Page 87 - Natural Antioxidants, Applications in Foods of Animal Origin

66 Natural Antioxidants: Applications in Foods of Animal Origin
VetBooks.ir (quercetin, catechin, and condensed tannins), phenolic acids (caffeic acid)

and esters (chlorogenic acid, gallotannins), stilbenes (resveratrol), phenolic
alcohols (hydroxytyrosol), and so forth (Laguerre et al., 2014). POH are
natural antioxidants and integral bioactive molecules of an oil or fat. POH
act as free radical scavengers and metal chelators (Dai & Mumper, 2010).
POH act as free radical acceptors and chain breakers. They interfere with
the oxidation of lipids and other molecules by rapid donation of a hydrogen
atom to radicals (R) ,that is, R + POH → RH + PO . The phenoxy radical
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intermediates (PO ) are relatively stable due to resonance and therefore a
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new chain reaction is not easily initiated. Moreover, the phenoxy radical
intermediates also act as terminators of propagation route by reacting with
other free radicals, that is, PO + R → POR. POH possess ideal structure
•
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chemistry for free RSAs because they have, (a) phenolic hydroxyl groups
that are prone to donate a hydrogen atom or an electron to a free radical and
(b) extended conjugated aromatic system to delocalize an unpaired electron.
Several relationships between structure and reduction potential of pheno-
lics have been established. For phenolic acids and their esters, the reduction
activity depends on the number of free hydroxyl groups in the molecule.
Hydroxycinnamic acids were found to be more effective than their hydroxy-
benzoic acid counterparts, possibly due to the aryloxy-radical stabilizing
effect of the ─CH═CH─COOH linked to the phenyl ring by resonance (Dai
& Mumper, 2010).

2.12 CAROTENOIDS

Carotenoids are a class of hydrocarbons consisting of eight isoprenoid units
joined in such a manner that the arrangement of isoprenoid units is reversed
at the center of the molecule so that the two central methyl groups are in a
1,6-positional relationship and the remaining non-terminal methyl groups
are in a 1,5-positional relationship. Carotenoids are defined by their chemical
structure. The majority of carotenoids are derived from a 40-carbon polyene
chain, which could be considered the backbone of the molecule. This chain
may be terminated by cyclic end-groups (rings) and may be complemented
with oxygen-containing functional groups (Zeb & Mehmood, 2004). These
hydrocarbons are commonly known as carotenes, while oxygenated deriva-
tives of these hydrocarbons are known as xanthophylls. β-carotene, the prin-
cipal carotenoid in carrots, is a familiar carotene, while lutein, the major
yellow pigment of marigold petals, is a common xanthophyll. The struc-
ture of a carotenoid ultimately determines what potential biological function