Carbohydrates - Disacharides

Disaccharides

1. Disaccharides are complex sugars. It consists of two monosaccharides joined together chemically.
2. Disaccharides are also known as double sugar.
3. Examples of disaccharides are maltose, sucrose and lactose.

Taste of Disaccharides
All disaccharides taste sweet and soluble in water.

Condensation and Hydrolysis Reaction

1. 2 monosacharides combine together to form 1 disaccharide molecule through a process call condensation.
2. Disaccharides can also be broken down to monosaccharides by hydrolysis.

Condensation of Monosacharides
Figure below shows the examples of formation of disaccharides from condensation of monosacharides.



Hydrolysis of Disaccharides
Hydrolysis is a chemical reaction that breaks up large molecules by adding water to them.



Sucrose (cane sugar)

Copyright statement

1. Sucrose is made up of glucose and fructose.
2. It is commonly found in sugar cane, sugar beet and sweet fruits.
3. It is generally extracted from sugar cane or sugar beet and then purified and crystallized to be used as a sweetener in beverages.

Lactose (milk sugar)

1. Consists of glucose and galactose.
2. Present in the milk.

Maltose (malt sugar)

1. Made up of two glucose molecules.
   
2. Product of the partial digestion of starch.

Reducing Sugar and Non-Reducing Sugar

Maltose and lactose are reducing sugars, while sucrose is a non-reducing sugar.

Chemical Test for Disaccharides

1. Since maltose and lactose are reducing sugar, they can be tested directly by Benedicts Solution.
2. When maltose or lactose is boiled with Benedicts Solution, a brick red precipitate will be produced, indicating the presence of reducing sugar.
3. Sucrose is non-reducing sugar. There is no direct test for non-reducing sugar.
4. However, the monomers of sucrose - glucose and fructose, are reducing sugar. Therefore, we can detect the presence of sucrose by breaking down sucrose into glucose and fructose through hydrolysis reaction (heating sucrose solution with hydroclhoric acid), follow by the heating the products with Benedicts Solution.
5. Formation of brick red precipitate indicates the presence of reducing sugar, and hence presence of sucrose in the solution.