Reactions of Lipids – Saponification, Hydrogenation, Peroxidation, Rancidity

Reactions of Lipids

As we have discussed lipids in the previous article so here are discussed only the reactions of lipids and tests for the purity of fats in lipids.


Hydrolysis of fat by alkali is called saponification. The products are glycerol and the alkali salts of the fatty acids, which are called soaps. Acid hydrolysis of fat yields free fats and glycerol.



Hydrogenation of unsaturated fats in the presence of a catalyst (nickel) is referred to as “hardening”. It is commercially important as a method of converting these liquid fats, usually of plant origin into solid fats as margarine, vegetable ghee, and so on.



Peroxidation (auto-oxidation) of lipids exposed to oxygen is accountable not just for deterioration of foods (rancidity) however also for damage to tissues in vivo, where it might be a cause of cancer. Lipid peroxidation is a domino effect creating complimentary radicals that initiate further peroxidation.

To manage and lower peroxidation, people utilize anti-oxidants. Naturally occurring anti-oxidants consist of vitamin E (tocopherol) and β-carotene (provitamin A), which are lipid-soluble, and vitamin C, which is water-soluble.


The undesirable smell and taste, developed by natural fats upon aging, is described as “rancidity”. Rancidity might be because of hydrolysis or oxidation of fat.

Rancidity due to hydrolysis

Naturally occurring fats, particularly those from animal sources, are polluted with enzyme lipase. The action of lipase brings about partial hydrolysis of glycerides of fat.

– Rancidity may likewise be brought on by different oxidative procedures. For example, oxidation at the double bonds of unsaturated fats of glycerides may form peroxides, which then disintegrate to form aldehydes of unpleasant odor and taste, this process is increased by exposure to light or heat.

Lots of natural vegetable fats and oils may include antioxidants like vitamin E which avoid the onset of rancidity. Therefore, vegetable fats can be preserved for a longer time than animal fats.

Tests for the purity of Fats

Fats are identified and their purity assayed by the following tests:

Saponification Number

It is specified as, number of mgs of KOH required to saponify one gm of fat. It is inversely proportional to the molecular weight of fat. This value is high in fats including a short-chain fatty acid. For example, the saponification number of:

– Butter = 220

– Coconut oil = 260

Iodine Number

The number of grams of iodine required to saturate 100 grams of a provided fat is called iodine number. Considering that iodine is taken up by the double bonds, a high iodine number shows a high degree of unsaturation of the fatty acids in fat, e.g.

– Butter fat = 27

– Coconut oil = 8

– Linseed oil = 200

Iodine number is important in the identification of the fat or oil in addition to is utilized for recognition of adulteration of oils. This unsaturation remains in the form of double bonds which react with iodine substances. The greater the iodine value, the more unsaturated fatty acid bonds are present in fat.

Acid Number

The number of mg of KOH required to reduce the effects of the free fatty acids present in one gm of fat is called acid number. The acid number indicates the degree of rancidity of the given fat. The acid number is directly proportional to the rancidity. The edibility of fat is inversely proportional to the acid number. Refined oil should not consist of totally free fatty acids. The existence of free fatty acids in any oil suggests that it is not pure.

Reichert Meissl Number

The number of ml of 0.1 N alkali, needed to reduce the effects of the unstable fatty acids distilled from 5 gm of fat. For example, the Reichert Meissl value for:

– Butter = 26

– Coconut oil = 7

It is less than one for other edible oils. The admixture of certain fatty acids may be utilized to prepare synthetic butter which might imitate butter in the majority of the constants except for RM value and for this reason, can be detected.