Disaccharides are compounds that contain a bond between carbon(1) of one sugar and a hydroxyl group at any position on the other sugar.

Disaccharides are sugars containing two hexose units, such as sucrose in cane sugar, maltose in malt sugar and lactose in milk sugar.

These sugars are hydrolyzed into monosaccharides in the digestive tract by specific enzymes and each of these specific sugars—sucrose, maltose and lactose—have a role to play in human nutrition.

• Sucrose
  The disaccharide important for the nutrition is—as other sugars too—not a rigid, but a flexible structure.

  Sucrose, ordinary table sugar, is probably the single most abundant pure organic chemical in the world and the one most widely known to nonchemists. Whether from sugar cane (20% by weight) or sugar beets (15% by weight), and whether raw or refined, common sugar is still sucrose.

  Sucrose is a disaccharide that yields 1 equiv of glucose and 1 equiv of fructose on acidic hydrolysis. This 1:1 mixture of glucose and fructose is often referred to as invert sugar, since the sign of optical rotation changes (inverts) during the hydrolysis from sucrose ([alpha]D = +66.5°) to a glucose fructose mixture ([alpha]D = -22.0°). Certain insects, particularly honeybees, have enzymes called invertases that catalyze the hydrolysis of sucrose to a glucose-fructose mixture. Honey, in fact, is primarily a mixture of these three sugars.

  Unlike most other disaccharides, sucrose is not a reducing sugar and does not exhibit mutarotation. These facts imply that sucrose has no hemiacetal linkages and that glucose and fructose must both be glycosides. This can happen only if the two sugars are joined by a glycoside link between C1 of glucose and C2 of fructose.

• Maltose
  The disaccharide obtained by enzyme-catalyzed hydrolysis of starch, consists of two D-glucopyranoses joined by a 1,4'-beta-glycoside bond. Both maltose and cellobiose are reducing sugars because the anomeric carbons on the right-hand sugar are part of a hemiacetal.

  Despite the similarities of their structures, cellobiose and maltose are dramatically different biologically. Cellobiose cannot be digested by humans and cannot be fermented by yeast. Maltose, however, is digested without difficulty and is fermented readily.

• Lactose
  Lactose is a disaccharide that occurs naturally in both human and cow's milk. It is widely used in baking and in commercial infant-milk formulas.

  Like cellobiose and maltose, lactose is a reducing sugar. It exhibits muta-rotation and is a 1,4'-beta-linked glycoside. Unlike cellobiose and maltose, however, lactose contains two different monosaccharide units. Acidic hydrolysis of lactose yields 1 equiv of D-glucose and 1 equiv of D-galactose; the two are joined by a beta-glycoside bond between C1 of galactose and C4 of glucose.