How does oxidation cause fats and oils to become rancid?

Rancidity in fats and oils is primarily caused by autoxidation, a chemical process where unsaturated fatty acids react with oxygen in the air.123

This process occurs through a free-radical chain reaction consisting of initiation, propagation, and termination steps. During initiation, factors such as light, heat, or trace metals trigger the formation of highly reactive lipid radicals. These radicals react with oxygen to form peroxy radicals, which then remove hydrogen atoms from other lipid molecules, creating hydroperoxides.231

These hydroperoxides eventually break down into volatile compounds, including aldehydes, ketones, esters, alcohols, and hydrocarbons. It is these specific volatile substances that produce the characteristic unpleasant odors and flavors associated with rancidity. While saturated fats are generally resistant to this process, unsaturated fats are highly susceptible. To delay this deterioration, manufacturers often add antioxidants, which terminate the chain reaction by donating hydrogen atoms to the reactive radicals.123