Food Lipids: Chemistry, Nutrition, and Biotechnology

For instance, the hydrogen of the hydroxyl group on �-tocopherol has a lower reduction
potential than the methylene-interrupted hydrogen of a polyunsaturated fatty
acid (E�� = 600 mV), thus allowing the �-tocopherol to react with peroxyl radicals
more rapidly than is possible for unsaturated fatty acids.
The efficiency of the FRS is also dependent on the energy of the resulting free
radical scavenger radical (FRS�). If the FRS� is a low energy radical, then the likelihood
that the FRS� will catalyze the oxidation of other molecules decreases. The
most efficient FRS have low energy radicals as a result of resonance delocalization
(Fig. 1) [5,6]. This can again be seen in standard reduction potentials, where FRS
such as �-tocopherol and catechol have lower reduction potentials than polyunsaturated
fatty acids and therefore do not efficiently abstract hydrogens from unsaturated
fatty acids (Table 1) [3]. Efficient FRS also produce radicals that do not react rapidly
with oxygen to form peroxides. When a radical scavenger forms peroxides during
oxidation, it is likely that it will autoxidize, thus depleting the system of the free
radical scavenger. Also, FRS peroxides can decompose into additional radicals species,
which could further promote oxidation. Thus, formation of FRS peroxides can
result in consumption of the antioxidant with no net decrease in free radicals numbers
[4].
FRS radicals may undergo additional reactions that remove radicals from the
system; examples include termination reactions with other FRS� or lipid radicals to
form nonradical species (Fig. 2). This means that each FRS is capable of inactivating
at least two free radicals, the first being inactivated when the FRS interacts with the
peroxyl radicals and the second when the FRS� enters a termination reaction with
another peroxyl radical.
Phenolics possess many of the properties of an efficient FRS. Hydrogen donation
generally occurs through the hydroxyl group, and the radical subsequently
formed is stabilized by resonance delocalization throughout the phenolic ring structure.
The effectiveness of phenolic FRS can be increased by substitution groups.
Alkyl groups in the ortho and para positions enhance the reactivity of the hydroxyl
hydrogen toward lipid radicals; bulky groups at the ortho position increase the stability
of phenoxy radicals; and a second hydroxy group at the ortho or para position
stabilizes the phenoxy radical through an intramolecular hydrogen bond [5]. In foods,
the efficiency of phenolic FRS depends on additional factors. Besides chemical reactivity,
factors such as volatility, pH sensitivity, and polarity can influence the retention
and activity of the FRS in stored and processed foods [6].
A. Tocopherols
Tocopherols are a group of phenolic FRS isomers originating in plants and eventually
ending up in animal foods via the diet [7]. Interactions between tocopherols and lipid
peroxyl radicals lead to the formation of a hydroperoxide and several resonance
structures of tocopheroxyl radicals (Fig. 3) [6]. Tocopheroxyl radicals can interact
with other compounds or with each other to form a variety of products. The types
and amounts of these products depend on oxidation rates, radical species, lipid state
(e.g., bulk vs. membrane lipids), and tocopherol concentration.
Under conditions of low oxidation rates in lipid membrane systems, tocopheroxyl
radicals primarily convert to tocopherylquinone. Tocopherylquinone can form
when the interaction of two tocopheroxyl radicals leads to the formation of to-
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