Why are unsaturated fats more likely to become rancid than saturated fats?

Unsaturated fats are more prone to becoming rancid than saturated fats because of their chemical structure. Specifically, unsaturated fatty acids contain one or more carbon-carbon double bonds, which are highly reactive sites susceptible to oxidation when exposed to air.123

This process, known as autoxidation, is an autocatalytic, free-radical chain reaction. During this reaction, oxygen attacks the double bonds, leading to the formation of hydroperoxides that eventually break down into volatile compounds like aldehydes, ketones, and alcohols. These breakdown products are responsible for the unpleasant odors and flavors associated with rancidity.41

In contrast, saturated fats lack these reactive double bonds, making them significantly more stable and resistant to oxidation at ordinary temperatures. Because of this, the food industry often uses hydrogenation to add hydrogen to the double bonds of unsaturated fats, effectively converting them into more stable, saturated-like structures that are less likely to spoil.123