Food Lipids: Chemistry, Nutrition, and Biotechnology

Membrane systems that reduce iron constitute another enzymic system of importance
in the process of lipid oxidation because they not only generate active
catalysts but do so in an environment consisting of highly unsaturated membrane
lipids. Membrane systems in muscle capable of generating active oxygen species in
the presence of NAD(P)H and ferric iron include sarcoplasmic reticulum [164] and
mitochondria [165]. Whereas the enzymic systems in beef and chicken utilize
NADPH preferentially [166,167], the enzymic system in fish utilizes NADH preferentially
[164]. In the latter case, both NADH-cytochrome b 5 reductase and cytochrome
b 5 have been associated with reduction of low and high concentrations of
ferric-histidine and low concentrations of ferric-ATP [168]. During storage, as NADH
concentrations drop to levels that are maximal to the enzymic system [164], stimulation
by this enzymic system would be expected to increase with time postmortem.
Myeloperoxidase is another enzyme that may be present in muscle systems
postmortem and capable of initiating lipid oxidation [169]. Normally found in neutrophils
of blood, myeloperoxidase may contaminate muscle tissues following
slaughter and spreading of blood over the surface of the product. Even when the
blood has been washed off, residual concentrations of myeloperoxidase may be sufficient
to accelerate oxidation in stored foods through the following reactions:
� �
HO 2 2 � Cl → HOCl � OH
�• � •
HOCl � O2 → O2 � Cl � OH
In lipoproteins and phospholipid liposomes, hypochlorous acid (HOCl) has been
found to initiate lipid oxidation [170] and may do so by interacting with organic
peroxides in vivo to form reactive radicals that subsequently initiate lipid oxidation
[171].
F. Antioxidants
By far the most important defense mechanism for lipid oxidation is the presence of
antioxidants, which can delay or slow the rate of oxidation of autoxidizable materials.
Inhibition may take two forms: a reduction in the rate at which the maximal level
of oxidation is approached or a reduction in the maximal level of oxidation. This
section focuses primarily on antioxidants endogenously present in muscle tissues,
with only limited discussion of antioxidants applied exogenously during processing.
1. Tocopherol
The main lipid-soluble antioxidant present in muscle tissue is tocopherol. ‘‘Tocopherol’’
is actually used as a generic description for mono-, di-, and trimethyl tocols
that contain a 6-chromanol ring structure with different numbers of methyl groups
at the 5-, 7-, and 8-positions and a saturated or unsaturated 16-carbon isoprenoid
side chain (Table 7). �-Tocopherol is the predominant form in muscle tissue of beef,
pork, chicken, and fish, although depending on the diet composition, �-tocopherol
and �-tocotrienol may also be present to varying degrees [53,172–175]. Studies
indicate that when �-tocopherol is supplied continuously in the diet, it accumulates
in the muscle but to a much smaller extent than when rats are fed similar levels of
�-tocopherol [176,177]. Under these conditions, �-tocopherol may instead tend to
be accumulated in fat deposits.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.