294 Part III: Muscle Foods
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. Based
on color, they can be classed as red, white, or inter-
mediate (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 exhib-
it glycolytic metabolism; and (3) intermediate fibers
exhibit intermediate properties. Red muscles con-
tain 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 fi-
bers, 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 connec-
tive tissue, contains around 1000 myofibrils, all of
them arranged in a parallel way and responsible for
contraction and relaxation. They are embedded in a
liquid known as sarcoplasm, which contains the sar-
coplasmic (water-soluble) proteins. Each myofibril
contains clear dark lines, known as Z-lines, regular-
ly located along the myofibril (see Fig. 13.1). The
distance between two consecutive Z-lines is 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 sarcomeres. All these
filaments are composed of proteins known as myo-
fibrillar proteins.
Muscle, fat, bones, and skin constitute the main
components of carcasses; muscle is the major com-
pound and is, furthermore, associated to the term of
meat. The average percentage of muscle in relation
to live weight varies depending on the species, de-Figure 13.1.Structure of a myofibril, with details of the main filaments in the sarcomere.