Handbook of Food Science and Technology 3

Figure 2.7
.
 Change in myoglobin and variation in meat color 
Three types of pigments (myoglobin, oxymyoglobin and metmyoglobin) 
coexist, but it is their relative proportion that gives meat its characteristic 
color. If, on the surface of the meat, more than 40% of the myoglobin is in 
the form of metmyoglobin, meat has a brown color and is therefore 
unsellable. 
Many factors affect the chemical structure of heme and meat color. Some 
are linked to the characteristics of the animal: 
– age: color tends to increase with age; 
– physiological maturity, which can differ with breed; 
– diet, fatness or iron deficiency tends to lighten meat color; 
– type of muscle: each muscle has a specific color.

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80 Handbook of Food Science and Technology 3 
These differences are mainly due to the amounts of pigment in the 
muscle. The pH can also change the color of meat when it reaches abnormal 
values, especially in a stressful situation prior to slaughter. In addition, the 
storage and packaging of meat influence meat color. Vacuum packaging 
promotes dark red myoglobin, while modified atmosphere packaging is 
conducive to the formation of bright red oxymyoglobin. Brown 
metmyoglobin increases with storage. 
Poorly soluble fraction 
–
 Review of myofibril structure 
Myofibrils are composed of protein myofilaments held in place by
Z-lines that mark the boundaries between sarcomeres. An internal skeleton 
or cytoskeleton also contributes to the stability of the structure. All these 
structures are composed of poorly soluble molecules in addition to the 
regulatory proteins of muscle contraction.
–
 Myosin 
Myosin forms the thick filaments. It has a molecular weight of 540 kDa 
and a pI close to 5.4. It is completely insoluble at the ionic strength of the 
sarcoplasm. 
The myosin molecule is made up of two parts (Figure 2.8): 
– a tail consisting of two polypeptide chains (two 
α
-helices intertwined 
about a common axis to form a double super helix); 
– a globular head composed of the two aforementioned chains randomly 
intertwined (heavy chains) to which two shorter polypeptide chains (light 
chains) are attached.
Myosin has two key properties: 
– Constituent molecules spontaneously assemble into filaments under 
physiological conditions. 
– Myosin behaves like a weak ATPase; it hydrolyses the terminal 
phosphate group of adenosin triphosphate (ATP) as well as guanosine 
triphosphate 
(GTP), 
inosine 
triphosphate 
(ITP) 
and 
cytidine
triphosphate (CTP). This activity is stimulated by Ca
2+
ions, inhibited by

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From Muscle to Meat and Meat Products 81 
Mg
2+
ions, influenced by the concentration of KCl, and has two optimum pH 
for activity (6.0 and 9.5). The hydrolysis of ATP can be summarized as 
follows: 
ATP + H
2
O 
→
ADP + Pi + H
+
[2.1]