Page 377 - Natural Antioxidants, Applications in Foods of Animal Origin

356 Natural Antioxidants: Applications in Foods of Animal Origin
VetBooks.ir with peroxyl radicals than BHA, due to the different nature of the radicals

formed on H-abstraction.
Tocopherols are approved as food additives with different E numbers:
E306 (tocopherol), E307 (α-tocopherol), E308 (γ-tocopherol), and E309
(δ-tocopherol).
Flavonoids are pigments widely distributed in vegetables where typically
impart a yellow color. Chemically, they have a general structure consisting
in a 15-carbon atoms skeleton integrated by two phenyl rings (named A and
B) and a heterocyclic ring (named C). Such carbon structure can be abbre-
viated C -C -C (A-C-B rings). The different classes of flavonoids differ in
3
6
6
the degree of oxidation and pattern of substitution in the C ring, while indi-
vidual compounds within a same class differ in the pattern of substitution
in the A and B rings (Pietta, 2000). Flavonoids, according to their chem-
ical structure are divided into five different classes: Anthoxanthins (which
include two subgroups, flavones, and flavonols), flavonones, flavanonols,
flavans (which include flavan-3-ols, flavan-4-ols, and flavan-3, 4-diols), and
anthocyanidins. The capacity of flavonoids to act as antioxidants in vitro has
been demonstrated by several studies, and important structure-antioxidant
activity relationships have been established (Pietta, 2000). Flavonoids are
generally primary antioxidants which act as free radical acceptors, breaking
the oxidation chain. Flavonols can also chelate metal ions at the 3-hydroxy-
4-keto-group, and/or the 5-hydroxy-4-keto-group (in the case in that the A
ring was hydroxylated at the fifth position).
It is generally recognized that the degree of hydroxylation and the posi-
tion of the hydroxyl groups determine the antioxidant activity of the flavo-
noids (Shahidi et al., 1992). The hydroxylation in the B ring is the major
factor for antioxidant activity. The o-dihydroxylation in the B ring actively
contributes to the antioxidant activity, and all the flavonoids with 3’-4’-dihy-
droxy configuration have antioxidant activity in more or less extent. Two
flavones, robinetin and myricetin, have an additional hydroxyl group placed
at their fifth position, which confers to these two molecules an enhanced
antioxidant activity in relation to the corresponding molecules that do not
possess such 5’-hydroxyl group (fisetin and quercetin). On the contrary, two
other flavones, naringenin, and hesperetin, have only a hydroxyl group in
the B ring, and due to this particularity they show little antioxidant activity.
Besides the hydroxylation in the B ring, other structural characteristics
affecting the A ring determine the antioxidant activity such as the presence
of a carbonyl group at the fourth position and a free hydroxyl group at third
and/or fifth positions.