Carotenoids are a chemically diverse group (>600 different compounds) of yellow to red colored polyenes consisting of 3–13 conjugates double bonds and in some cases, six carbon hydroxylated ring structures at one or both ends of the molecule. β-Carotene is the most extensively studied carotenoid antioxidant (Fig. 2). β-Carotene will react with lipid peroxyl radicals to form a carotenoid radical. Whether this reaction is truly antioxidative seems to depend on oxygen concentrations, with high oxygen concentrations resulting in a reduction of antioxidant activity. The proposed reason for loss of antioxidant activity with increasing oxygen concentrations involves the formation of carotenoid peroxyl radicals that autoxidizes into additional free radicals. Under conditions of low oxygen tension, the carotenoid radical would be less likely to autooxidize and thus could react with other free radicals thereby forming nonradical species with in a net reduction of radical numbers.
The major antioxidant function of carotenoids in foods is not due to free radical scavenging but instead is through its ability to inactivate singlet oxygen. Singlet oxygen is an excited state of oxygen in which two electrons in the outer orbitals have opposite spin directions. Initiation of lipid oxidation by singlet oxygen is due to its electrophillic nature, which will allow it to add to the double bonds of unsaturated fatty acids leading to the formation of lipid hydroperoxides. Carotenoids can inactivate singlet oxygen by both chemical and physical quenching. Chemical quenching results in the direct addition of singlet oxygen to the carotenoid, leading to the formation of carotenoid breakdown products and loss of antioxidant activity.Amore effective antioxidative mechanism of carotenoids is physical quenching. The most common energy states of singlet oxygen are 22.4 and 37.5 kcal above ground state. Carotenoids physically quench singlet oxygen by a transfer of energy from singlet oxygen to the carotenoid, resulting in an excited state of the carotenoid and ground state, triplet oxygen. Harmless transfer of energy from the excited state of the carotenoid to the surrounding medium by vibrational and rotational mechanisms then takes place. Nine or more conjugated double bonds are necessary for physical quenching, with the presence of six carbon oxygenated ring structures at the end the molecule increasing the effectiveness of singlet oxygen quenching.
In foods, light will activate chlorophyll, riboflavin, and heme-containing proteins to high energy excited states. These photoactivated molecules can promote oxidation by direct interactions with an oxidizable compounds to produce free radicals, by transferring energy to triplet oxygen to form singlet oxygen or by transfer of an electron to triplet oxygen to form the superoxide anion. Carotenoids inactivate photoactivated sensitizers by physically absorbing their energy to form the excited state of the carotenoid that then returns to the ground state by transfer of energy into the surrounding solvent.
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