The disaccharides are sugars composed of two monosaccharide residues united by a glycosidic linkage. Their chemical name reflects their component monosaccharide. The important disaccharides of food are maltose, sucrose, and lactose.

a. Structure of Sucrose

Sucrose, common table sugar, is the most abundant of all disaccharides and occurs throughout the plant kingdom. It is made up of one glucose unit and a fructose unit linked by α 1-2 β glycosidic bond between the aldehyde or keto groups. Hence it is a nonreducing sugar. Sucrose occurs in the juices of sugar beets, sugar cane, sorghum, sugar maple and juices of pineapple and other fruits. Ripe fruits are rich in sucrose. It is the most abundantly distributed sugar.  

Hydrolysis of sucrose yields a crude mixture called “invert sugar” because the optical activity of dextro rotatory sucrose (+66.5^o) changes (inverts )to -19.5^o due to the production of the strongly levorotatory fructose. Because of the inversion of the sign of rotation in the reaction, the process is referred to as “inversion”, and the mixture of glucose and fructose obtained is called “invert sugar”.  Honey contains a large proportion of invert sugar.

b. Structure of Maltose

Maltose is composed of two glucose units linked by α 1-4 glycosidic bond bond.                                  

It is formed when starch is hydrolysed by the action of salivary amylase (ptyalin) and pancreatic amylase during the processes of digestion. Maltose is also formed as an intermediate product in the acid hydrolysis of starch and consequently is an important constituent of corn syrups, which are prepared by partial hydrolysis of starch with dilute acid. These syrups are complex mixtures of dextrins, maltose, and glucose. Commercial malt sugar is a mixture of maltose and dextrins formed from starch by hydrolysis with amylase.        

c. Structure of Lactose