Antioxidants slow down the oxidation rates of foods by a combination of scavenging free radicals, chelating pro-oxidative metals, quenching singlet oxygen and photo-sensitizers, and inactivating lipoxygenase (LOX).
Free radical scavenging
The effectiveness of antioxidants to scavenge free radicals in foods depends on the bond dissociation energy between oxygen and a phenolic hydrogen, and reduction potential and delocalization of the antioxidant radicals. Vitamin E or α-tocopherol is a methylated phenol required in the human diet. Phenolic compounds primarily inhibit lipid oxidation through their ability to scavenge free radicals and convert the resulting phenolic radicals into a low-energy form that does not further promote oxidation. There is a synergistic effect between α-tocopherol and ascorbic acid. The tocopherol radical sometimes reacts with lipid peroxy radicals at very high concentration and produces tocopherol peroxide. Tocopherol peroxide produces two isomers of epoxy-8α-hydroperoxytocopherone which becomes epoxyquinones upon hydrolysis. This reaction produces alkoxy radicals, instead of peroxy radicals, and loses only tocopherol. Since there is no net decrease in free radicals in the system, tocopherol does not act as an antioxidant.
Pro-oxidative metals as chelating agents
Metals reduce the activation energy of the oxidation reaction, especially in the initiation step, to accelerate oil oxidation. Metals catalyze food radical formation by abstracting hydrogen. They also produce hydroxyl radicals by catalyzing decomposition of hydrogen peroxide or hydroperoxides. Ferric ions decrease the oxidative stability of olive oil by decomposing phenolic antioxidants such as caeic acid. Flavonoids are known to exhibit a strong metal chelating activity in addition to their antioxidant properties, with the arrangement of 4-keto and 5-OH, or 3′ and 4′-OH substituents resulting in the formation of chelating complexes between flavonoids and divalent cations. The extract of roots of Delphinium linearilobum (Trautv.) N. contains the alkaloid lycoctonine which exhibits the highest metal chelating activity.
Quenching Singlet Oxygen
Carotenoids with nine or more conjugated double bonds are good singlet oxygen quenchers by energy transfer. The singlet oxygen quenching activity of carotenoids depends on the number of conjugated double bonds in the structure and the substituent in the β-ionone ring. β-carotene and lycopene which have 11 conjugated double bonds are more effective singlet oxygen quenchers than lutein which has 10 conjugated double bonds. The presence of oxo and conjugated keto-groups, or cyclopentane ring in the structure increases the singlet oxygen quenching ability. β-carotene, tocopherols, ascorbic acid, amino acids, peptides and phenolics are oxidized by singlet oxygen and they are all chemical quenchers of singlet oxygen.