Edible oil is oxidized during processing and storage via autoxidation and photosensitized oxidation, in which triplet oxygen ( 3 O 2 ) and singlet oxygen ( 1 O 2 ) react with the oil, respectively. Autoxidation of oils requires radical forms of acylglycerols, whereas photosensitized oxidation does not require lipid radicals since 1 O 2 reacts directly with double bonds. Lipid hydroperoxides formed by 3 O 2 are conjugated dienes, whereas 1 O 2 produces both conjugated and nonconjugated dienes. The hydroperoxides are decomposed to produce off-flavor compounds and the oil quality decreases. Autoxidation of oil is accelerated by the presence of free fatty acids, mono-and diacylglycerols, metals such as iron, and thermally oxidized compounds. Chlorophylls and phenolic compounds decrease the autoxidation of oil in the dark, and carotenoids, tocopherols, and phospholipids demonstrate both antioxidant and prooxidant activity depending on the oil system. In photosensitized oxidation chlorophyll acts as a photosensitizer for the formation of 1 O 2 ; however, carotenoids and tocopherols decrease the oxidation through 1 O 2 quenching. Temperature, light, oxygen concentration, oil processing, and fatty acid composition also affect the oxidative stability of edible oil.

Deep-fat frying produces desirable or undesirable flavor compounds and changes the flavor stability and quality of the oil by hydrolysis, oxidation, and polymerization. Tocopherols, essential amino acids, and fatty acids in foods are degraded during deep-fat frying. The reactions in deep-fat frying depend on factors such as replenishment of fresh oil, frying conditions, original quality of frying oil, food materials, type of fryer, antioxidants, and oxygen concentration. High frying temperature, the number of fryings, the contents of free fatty acids, polyvalent metals, and unsaturated fatty acids of oil decrease the oxidative stability and flavor quality of oil. Antioxidant decreases the frying oil oxidation, but the effectiveness of antioxidant decreases with high frying temperature. Lignan compounds in sesame oil are effective antioxidants in deep-fat frying.

Antioxidants delay or inhibit lipid oxidation at low concentration. Tocopherols, ascorbic acid, carotenoids, flavonoids, amino acids, phospholipids, and sterols are natural antioxidants in foods. Antioxidants inhibit the oxidation of foods by scavenging free radicals, chelating prooxidative metals, quenching singlet oxygen and photosensitizers, and inactivating lipoxygenase. Antioxidants show interactions, such as synergism (tocopherols and ascorbic acids), antagonism (␣-tocopherol and caffeic acid), and simple addition. Synergism occurs when one antioxidant is regenerated by others, when one antioxidant protects another antioxidant by its sacrificial oxidation, and when 2 or more antioxidants show different antioxidant mechanisms.

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Reactive oxygen species (ROS) are formed enzymatically, chemically, photochemically, and by irradiation of food. They are also formed by the decomposition and the inter-reactions of ROS. Hydroxy radical is the most reactive ROS, followed by singlet oxygen. Reactions of ROS with food components produce undesirable volatile compounds and carcinogens, destroy essential nutrients, and change the functionalities of proteins, lipids, and carbohydrates. Lipid oxidation by ROS produces low molecular volatile aldehydes, alcohols, and hydrocarbons. ROS causes crosslink or cleavage of proteins and produces low molecular carbonyls from carbohydrates. Vitamins are easily oxidized by ROS, especially singlet oxygen. The singlet oxygen reaction rate was the highest in ss-carotene, followed by tocopherol, riboflavin, vitamin D, and ascorbic acid.