Chemistry Reference
In-Depth Information
3.4.2 -tocopherol
Vitamin E or -tocopherol is generally considered to be a very potent natural
antioxidant. However, the antioxidant activity of tocopherols decreases at higher
levels and becomes prooxidant above the optimum concentration (Jung and Min,
1990). Oxidation products from -tocopherol have been shown to generate
singlet oxygen (Barclay et al., 1989).
Kim et al. (2007) oxidized -tocopherol in methanol containing methylene
blue for 30 hours under light. Afterwards the effect of different concentrations of
oxidized -tocopherol on the oxidative stability of purified soybean oil was
studied in the absence of light. A mechanism was proposed for the reaction of
singlet oxygen with -tocopherol. Two moles of -tocopherol peroxy radical
form dimerized -tocopherol peroxide and singlet oxygen. The prooxidant
effect of -tocopherol at high concentration could be due to high amounts of
peroxy radical and oxy radical of -tocopherol, hydroxyl radical, and singlet
oxygen formed from tocopherol during storage. This shows that that reaction of
vitamin E with singlet oxygen generated during storage in light can even lead to
the formation of singlet oxygen and free radicals during storage in the dark.
3.4.3 Food colorants
Many food colorants were found to be potent photosensitizers (Pan, 2005b). The
effect of a wide variety of food colorants on the photosensitized oxidation of
methyl linoleate has been investigated. Rose Bengal, erythrosine B and phloxine
B accelerated oxidation of methyl linoleate under light exposure and their
prooxidative effects were concentration-dependent. Light exposure of methyl
linoleate with added colorants induced the generation of hydroperoxide isomers,
including 10-cis,trans- and 12-cis,trans-methyl linoleate hydroperoxide. Based
on the different reactivity of singlet and triplet oxygen, the introduction of
oxygen at these positions suggests that the food colorants served as photo-
sensitizers. The addition of the singlet oxygen quenchers -tocopherol or -
carotene effectively suppressed the oxidation of methyl linoleate, and their
antioxidative effects were concentration-dependent. The study also revealed that
the presence of a xanthene skeleton with an increased number of halogen
substituents increased the potential as photosensitizer. Because agricultural food
products and seafood products often contain these food colorants, photo-
sensitized lipid oxidation can shorten shelf life significantly if packaged in
transparent material.
3.4.4 Emulsifiers
Phosphatidylcholine (PC), a principal component of the food emulsifier lecithin
has been shown to quench singlet oxygen in two model systems (Lee and Choe,
2008). In a non-food microemulsion it could be shown that PC added at different
concentrations clearly quenched singlet oxygen. Also a food emulsion model
was prepared with sunflower oil, distilled water, and xanthan gum. Also PC was
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