Oxidative degradation of a substance by photoreaction. (Poly)unsaturated fatty acids can undergo two types of photooxidation: (1) a free-radical chain reaction, which starts from the excited state of another molecule, and (2) a singlet oxygen (IO2) reaction in which the absorption of photons by molecules of another food component is followed by energy transfer to ground-state oxygen, leading to the formation of singlet oxygen. Photooxidation of foods can be prevented by using packaging materials that absorb the photochemically active light and by removing endogenous photosensitizers and oxygen from the food.