Understanding antioxidant mechanisms in preventing oxidation in foods - Oxidation in Foods and Beverages and Antioxidant Applications - page 234

10.2.3 Other naturally occurring free radical scavengers
Carotenoids
Carotenoids contain 3-13 conjugated double bonds and in some cases 6 carbon
hydroxylated ring structures at one or both ends of the molecule (Olson, 1993).
The carotenoid, -carotene, is the major dietary source of vitamin A. Vitamin A
is often referred to as an antioxidant vitamin but its effectiveness along with the
effectiveness of other carotenoids is extremely dependent on environmental
conditions and the nature of oxidation catalyst. For example, carotenoids can be
effective antioxidants in the presence of singlet oxygen (discussed in later
sections) and can inhibit free radical promoted oxidation under conditions of low
oxygen concentrations.
The free radical scavenging mechanism of -carotene involves its reaction
with lipid peroxyl radicals to form a carotenoid radical. Under conditions of high
oxygen tension, the antioxidant activity of -carotene is diminished due to the
increased formation of carotenoid peroxyl radicals that favor autoxidation over
inactivation of lipid peroxyl radicals. Under conditions of low oxygen tension,
the lifetime of the carotenoid radical is long enough so that it may react with
another peroxyl radical thus forming a nonradical species and effectively
inhibiting oxidation by eliminating radicals from the system (Burton, 1984).
Incubation of -carotene with peroxyl radical generators in organic solvents
at high (atmospheric) oxygen tensions leads to additional reactions to form
carotenoid-peroxyl adducts. Addition of a peroxyl radical to the cyclic end
group or the polyene chain followed by loss of alkoxyl radicals leads to the
formation of 5, 6 and 15, 15 0 epoxides. Elimination of the alkoxyl radical from
the 15, 15 0 positions can also cause cleavage of the polyene chain resulting in
formation of aldehydes. Since the formation of -carotene epoxides from the
addition of peroxyl radicals results in the formation of an alkoxyl radical, the net
change in radical number is zero and thus an antioxidant effect is not expected
(Liebler, 1993b).
-Carotene can also inactivate peroxyl radicals by donation of an electron to
produce a -carotene cation radical and a peroxyl anion. The resulting cation
radical is less likely to react with oxygen to form hydroperoxides due to its
resonance stabilization over the conjugated double bond system. However, the
-carotene cation radical has sufficient energy to oxidize other lipophilic
hydrogen donors, including tocopherols and ubiquinone (Liebler, 1993b).
Overall, carotenoids are not very effective free radical scavengers in food
systems. In addition, carotenoids can be prooxidative through their ability to
reduce metals (Boon, 2009). Finally, even if conditions in a food product were
favorable to allow effective free radical scavenging, these reactions lead to the
bleaching of the carotenoid and thus result in a change in the color of the food
during storage.
Ascorbic acid
Biological systems contain both lipid- and water-soluble free radicals. Water
soluble free radicals are generated by processes such as the Fenton reaction