engross, gross, Grosz, jocose, morose, Rhos, verbose •grandiose • religiose • otiose •globose • viscose • bellicose • varicose •vorticose • cellulose • lachrymose •lactose • comatose • siliquose

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lactose (milk sugar) A disaccharide sugar comprising glucose and galactose; it is the principal sugar in milk.

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lactose (milk sugar) A sugar comprising one glucose molecule linked to a galactose molecule. Lactose is manufactured by the mammary gland and occurs only in milk. For example, cows' milk contains about 4.7% lactose. It is less sweet than sucrose (cane sugar).

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lactose (milk sugar) (lak-tohz) n. a sugar, consisting of one molecule of glucose and one of galactose, found only in milk. l. intolerance inability to absorb lactose, caused by absence or low activity of the enzyme lactase.