bioses, carbohydrates whose molecules consist of two monosaccharide groups. All disaccharides have a glycoside type of structure in which the hydrogen atom of a glycoside hydroxyl of one monosaccharide molecule is replaced by a group from another monosaccharide molecule bonded by a hemiacetal or alcohol hydroxyl. In the former case the disaccharides that form lack reducing properties, but in the second case they have them (I). The group of irreducible disaccharides includes trehalose (mycose, or fungus sugar), which consists of two glucose groups (II), and saccharose, which consists of glucose and fructose groups (III). The group of reducible disaccharides includes maltose (IV), cellobiose (V), and lactose (VI).

Disaccharides may contain the five- or six-membered rings of the monosaccharides (pentoses and hexoses) and may differ with respect to the configuration of the glycoside bond (α- or β-glycosides). The spatial forms (conformations) of the rings of the monosaccharide groups in the various disaccharides can vary. Thus, cellobiose and maltose differ not only in the configuration of the glycoside bond (α in maltose and β in cellobiose) but also in the fact that in cellobiose both groups have the same conformation, whereas in maltose they are different.

Disaccharides crystallize well, are readily soluble in water and 45-48-percent alcohol, only slightly soluble in 96-percent alcohol, optically active, and sweet to the taste. They are hydrolyzed by acids (in the case of saccharose it is called inversion); if the monosaccharide group has a five-membered ring the rate of acid hydrolysis of the disaccharide is increased. Disaccharides are also hydrolyzed by enzymes (carbohydrases), such as α- and β-glucosidases (depending on the type of glycoside bond in the disaccharide). Hydrolysis produces monosaccharides.

Disaccharides are widespread in animals and plants. They occur in the free state (as products of biosynthesis or of partial hydrolysis of polysaccharides) and in the form of structural components of glycosides and other compounds. Disaccharides are usually produced from natural sources— for example, saccharose from the sugar beet or from sugar-cane, and lactose from milk. Many disaccharides can be synthesized by chemical and biochemical means.

Saccharose, lactose, and maltose are valuable food and flavoring substances. The sugar industry produces saccharose.