Food Lipids: Chemistry, Nutrition, and Biotechnology

A. Initiation
The direct reaction of a lipid molecule with a molecule of oxygen is highly improbable
because the lipid molecule is in a singlet electronic state and the oxygen molecule
has a triplet ground state. To circumvent this spin restriction, oxygen can be
activated by any of the following three initiation mechanisms: (1) formation of singlet
oxygen; (2) formation of partially reduced or activated oxygen species such as
hydrogen peroxide, superoxide anion, or hydroxyl radical; and/or (3) formation of
active oxygen–iron complexes (ferryl iron or ferric–oxygen–ferrous complex). In
addition, the oxidation of fatty acids may occur either directly or indirectly through
the action of enzyme systems, of which three major groups are involved: microsomal
enzymes, peroxidases, and dioxygenases, such as lipoxygenase or cyclooxygenase.
That such chemical and enzymatic reactions exist in living tissue is evidenced by
the occurrence in aerobic organisms of enzymes that can eliminate or detoxify these
compounds (e.g., superoxide dismutase, catalase, glutathione peroxidase). Therefore,
activated oxygen species are likely to be present in the food item even before it is
harvested, not just produced during processing and storage. As for which mechanism
of initiation is primarily responsible, a large volume of research has been published
exploring this issue but no consensus has arisen. The reader is therefore encouraged
to look in the reviews of Kanner et al. [21], Hsieh and Kinsella [22], Kappus [23],
and Bradley and Min [24] for a more in-depth look at mechanisms of initiation.
However, additional information on specific sources of initiation will be presented
in Section III.E.
B. Propagation
Propagation reactions form the basis of the chain reaction process and in general
include the following:
Radical coupling with oxygen: R • � O 2 → ROO •
Atom or group transfer: ROO • � RH → ROOH � R •
Fragmentation: ROO • → R • � O 2
Rearrangement:
Cyclization:
In oxygen radical coupling, molecular oxygen reacts with the carbon-centered free
radical at or near the diffusion-controlled rate of approximately 10 9 M �1 s �1 . A major
consequence of this reactivity is that the concentration of R • is much smaller than
• •
that of RO 2. In atom transfer, whereas a peroxyl radical, RO 2,
will not readily abstract
hydrogen from a saturated hydrocarbon, it will do so very readily from allylic and
bisallylic C–H bonds of unsaturated fatty acids.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.