Food Lipids: Chemistry, Nutrition, and Biotechnology

alkyl bisallylic
↓ ↓
R— CH — — — — — —
2 CH2 CH— CH CH2 CH— CH— R�
↑
allylic
Lower bond energies for bisallylic and allylic hydrogens versus methylene hydrogens
(75 and 88 vs. 100 kcal/mol, respectively), as well as resonance stabilization of the
radical intermediate, contributes to ease of abstraction from unsaturated fatty acids
[25,26]. The newly formed hydroperoxy radical can in turn abstract hydrogen from
an adjacent unsaturated fatty acid such that the reaction sequence goes through 8 to
14 propagation cycles before termination [27]. Conditions that determine the chain
propagation length include initiation rate, structures of aggregates (increasing with
increasing structure of the aggregates), temperature, presence of antioxidants, and
chain branching. Chain branching involves the breakdown of fatty acid hydroperoxides
to the lipid peroxyl or alkoxyl radical. Given the bond dissociation energies of
LOO–H (about 90 kcal/mol) and LO–OH (about 44 kcal/mol), spontaneous decomposition
is unlikely at refrigerated or freezing temperatures [28]. Instead, breakdown
of hydroperoxides would be dominated by one-electron transfers from metal ions
during low temperature storage.
2� 3� • �
Fe � LOOH → Fe � LO � OH
The major contributors to decomposition of lipid hydroperoxides in food and biological
systems would be heme and nonheme iron, with reactions involving the
ferrous ion occurring much more quickly than those involving ferric ion.
C. Termination
To break the repeating sequence of propagating steps, two types of termination reactions
are encountered: radical–radical coupling and radical–radical disproportionation,
a process in which two stable products are formed from A • and B • by an atom
or group transfer process. In both cases, nonradical products are formed. However,
the termination reactions are not always efficient. When coupling gives rise to tertiary
tetroxides, they decompose to peroxyl radicals at temperatures above �80�C and to
alkoxyl radicals at temperatures above �30�C [29]. Secondary and primary peroxyl
radicals, on the other hand, terminate efficiently by a mechanism in which the tetroxide
decomposes to give molecular oxygen, an alcohol, and a carbonyl compound.
III. MUSCLE COMPOSITION AND LIPID OXIDATION
A. Muscle Structure and Function
Before discussing individual constituents of muscle tissue, a review of the structural
and chemical features that contribute to muscle tissue’s oxidative stability will be
taken. Within the animal body, there are more than 600 muscles varying widely in
shape, size, and activity. However, at the cellular level there is close resemblance
among muscles from a wide variety of organisms. The typical arrangement of a
skeletal muscle in cross section consists of epimysium (connective tissue surrounding
the entire muscle), perimysium (connective tissue separating the groups of fibers into
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.