Prolonged exposure of fat or oil in moist air and light causes bad smell (rancidity). It is due to

Step-by-step solution

On long storage in contact with air and moisture, oils and fats develop unpleasant smell. This process is known as ranicidification. It is due to (a) Enzymatic hydrolysis - producing bad smelling lower fatty acids. (b) Oxidation of unsaturated acids - producing aldehydes and ketones.