Disaccharides (di- = “two”) form when two monosaccharides undergo a dehydration reaction (also known as a condensation reaction or dehydration synthesis). During this process, the hydroxyl group of one monosaccharide combines with the hydrogen of another monosaccharide, thus, releasing a molecule of water and forming a covalent bond.
A covalent bond formed between a carbohydrate molecule and another molecule (in this case, between two monosaccharides) is known as a glycosidic bond (see image above). Glycosidic bonds (also called glycosidic linkages) can be of the alpha or the beta type.
Common disaccharides include lactose, maltose, and sucrose (see image above). Lactose is a disaccharide consisting of the monomers glucose and galactose. You can find it naturally in milk. Maltose, or malt sugar, is a disaccharide formed by a dehydration reaction between two glucose molecules. The most common disaccharide is sucrose, or table sugar, which is composed of the monomers glucose and fructose.