2009_Book_FoodChemistry

3.7 Changes in Acyl Lipids of Food 217
In the reaction with peroxyl radicals, the higher
rate of tocopherols compared with DBHA
(cf. Table 3.39) is based on the fact that the
chromanoxyl radical formed on H-abstraction
is more stable than the phenoxyl radical. Both
types of radical are stabilized by the following
resonance:
(3.83)
This resonance effect is the highest when the
orbital of the 2p electron pair of the ether oxygen
and the half occupied molecule orbital of the
radical oxygen are aligned parallel to each other,
i. e., vertical to the plane of the aromatic ring.
Any deviation lowers the stability, slowing
down H-abstraction. Due to incorporation into
a six-membered ring present in the half-chair
conformation, the ether oxygen is so strongly
fixed in the chromanoxyl radical that the deviation
is only 17 ◦ . The methoxy group in the
DBHA phenoxyl radical is freely rotatable so that
the orbital of the 2p electron pair is oriented to
the plane of the aromatic ring; thus the deviation
is ∼90 ◦ . BHT reacts even slower than DBHA
(Table 3.39) because there is no ether oxygen.
Ascorbic acid (cf. 6.3.9) is active as an antioxidant
in aqueous media, but only at higher concentrations
(∼10 −3 mol/l). A prooxidant activity is
observed at lower levels (10 −5 mol/l), especially
in the presence of heavy metal ions. The effect of
tocopherols is enhanced by the addition of fat soluble
ascorbyl palmitate or ascorbic acid in combination
with an emulsifier (e. g. lecithin) since
the formed tocopherol radical from reaction 2 in
Fig. 3.35 is rapidly reduced to α-tocopherol by
vitamin C.
(3.84)
Carotinoids also can act as scavengers for
alkyl radicals. Radicals stabilized by resonance
are formed (Formula 3.84), unable to initiate
lipid peroxidation. β-Carotenes are most active
at a concentration of 5 · 10 −5 mol/l, while at
higher concentrations the prooxidative effect is
predominant. Also the partial pressure of oxygen
is critical, it should be below 150 mm Hg.
Phenolic compounds (cf. 18.1.2.5) which are
widely distributed in plant tissues, act as natural
antioxidants. The protective effect of several
herbs, spices (e. g. sage or rosemary) and tea
extracts against fat (oil) oxidation is based
on the presence of such natural antioxidants
(cf. 21.2.5.1 and 22.1.1.4). The antioxidative
effect of phenols depends on the pH. It is low in
an acidic medium (pH 4) and high in an alkaline
medium (pH 8) when phenolation occurs.
In the protection of linoleic acid micelles,
the antioxidative activity of quercetin is approximately
as high as that of α-tocopherol
(Table 3.40). The activity of the two synthetic
dihydroxyflavones at 70% and 63% is also
high. Therefore, it is not only the number of
OH-groups in the molecule, but the presence of
OH-groups in the ortho position that is important.
But this characteristic feature is not enough to
explain the high activity of quercetin compared
with that of catechin, which is four times less
active (Table 3.40) although the OH patterns
correspond. Obviously the carbonyl group, which
is absent in catechin, increases the stability of
the phenoxyl radical by electron attraction which
Table 3.40. Relative antioxidative activity (RAA) of
flavonoids, cumarins and hydroxycinnamic acids a,b
Compound RAA × 100
α-Tocopherol 100
Quercetin (cf. Formula 18.32) 90
Cyanidin 90
Catechin (cf. Formula 18.20) 22
6,7-Dihydroxyflavonc 70
7,8-Dihydroxyflavone 63
7,8-Dihydroxycumarin 3.3
Ferulic acid < 0.1
Caffeic acid < 0.1
a Test system: linoleic acid micelles stabilized with Na
dodecyl sulfate (pH 7.4, T: 50 ◦ C).
b RAA with reference to the activity of α-tocopherol.