Food Lipids: Chemistry, Nutrition, and Biotechnology

Despite the appearance of oxidative products in stored muscle foods, in some
studies investigators were not able to detect losses of unsaturated fatty acids [46–
48]. Failure to observe measurable losses in PUFAs during frozen storage of carp
may have been due to considerable fish-to-fish variation in lipid composition [49].
Even a paired-fillet technique designed to minimize the variability in triacylglycerol
composition of mackerel or catfish was insufficient to detect losses in total lipids or
the triacylglycerol fraction [50]. However, focusing on specific lipid fractions rather
than total lipid did lead to greater sensitivity in detection of losses of unsaturated
fatty acids, with losses occurring primarily in the phospholipid fraction [47,51,52].
2. Lipid Composition of Muscle
A wide degree of variation in lipid content exists among muscles from different
species (Table 4) [51,53,54]. Typically, in these muscles, the quantity of phospholipids
is 500 mg/100 g muscle, directly paralleling the actual amount of membrane [55].
The remainder of the lipid may therefore be considered to be primarily nonpolar
triacylglycerols. However, the level of triacylglycerol does not determine the oxidative
susceptibility of that sample. Rather, the relative reactivity and accessibility
to catalysts and inhibitors constitute the major determinants for identification of the
critical site of oxidation. Membrane lipids are distributed throughout the tissue, but
the triacylglycerols or storage lipids often are not. In chicken and trout muscle,
adipose cells containing the triacylglycerols were primarily found in peripheral subcutaneous
fat; in red meat muscle, adipose cells were found both between and within
muscle fibers; and in salmon muscle, adipose cells were mainly distributed in the
myosepta and, to a lesser extent, in the connective tissue surrounding bundles of
white muscle fibers [56,57]. One must keep in mind, though, that adipose cells are
bounded by membrane phospholipids. Oxidation of either fraction may then spread
to the other lipid site. When such spreading occurred for a peroxidizing fish microsomal
fraction system, emulsified lipids added to the system were oxidized [58].
Carbon-centered lipid radicals, which have been found in the extracellular medium
of oxidizing cells, may be the vehicle by which oxidation of other adjacent lipid
structures occurs [59].
As shown in Table 4, the degree of polyunsaturation in the muscle varies
depending on the species. To understand the importance of this component to oxi-
Table 4 Lipid Composition of Raw Muscle Tissues
Muscle/strain/variety % Fat % PUFA Ref.
Channel Aqua 3.7 24.4 51
LSU 5.4 22.1 51
Tilapia Red 1.7 26.1 53
Blue 1.9 24.5 53
Beef Longissimus dorsi 4.4 4.9 54
Semimembranosus 3.5 6.6 54
Pork Longissimus dorsi 4.5 5.3 54
Semimembranosus 3.3 9.2 54
Chicken Breast 1.4 18.9 54
Thigh 6.0 15.5 54
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