Disaccharides and the glycosidic link

Two monosaccharides join by losing a water molecule, leaving an oxide bridge between them. A link between sugar units through oxygen is a glycosidic linkage. Dilute acid or enzymes hydrolyse it back.

If both reducing groups (the aldehydic or ketonic carbons) are used up in the link, the disaccharide is non-reducing. If one stays free it is reducing.

Sucrose: C1 of α-D-glucose joined to C2 of β-D-fructose. Both reducing carbons are in the bond, so sucrose is a non-reducing sugar.

Sucrose is dextrorotatory. Its hydrolysis gives equal moles of D-(+)-glucose (+52.5°) and D-(–)-fructose (–92.4°). Fructose's laevorotation is the larger, so the mixture is laevorotatory.

The sign of rotation flips from (+) to (–) on hydrolysis; the product is called invert sugar.

Maltose: two α-D-glucose units, C1 of one joined to C4 of the other. The second unit's C1 can open to a free aldehyde in solution, so maltose is reducing.

Lactose (milk sugar): β-D-galactose and β-D-glucose linked from C1 of galactose to C4 of glucose. The glucose unit's C1 can give a free aldehyde, so lactose is reducing.