Final answer:
Fats used in frying can become rancid due to the oxidation of unsaturated fatty acids.
Step-by-step explanation:
Rancidity occurs in fats used in frying due to the oxidation of unsaturated fatty acids. This oxidation process occurs when the double bonds in the unsaturated fatty acids react with oxygen in the presence of heat, leading to the formation of free radicals. These free radicals can further react with other fats, causing the fats to become rancid and develop unpleasant odors and tastes.