6.4.4 Oxidative and hydrolytic rancidity

6.4.4 Oxidative and hydrolytic rancidity

The unpleasant odor and taste developed by most natural fats on aging is referred to as “rancidity”. Nearly all natural fats are oxidized when exposed to air, light and moisture. Oxidation of unsaturated bonds in the glycerides of fats proceeds as that for the fatty acids. Oxygen may add at the double bonds to form peroxides, which may then decompose into other substances.

a) Hydrolytic rancidity: Rancidity may due to hydrolysis of component glycerides of a fat into free fatty acids and glycerol or a monoglycerides and diglycerides (Hydrolytic rancidity). This is often hastened by the presence of lipolytic enzymes (lipases) in the tissue and microbes, which in the presence of moisture and warm temperature bring about hydrolysis. Free fatty acid value is a measure of hydrolytic rancidity.

b) Oxidative rancidity: Rancidity may also be caused by oxidative processes (Oxidative rancidity). Oxidation at the double bonds on the unsaturated glycerides may form peroxides which then decompose by hydrolysis to from aldehydes, ketones and acids of smaller carbon chain causing objectionable odor and taste. This process is called oxidative rancidity. It is greatly increased by exposure to light. Nearly all natural fats are oxidized when exposed to air, light and moisture. It has been observed that highly refined fats and distilled fatty acids or their esters begin to absorb atmospheric oxygen almost immediately upon exposure to it, whereas natural fats often exhibit an “induction period” of variable duration before oxidation begins. This oxidation of unsaturated glycerides are prevented by substances which are called “antioxidants”.

Antioxidants: Antioxidants are those compounds that prevent the oxidation of fat. a Tocopherol is the natural antioxidant present in plant and animal tissues. The addition of minute amounts of certain synthetic compounds also prevents oxidation and they are called synthetic antioxidants.

e.g. Propyl gallate, Isoamyl gallate, BHT, BHA, Nordiguaretic acid.

Certain compounds induce oxidation by their presence and they are called as pro-oxidants e.g. iron, copper and other metal ions.


Some compounds increase the antioxidant activity of the antioxidants and they are called as synergies e.g. ascorbic acid, citric acid and phosphoric acids.

Last modified: Tuesday, 6 March 2012, 4:57 AM