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288 Part 3: Meat, Poultry and SeafoodsSTRUCTURE OF MUSCLE
A good knowledge of the structure of muscle is essential for
the use of muscle as meat. The structure is important for the
properties of the muscle, and its changes during postmortem
events influence the quality properties of the meat.
Muscle has different colors within the range of white to
red, depending on the proportion of fibers. There are different
classifications for fibers. On the basis of color, they can be
classed as red, white, or intermediate (Moody and Cassens
1968): (1) red fibers are characterized by a higher content of
myoglobin and higher numbers of capillaries and mitochondria,
and they exhibit oxidative metabolism; (2) white fibers contain
low amounts of myoglobin and exhibit glycolytic metabolism;
and (3) intermediate fibers exhibit intermediate properties. Red
muscles contain a high proportion of red fibers and are mostly
related to locomotion, while white muscles contain a higher
proportion of white fibers and are engaged in support tasks
(Urich 1994). Other classifications are based on the speed of
contraction (Pearson and Young 1989): Type I for slow-twitch
oxidative fibers, type IIA for fast-twitch oxidative fibers, and
IIB for fast-twitch glycolytic fibers.
The skeletal muscle contains a great number of fibers. Each
fiber, which is surrounded by connective tissue, contains around
1000 myofibrils, all of them arranged in a parallel way and re-
sponsible for contraction and relaxation. They are embedded in
a liquid known as sarcoplasm, which contains the sarcoplas-
mic (water-soluble) proteins. Each myofibril contains clear dark
lines, known as Z-lines, regularly located along the myofibril
(see Fig. 15.1). The distance between two consecutive Z-lines isFigure 15.1.Structure of a myofibril with details of main filaments in
the sarcomere.known as a sarcomere. In addition, myofibrils contain thick and
thin filaments, partly overlapped and giving rise to alternating
dark (A band) and light (I band) areas. The thin filaments extend
into the Z-line that serves as linkage between consecutive sar-
comeres. All these filaments are composed of proteins known as
myofibrillar proteins.
Muscle, fat, bones, and skin constitute the main components
of carcasses; muscle is the major compound and is, furthermore,
associated to the term of meat. The average percentage of muscle
in relation to live weight varies depending on the species, degree
of fatness, and dressing method: 35% for beef, 32% for veal,
36% for pork, 25% for lamb, 50% for turkey, and 39% for
broiler chicken. The muscle to bone ratio is also an important
parameter representative of muscling: 3.5 for beef, 2.1 for veal,
4.0 for pork, 2.5 for lamb, 2.9 for turkey, and 1.8 for poultry
(Kauffman 2001).MUSCLE COMPOSITION
Essentially, meat is basically composed of water, protein, lipids,
minerals, and carbohydrates. Lean muscle tissue contains ap-
proximately 72–74% moisture, 20–22% protein, 3–5% fat, 1%
ash, and 0.5% of carbohydrate. An example of typical pork
meat composition is shown in Table 15.1. These proportions are
largely variable, especially in the lipid content, which depends
on species, amount of fattening, inclusion of the adipose tissue,
and so on. There is an inverse relationship between the percent-
ages of protein and moisture and the percentage of fat so thatTable 15.1.Example of the Approximate Composition
of Pork MuscleLongissimus Dorsi. Expressed as
Means and Standard Deviation (SD)Units Mean SDGross composition
Moisture g/100g 74.5 0.5
Protein g/100g 21.4 2.1
Lipid g/100g 2.7 0.3
Carbohydrate g/100g 0.5 0.1
Ash g/100g 0.9 0.1
Proteins
Myofibrillar g/100g 9.5 0.8
Sarcoplasmic g/100g 9.1 0.6
Connective g/100g 3.0 0.4
Lipids
Phospholipids g/100g 0.586 0.040
Triglycerides g/100g 2.12 0.22
Free fatty acids g/100g 0.025 0.005
Some minor compounds
Cholesterol mg/100g 46.1 6.1
Haem content mg/100g 400 30
Dipeptides mg/100g 347.6 35.6
Free amino acids mg/100g 90.2 5.8Source:Aristoy and Toldr ́a 1998, Hern ́andez et al. 1998, Toldr ́a 1999,
Unpublished.