Understanding antioxidant mechanisms in preventing oxidation in foods - Oxidation in Foods and Beverages and Antioxidant Applications - page 225

species such as fatty acid peroxyl (LOO · ) or alkoxyl (LO · ) radicals by the
following reactions (Bolland, 1947).
LOO · or LO · + FRS ÿ! LOOH or LOH + FRS ·
Peroxyl radicals are thought to be the radicals most likely to interact with FRS
since the lower energy of peroxyl radicals limits them to interacting with
compounds with a labile hydrogen such as an unsaturated fatty acids or FRS.
Higher energy free radicals such as alkoxyl radicals and hydroxyl radicals can
abstract hydrogen from antioxidants but they will also undergo competing
reactions such the involvement of alkoxyl radicals in -scission reactions and
the interaction of high energy hydroxyl radicals in their immediate vicinity
(Buettner, 1993; Liebler, 1993a).
The ability of a FRS to inhibit lipid oxidation is linked to its ability to interact
with free radicals more efficiently than unsaturated fatty acids, and to produce
lower energy antioxidant radicals that do not readily promote further oxidation.
Chemical properties that are important to the effectiveness of a FRS include
reduction potential, hydrogen bond energies, resonance delocalization and
susceptibility to autoxidation. Initially, antioxidant efficiency is dependent on
the ability of the FRS to donate hydrogen to the free radical. As the bond energy
of the donating hydrogen on the FRS decreases, the transfer of the hydrogen to
the free radical is more energetically favorable and thus more rapid. One way to
predict the ability of a FRS to donate a hydrogen to a free radical is from
standard one electron reduction potentials (Buettner, 1993). Any compound
which has a reduction potential lower than the reduction potential of a free
radical (or oxidized species) is capable of donating a hydrogen to that free
radical unless the reaction is kinetically unfeasible (Table 10.1). Examples of
compounds with reduction potentials lower than peroxyl radicals (E 0
1000mV) include epigallocatechin 3-gallate (E 0 530 mV), -tocopherol
(E 0 480mV), quercetin (E 0 330 mV), and ascorbate (E 0 282 mV).
These lower reduction potentials mean that these compounds can donate a
hydrogen to peroxyl radicals to form a hydroperoxide. Standard reduction
potentials can also be used to predict the rate by which a compound can donate a
hydrogen to the peroxyl radical. For instance, the above antioxidants (282±
530 mV) have lower reduction potentials than the methylene interrupted
hydrogen of a polyunsaturated fatty acid (E 0 600 mV),allowing them to
react more rapidly with peroxyl radicals (E 0 1000mV) and, thus, being
preferentially oxidized before unsaturated fatty acids.
The ability of a FRS to inhibit lipid oxidation is also dependent on the energy
of the resulting free radical scavenger radical (FRS · ). If the FRS · is a low energy
radical, than the likelihood of the FRS · catalyzing the oxidation of other
molecules decreases. The most efficient FRS have low energy radicals due to
resonance delocalization (Fig. 10.1; Nawar, 1996; Shahidi, 1992). The low
energy of the FRS · can also be shown by standard reduction potentials where
FRS such as -tocopherol and catechol (480 and 530 mV respectively) have
lower reduction potentials than polyunsaturated fatty acids (600mV) and