Antioxidants and Shelf Life of Foods - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 189

M ECHANISMOF A CTION

Phenolic compounds function as primary antioxidants by performing the role of free
radical terminators. They interfere with lipid oxidation by rapidly donating a hydrogen
atom to the lipid radicals, and the efficiency of these antioxidants (AH) increases
with decreasing A-H bond strength. Phenolic antioxidants are excellent hydrogen or
electron donors, and their radical intermediates are relatively stable due to resonance
delocalization and general lack of suitable sites for attack by molecular oxygen. 31
The reaction of a phenol with a lipid radical forms a phenoxy radical, which is
stabilized by delocalization of unpaired electrons around the aromatic ring. 13,31
While phenol itself is not active as an antioxidant, substitution in the ortho and
para positions with alkyl groups (ethyl or n-butyl) increases the electron density of
the OH moiety by inductive effect enhancing its reactivity toward lipid radicals. The
introduction of a second hydroxyl group at the ortho or para position of a phenol
also increases its antioxidant activity. 13,31,73 The stability of the phenoxy radical is
increased by bulky groups at the ortho positions. 13,31 However, the presence of bulky
substituents in the 2 and 6 positions reduces the rate of reaction of the phenol with
lipid radicals. This steric effect opposes the increased stabilization of the radical,
and both effects must be considered in assessing the overall activity of an antioxi-
dant. 13 Namicki 74 confirmed that o -dihydroxylation enhanced antioxidant activity of
phenolic compounds while methoxylation of the hydroxyl groups drastically reduced
this effect. Milic and co-workers 75 recently showed that the ability of phenolic acids
to scavenge lipid alkoxy radicals depended on their structure and the number and
position of the hydroxyl groups. Using an ESR spin trapping technique they showed
the antioxidant effect increased in the order of gallic > caffeic > chlorogenic >
vanillic > salicylic acid for a hydroperoxide-enriched sunflower oil model system
( Figure 6.13 ).
Luzia et al. 76 showed that 5-caffeoylquinic acid, an ester of quinic acid with
caffeic acid found in some vegetables, behaved as a primary antioxidant in soybean
oil. Further work on the oxidative stability of soybean oil by Luzia et al. 77 suggested
5-caffeoylquinic acid acted as both a primary antioxidant and a metal chelator. The
antioxidant activity was attributed to the ortho-dihydroxy grouping in the structure
of 5-caffeoylquinic acid ( Figure 6.14 ).
It is generally accepted that the position and degree of hydroxylation is of primary
importance in determining the effectiveness of flavonoids as antioxidants and that
hydroxylation of the B-ring is the major consideration for antioxidant activity. 31,78
Studies by Foti and co-workers 73 on flavonoids in micelles, however, did not find the
number of hydroxy groups or their location on the B vs. A ring to be significant. All
flavonoids with o -dihydroxylation of the B ring, with 3

dihydroxy configura-
tion, possess antioxidant activity. 13,31,78 An additional hydroxyl group at the 5

′ and 4 ′

position
further enhanced antioxidant activity, which explains why flavonols such as myricetin
are more effective than quercetin ( Figure 6.11 ). 31 A carbonyl group at position 4 and
a free hydroxyl group at positions 3 and/or 5 also had an effect. In addition to the
carbonyl group, a 2,3 double bond in the central ring participates in the radical
stabilization, resulting in increased antioxidant activity. 73,79 The o -dihydroxy grouping

′