How does oxidation cause fats and oils in foods to become rancid?

Fats and oils become rancid through a process called autoxidation, a chemical deterioration where unsaturated fatty acids react with oxygen. This process is an autocatalytic, free-radical chain reaction that proceeds in three main stages:12

• Initiation: A free radical removes a hydrogen atom from a lipid molecule, creating a highly reactive lipid radical. This can be triggered by light, heat, or the presence of trace metals.12
• Propagation: The lipid radical reacts with molecular oxygen to form a peroxy radical, which then strips a hydrogen atom from another lipid molecule, continuing the cycle. This stage produces hydroperoxides, which further break down into alkoxy and peroxy radicals.1
• Termination: Radicals combine to form stable, nonpropagating products, including volatile aldehydes, ketones, and acids.12

These final compounds are responsible for the development of unpleasant, rancid off-flavours in foods such as nuts, fried snacks, and cooking oils. To mitigate this, antioxidants are often added to foods to terminate the chain reaction by donating hydrogen atoms to the chain carriers.12