Contrary to antioxidants, prooxidants decrease the stability of food lipids on storage. The most important prooxidants are heavy metals of transient valency, especially copper, manganese and iron ions or complexes. They catalyze the decomposition of lipid hydroperoxides into free radicals, which initiate further oxidation of unsaturated fatty acid derivatives. Another type of prooxidants are photosensitizers. Green parts of plant material always contain chlorophyll pigments, which are photosensitizers in presence of light, catalyzing the oxidation of polyunsaturated lipids by converting the triplet oxygen into substantially more reactive singlet oxygen. The presence of carotenoids can transform the singlet oxygen back into its less reactive triplet form. Stability against oxidation is thus much improved. Carotenes and different carotenoids have thus an antioxidative activity, at least in the presence of light.
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