Food Chemistry

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 thehalf-chair
conformation, the ether oxygen is so strongly
fixed in the chromanoxyl radical that the de-
viation 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 antioxi-
dant in aqueous media, but only at higher concen-
trations (∼ 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 enhancedby the addition of fat sol-
uble ascorbyl palmitate or ascorbic acid in com-
bination with an emulsifier (e. g. lecithin) since
the formed tocopherol radical from reaction 2 in
Fig. 3.35 is rapidly reduced toα-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 ap-

proximately as high as that ofα-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 acidsa,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

aTest system: linoleic acid micelles stabilized with Na

dodecyl sulfate (pH 7.4, T: 50◦C).

bRAA with reference to the activity ofα-tocopherol.