Disaccharides:-

These are the carbohydrates which give two units of monosaccharides on hydrolysis. Some examples are: Sucrose, Lactose, Maltose.

C₁₂H₂₂O₁₁ + H₂O C₆H₁₂O₆           +             C₆H₁₂O₆

  Sucrose                              α-D-Glucose      β-D-Fluctose

C₁₂H₂₂O₁₁ + H₂O C₆H₁₂O₆          +             C₆H₁₂O₆

  Lactose                               β-D-Glucose       α-D-Glucose

_{C12H22O11 + H2O 2 C6H12O6}

  Maltose                             α-D-Glucose

This implies that they are formed by the condensation of two monosaccharides. The linkage which holds monosaccharide units in a disaccharide (or polysaccharide) molecule is called Glycoside Linkage. e.g.

(i) Sucrose:- In sucrose, α-D-Glucose and β-D-Fructose condense together: (C₁ – C₂ linkage).

(ii) Lactose:- In Lactose, Glactose unit and glucose unit condense together (C₁ – C₄ linkage).

(iii) Maltose:- In Maltose two units of α-D-Glucose condense together (C₁ – C₄ linkage)

The Disaccharides which involve the carbonyl group in the glycoside linkage is called non reducing sugar e.g. sucrose and the disaccharides which involve no carbonyl group and is free are called reducing sugars e.g. Maltose and Lactose.