Foods prepared with edible oil develop a kind of smell and undergo changes in taste. What is the reason for this?

Edible oils are rich in fats. Auto oxidation occurring in natural edible fats is called 'rancidification'. The chemical changes occur during rancidification is known as 'rancidity'. Rancidity occurs when fats or edible oils are exposed to heat, light, air and moisture. The fat, which has become rancid has a unpleasant odour and taste. When a high level of rancidity occurs, the fat is unfit for consumption.

Rancidity occurs more frequently in summer. There are two types of rancidity, 'hydrolytic' and 'oxidative'. The fats contain triglycerids, a complex of glycerol and fatty acids. The fatty acids are saturated or unsaturated. Hydrolytic rancidity involves partial hydrolysis of the glycerids to mono and diglycerids, while glycerol and fatty acids are also liberated. This hydrolysis is hastened by the presence of moisture, heat and lipases, the fat hydrolysing enzymes present in fats. Fats like butter contain a high percentage of volatile fatty acid, butyric acid. Hydrolytic rancidity produces disagreeable odour and taste due to the liberation of the volatile butyric acid. Butter undergoes rancidification more easily in summer. The unsaturated fatty acids have double bonds in their chemical structure.

In oxidative rancidity, the unsaturated fatty acids are oxidised at the double bond position to form 'peroxides' which then decompose to form aldehydes and ketones of objectionable odour and taste. Rancidity can be considerably prevented by addition of traces of organic compounds like phenols, gallic acid and vitamins like 'C' and 'E'. Such substances are called 'antioxidants'. The antioxidants prevent oxidation and peroxide formation. Rancidity can also be prevented by proper storage of the fats and oils in air tight non-metallic containers kept in a cool place, away from light, moisture and warmth. Storing in non-metallic vessels is necessary, because, metals like copper (Cu) and lead (Pb) catalyse the rancidity.