Disaccharide

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A disaccharide is a sugar (a carbohydrate) composed of two monosaccharides. It is formed when two sugars are joined together and a molecule of water is removed. For example, milk sugar (lactose) is made from glucose and galactose whereas cane sugar (sucrose) is made from glucose and fructose.

[edit] Chemistry

The two monosaccharides are bonded via a dehydration reaction that leads to the loss of a molecule of water. The glycosidic bond can be formed between any hydroxyl group on the component monosaccharide. So, even if both component sugars are the same (e.g., glucose), different bond combinations (regiochemistry) and stereochemistry (alpha- or beta-) result in disaccharides that are diastereoisomers with different chemical and physical properties.

Depending on the monosaccharide constituents, disaccharides are sometimes crystalline, sometimes water-soluble, and sometimes sweet-tasting. 'Disaccharide' is one of the four chemical groupings of carbohydrates (monosaccharide, disaccharide, oligosaccharide, and polysaccharide).

[edit] Common disaccharides

Disaccharide	Unit 1	Unit 2	Bond	Notes
Sucrose (table sugar, cane sugar, saccharose, or beet sugar)	glucose	fructose	α(1→2)	-
Lactose (milk sugar)	glucose	galactose	β(1→4)	-
Maltose	glucose	glucose	α(1→4)	Produced during the malting of barley
Trehalose	glucose	glucose	α(1→1)α	Present in fungi and insects. Successfully produced at an industrial scale by enzymatic treatment of starch for use as a food ingredient.
Cellobiose	glucose	glucose	β(1→4)	-

Maltose and cellobiose are hydrolysis products of the polysaccharides, starch and cellulose, respectively.