10 Part I: Principles
myoglobin, and other substances. Examination of
myofibrils under a phase contrast light microscope
shows them to be cross-striated due to the presence
of alternating dark or A-bands and light or I-bands.
These structures in the myofibril appear to be very
similar in both fish and meat. A lighter band or H-
zone transverses the A-band, while the I-band has a
dark line in the middle known as the Z-line. A further
dark line, the M-line, can also be observed at the
center of the H-zone in some cases (not shown in
Fig. 13.1). The basic unit of the myofibril is the sar-
comere, defined as the unit between adjacent Z-lines.
Examination of the sarcomere by electron micro-
scope reveals two sets of filaments within the fibrils, a
thick set consisting mainly of myosin and a thin set
containing primarily of F-actin. In addition to the
paracrystalline arrangement of the thick and thin set
of filaments, there is a filamentous “cytoskeletal
structure” composed of connectin and desmin.
Meat tenderization is the result of the synergetic
effect of glycolysis and actions of proteases such as
cathepsins and calpains. Meat tenderization is a very
complex multifactorial process controlled by a num-
ber of endogenous proteases and some as yet poorly
understood biological parameters. With currently
available literature, the following explanation can be
considered. At the initial postmortem stage, cal-
pains, having optimal near neutral pH, attack certain
proteins of the Z-line, such as desmin, filamin, neb-
ulin, and to a lesser extent, connectin. With the pro-
gression of postmortem glycolysis, the pH drops to
5.5 to 6.5, which favors the action of cathepsins on
myosin heavy chains, myosin light chains,-actinin,
tropnin C, and actin. This explanation does not rule
out the roles played by other postmortem proteolytic
systems that can contribute to this tenderization.
(See Eskin 1990, Haard 1990, Huff-Lonergan and
Lonergan 1999, Gopakumar 2000, Jiang 2000, Simp-
son 2000, Lowrie 1992, and Greaser 2001.)
Table 1.5 lists some of the more common enzymes
used in meat tenderization. Papain, ficin, and brome-
lain are proteases of plant origin that can breakdown
animal proteins. They have been applied in meat ten-
derization or in tenderizer formulations industrially
or at the household or restaurant levels. Enzymes
such as pepsins, trypsins, cathepsins, are well known
in the degradation of animal tissues at various sites
of the protein peptide chains. Enteropeptidase (en-
terokinase) is also known to activate trypsinogen
by cleaving its peptide bond at Lys^6 -Ile^7. Plasmin,Table 1.4.Locations and Major Functions of Myofibrillar Proteins Associated with the Contractile
Apparatus and Cytoskeletal Framework
Location Protein Major Function
Contractile apparatus
A-band Myosin Muscle contraction
c-protein Binds myosin filaments
F-, H-, I-proteins Binds myosin filaments
M-line M-protein Binds myosin filaments
Myomesin Binds myosin filaments
Creatine kinase ATP synthesis
I-band Actin Muscle contraction
Tropomyosin Regulates muscle contraction
Troponins T, I, C Regulates muscle contraction
-, -actinins Regulates actin filaments
Cytoskeletal framework
GAP filaments Connectin (titin) Links myosin filaments to Z-line
N 2 -Line Nebulin Unknown
By sarcolemma Vinculin Links myofibrils to sarcolemma
Z-line -actinin Links actin filaments to Z-line
Eu-actinin, filamin Links actin filaments to Z-line
Desmin, vimmentin Peripheral structure to Z-line
Synemin, Z-protein, Z-nin Lattice structure of Z-line
Sources:Eskin 1990, Lowrie 1992, Huff-Lonergan and Lonergan 1999, Greaser 2001.