Page 295 - Natural Antioxidants, Applications in Foods of Animal Origin

274 Natural Antioxidants: Applications in Foods of Animal Origin
VetBooks.ir in foods depends on the bond dissociation energy between oxygen and a

phenolic hydrogen, reduction potential, and delocalization of the antioxi-
dant radicals (Choe & Min, 2006; Cao et al., 2007). Phenolic compounds
primarily inhibit lipid oxidation through their ability to scavenge free radi-
cals and convert the resulting phenolic radicals into a low-energy form
that does not further promote oxidation. Flavonoids are known to exhibit
a strong metal chelating activity in addition to their antioxidant properties,
with the arrangement of 4-keto and 5-OH, or 3' and 4'-OH substituents
resulting in the formation of chelating complexes between flavonoids and
divalent cations (Cheng & Breen, 2000). Carotenoids with nine or more
conjugated double bonds are good O quenchers by energy transfer. The
1
2
1 O quenching activity of carotenoids depends on the number of conju-
2
gated double bonds in the structure (Min & Boff, 2002; Foss et al., 2004)
and the substituent in the β-ionone ring (Di Mascio & Sies, 1989). Beta-
carotene and lycopene which have 11 conjugated double bonds are more
effective O quenchers than lutein which has 10 conjugated double bonds
1
2
(Viljanen et al., 2002). The reaction mechanisms of a primary antioxidant,
AH (Antunes et al., 1999) and secondary antioxidant BH, is shown in
Figure 7.1.

a) Reaction of primary antioxidant (AH) with lipid radical
AH + ROO•  ROOH + A•
RH + A•  AH + A•
AH + ROO•  [ROO•AH] Complex

b) Termination reaction
[ROO•AH]  non-radical product

A• + A•  AA
A• + R•  RA
A• + ROO•  ROOA

c) Regeneration of primary antioxidant
A• + BH  AH + B•

FIGURE 7.1 Reaction mechanism of primary antioxidant with free radical. AH, antioxidant;
ROO•, lipid peroxyl radical; ROOH, hydroperoxide; A•, antioxidant free radical; RH,
unsaturated lipid; R•, lipid radical; ROO• AH, stable compound (non-radical product); BH,
secondary hydrogen donor; B•, secondary antioxidant free radical (Antunes et al., 1999).