Food Lipids: Chemistry, Nutrition, and Biotechnology

Table 2 Biochemical Changes and Negative Consequences in Postmortem Muscle Food
Biochemical change Consequence
Decrease in ATP Loss of energy source needed for reduction of
many compounds
Increase in hypoxanthine Off-flavor produced by ATP degradation
Conversion of hypoxanthine
Xanthine oxidase can initiate oxidation when
dehydrogenase to xanthine oxidase molecular oxygen is present in system
Decrease in ascorbate, glutathione Loss of secondary antioxidants and cofactors
Increase in low molecular weight iron Initiator of oxidation
Oxidation of myoglobin to porphyrin Can react with hydrogen peroxide to produce
radical
ferryl oxene (Fe 4� ), which can initiate lipid
oxidation
Loss of tocopherol Loss of primary antioxidant
Disintegration of membranes Could cause hydrolysis of phospholipids,
uneven maintenance of ions
Loss of Ca 2� sequestration Increased calcium ion content in aqueous phase
causes many inactive processes to become
active
white muscles (i.e., muscles dominated by white fibers) presumably best suited for
vigorous activity for a short period and red muscles for sustained activity. Red fibers
tend to be smaller than white fibers and contain more mitochondria, possess greater
concentrations of myoglobin and lipid, have a thicker sarcolemma and a much less
extensive and more poorly developed SR, and have less sarcoplasm; however, red
fibers are more generously supplied with blood than are white fibers. All of these
differences lead to differences in oxidative stability.
Fish muscle is noticeably different from avian or mammalian muscles. Containing
a larger percentage of myofibrillar protein than mammalian skeletal muscle,
fish are characterized by a large percentage of unsaturated fatty acids. Changes in
composition in response to environment are common. For example, fish that live in
a low temperature environment have a larger fraction of dark muscle than fish that
live in a warmer environment [32]. Caution is necessary when sampling fish for
oxidative stability as local differences exist [32–35]. The most susceptible portion
of herring fillets to oxidation during ice and frozen storage was found under the skin
and was attributed to the large proportion of dark muscle in the sample compared
with the middle and inner parts [33,34]. However, isolation and subsequent analysis
of white and dark muscle will not always ensure a uniform response to oxidation.
While white muscle is considered to be very uniform in composition no matter where
it is located on the fish, dark muscle varies in composition as a function of its
location, containing more lipid in the anterior part of the fish and more water and
protein in the posterior part [32].
To circumvent the inherent variability in composition that occurs between meat
cuts, experimental studies often isolate subcellular membranes from muscles and
measure their response to oxidative catalysts. Depending on the degree of purification,
wide variability may exist among isolated membranes. For instance, a microsomal
fraction may contain membranes from SR, mitochondria, Golgi bodies, ly-
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.