Principles of Biochemistry

6.4.5.2 Saponification Number

The saponification number is defined as number of the milligrams of KOH required to saponify one gram of fat or oil.

Boiling with an alcoholic solution of strong alkali hydrolyzes triglycerides into glycerol and fatty acids. This process is called saponification. The glyceride of the fat may be readily decomposed into glycerol and salts of the constituent fatty acids (Soaps) by boiling with strong bases such as sodium or potassium hydroxide. Since fats are insoluble in water, the process is facilitated by the addition of alcohol which dissolves the fat.

The saponification of fats is important in the chemical examination of fats. Since fats are mixtures of glycerides and the glycerides in turn contain various chain length fatty acids, the saponification number is an index of the average molecular size of the fatty acids present. It is inversely proportional to the average molecular weight of the fatty acids in the fat.