What happens to oils when they become rancid?

When oils become rancid, they undergo chemical changes that primarily lead to the development of off-flavors and unpleasant odors. This process is often caused by the oxidation of fats within the oil, which can result from exposure to air, light, and heat. The breakdown of fatty acids produces volatile compounds that contribute to the characteristic rancid smell and taste.

While it is true that rancid oils may lose some of their nutritional value as a result of these changes, the most noticeable and defining feature of rancidity is indeed the changes in flavor and odor. Rancid oils can emit a strong, unpleasant scent similar to that of spoiled food, making them unpalatable. Thus, the correct answer emphasizes the sensory changes that are most commonly associated with rancid oils.