disaccharides

Disaccharides are a class of carbohydrates composed of two monosaccharide units joined by a glycosidic bond. They form when two sugar molecules undergo a dehydration reaction, removing a molecule of water. The linkage is typically between the anomeric carbon of one sugar and a hydroxyl group of the other, most often creating an alpha or beta glycosidic bond. Depending on which anomeric carbon is involved, the disaccharide may be reducing or non-reducing.

Common disaccharides include sucrose, lactose, maltose, and cellobiose. Sucrose consists of glucose and fructose linked by an alpha-1,2 glycosidic bond and is non-reducing because both anomeric carbons participate in the bond. Lactose is galactose linked to glucose via beta-1,4 and is a reducing sugar. Maltose is two glucose units connected by an alpha-1,4 bond and is reducing. Cellobiose, a glucose–glucose disaccharide linked beta-1,4, is also considered reducing. The diversity of linkages among two monosaccharides determines properties such as sweetness, digestibility, and reactivity.

In biology and industry, disaccharides serve as energy sources and transport sugars. They are digested by specific enzymes: sucrase cleaves sucrose, lactase cleaves lactose, and maltase (and isomaltase) cleave maltose and related linkages. Hydrolysis of disaccharides yields monosaccharides, which are readily absorbed in the small intestine. In foods, disaccharides act as sweeteners and contribute to texture and browning reactions, while naturally occurring disaccharides appear in milk, sugar crops, and starch-processing products.