Food Lipids: Chemistry, Nutrition, and Biotechnology

undles), and endomysia (sheaths of connective tissue surrounding each muscle fiber).
Also surrounding each muscle fiber is the sarcolemma membrane, which periodically,
along the length of the fiber, forms invaginations usually referred to as T
tubules.
Within the muscle fiber, the sarcoplasm serves to suspend organelles such as
mitochondria and lysosomes. Water constitutes 75–80% of the sarcoplasm but in
addition may contain lipid droplets, variable quantities of glycogen granules, ribosomes,
proteins, nonprotein nitrogenous compounds, and a number of inorganic
constituents.
Myofibrils, which are organelles found only in muscle cells, are long, thin,
cylindrical rods that extend the entire length of the muscle fiber and constitute the
contractile apparatus, which is composed of primarily myosin and actin proteins.
Also intracellular in nature is the sarcoplasmic reticulum (SR). Forming a closely
meshed network around each myofibril, SR membranes serve as the storage site for
Ca 2� in resting muscle fibers.
The contractile process is a complex mechanism that starts with an action
potential spreading through the T tubules and sarcolemma, finally reaching the sarcoplasmic
reticulum. At the SR membrane, depolarization occurs, stimulating the
release of calcium. The released calcium binds to troponin causing a conformational
change in the protein, which in turn triggers myosin ATPase to hydrolyze ATP to
ADP. The chemical energy released from the hydrolysis is utilized by the myosin
and actin to initiate the sliding mechanism resulting in the contraction of the muscle.
When the impulse that started the potential subsides, the sarcolemma polarizes, stimulating
the energy-dependent sequestration of calcium by the SR membranes.
B. Biochemical Changes in Muscle Postmortem
After death of an animal, all circulation ceases—an event that rapidly brings about
important changes in the muscle tissue (Table 2). The principal changes are attributable
to a lack of oxygen (anaerobic conditions) and the accumulation of certain
waste products, especially lactate and H � . In a short time, the mitochondrial system
ceases to function in all but surface cells because internal oxygen is rapidly depleted.
Anaerobic glycolysis continues to regenerate ATP and lactate, but eventually the
decrease in pH caused by the presence of lactate disrupts glycolytic activity. When
ATP is depleted, the most immediate response seen is the onset of rigor whereby
actin and myosin remain in a contracted state as a result of the absence of a plasticizing
agent (ADP or ATP). A more important response with regard to lipid oxidation
is the cellular membranes’ inability to maintain their integrity. Consequently,
lysosomal enzymes, such as phospholipase and lipase, may be released, affecting in
turn the susceptibility of lipids to oxidize. Calcium leakage is a noted response to
the increased membrane permeability of SR and mitochondria [30,31]. Increased
calcium concentrations, in turn, could activate enzymic systems, such as phospholipase,
which would not normally be turned on.
C. Variability Between and Within Muscles
Although the muscle function of locomotion is similar throughout the animal kingdom,
compositional differences exist between species and even within different muscles
of the same species. Red and white muscles present a classic example, with
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