Handbook of Food Science and Technology 3

2.1.4.
 Carbohydrates
Meat and fish are low in sugar; these carbohydrates, serving primarily as 
energy reserves, are mostly found in the form of glycogen granules but also, 
after the death of the animal, in the form of ribose from the degradation of 
ATP. The glycogen content of the muscle depends on species, muscle type, 
the physiological state of the animal, and is generally lower in fish muscle 
(1% in meat and less than 1% in fish).
2.1.5.
 Vitamins and minerals 
The vitamin content of mammalian and fish muscle is very similar except 
for vitamins A and D, present in large quantities in the muscle of oily fish 
and the liver of white fish (cod, halibut), but present only in trace amounts in 
meat. Fish is a good source of minerals, especially calcium, phosphorus, iron 
and copper, as well as iodine in sea fish. Meat, and in particular beef, is rich 
in iron.

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From Muscle to Meat and Meat Products 89 
2.2. Biological and physicochemical changes in muscle 
2.2.1.
 Muscle contraction 
2.2.1.1.
 Excitation and contraction coupling 
The stimulus for muscle contraction is the transmission of an impulse 
from the motor nerve to the motor end plate of the sarcolemma, which 
occurs very rapidly. Normally, the potential difference (pd) between the 
inside and outside of the muscle cell is +60 mV. During the transmission of 
the nerve impulse, pd is zero; this is known as depolarization. It is caused by 
changes in cell membrane permeability to K+, Na
+
and Ca
2+
ions.
For a long time, it was not understood how all the myofibrils in the 
muscle fiber could contract simultaneously. The diffusion of a chemical 
mediator from the sarcolemma to the myofibrils did not explain the rapidity 
of the response (<1 s). Electron microscopy provided the solution. 
Sarcolemmal invaginations run along the Z-line across the muscle cell so 
that these tubules are in close contact with all the myofibrils in the cell. This 
system of transverse tubules is known as the T (triad) system. When a nerve 
impulse reaches the sarcolemma and depolarization occurs, the entire T 
system is also stimulated, thus communicating nerve impulses to all the 
sarcomeres in the muscle fiber.
Electron microscopy was also used to explain the conversion of electrical 
signals in the T system to chemical signals in the myofibrils. Surrounding 
each set of adjacent sarcomeres are double-membrane vesicles, which are 
arranged longitudinally and contain many perforations. Each vesicle extends 
from one A-I junction to the next (Figure 2.5). These vesicles form the 
sarcoplasmic reticulum. The internal compartments of the vesicles or 
cisternae are linked to each other by sac-like channels called terminal 
cisternae. Parallel pairs of terminal cisternae run along the myofibrils in 
close contact with the T system.
When the sarcolemma is excited and the T system is depolarized, the 
permeability of the membranes next to the sarcoplasmic reticulum increases. 
As a result, Ca
2+
ions are released from the cisternae of the reticulum into the 
sarcoplasm, where they stimulate myosin ATPase activity, thereby triggering 
muscle contraction. In the rest of the cell, the lack of calcium prevents the 
hydrolysis of ATP by myosin. The concentration of Ca
2+
in the sarcoplasm 
of relaxed muscles is estimated to be less than 1 
μ
M. The minimum 

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90 Handbook of Food Science and Technology 3 
concentration of Ca
2+
required to trigger muscle contraction is in the range of 
10 
μ
M. 
2.2.1.2.
 Relaxation 
When the nerve impulse has crossed the sarcoplasmic reticulum causing 
Ca
2+
ions to be released into the sarcoplasm, the sarcolemma and the 
sarcoplasmic reticulum return to their initial state of polarization (+60 mV 
between the inside and outside of the cell). Calcium is therefore retained in 
the reticulum cisternae; the muscle is ready to receive further excitation.
The accumulation of calcium in the sarcoplasmic reticulum is due to the 
action of an ATP-dependent calcium pump located in the membranes of the 
vesicles. This transfer occurs against a concentration gradient via active 
transport. The energy required for this transport comes from ATP hydrolysis 
by sarcoplasmic ATPases in the reticulum membranes. Vesicles are able to 
pump calcium from the surrounding medium up to a Ca
2+
ion concentration 
of 1 
μ
M. The Ca
2+
ions in the vesicles are then released into the sarcoplasm 
when a new impulse arrives.
2.2.1.3.
 Sources of energy for muscle contraction
During muscle contraction, lactic acid is formed by glycogenolysis and 
the glycogen content of the muscle decreases. It was thought, therefore, that 
glycogenolysis provided the energy needed for muscle contraction. 
However, if this metabolic pathway is inhibited, contraction still occurs. So 
it was then assumed that ATP was the energy source, which also proved to 
be wrong for two reasons: 
– the ATP concentration of the muscle is insufficient to supply the energy 
needed for contraction. For every minute of muscle contraction, 10
-3
mol of 
ATP per g of muscle is required. However, it only contains 5×10
-6
mol.g
-1
, 
which corresponds to 0.5 second of activity;
– before and after muscle contraction, concentrations of ATP and ADP 
hardly vary in the muscle. 
The energy required for muscle contraction is generated mainly by 
phosphocreatine, a compound capable of storing energy and found in all 
muscles of vertebrates, at a concentration four to five times higher than that 
of ATP.

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From Muscle to Meat and Meat Products 91 
The phosphate group of phosphocreatine can be quickly transferred to 
ADP by the action of creatine kinase: 
Creatine-P + ADP 
↔
Creatine + ATP 
[2.4] 
At the pH of the sarcoplasm (pH 7), the equilibrium is shifted towards
the formation of ATP; therefore the concentration of ATP does not drop 
during muscle contraction. If glycolysis and respiration are blocked, 
phosphocreatine may be deficient and the ATP concentration drops.
Rephosphorylation of ATP and phosphocreatine varies with muscle type: 
for very active or red muscles containing a lot of myoglobin and respiratory 
pigments (cytochromes), respiration is the main supplier of energy through 
oxidative phosphorylation like, for example, in the flight muscles of birds, 
the leg muscles of mammalian runners and the muscles of pelagic fish. For 
less active or white muscles containing a small amount of myoglobin and 
respiratory pigments (pectoral muscles of birds or benthic flatfish), 
glycolysis is the main source of energy.
Some ADP can also be converted to ATP using adenylate kinase as 
follows: 
2 ADP 
↔
ATP + AMP 
[2.5]