2009_Book_FoodChemistry

590 12 Meat
Table 12.15. Endopeptidases involved in the aging of meat
Origin Enzyme M r pH range Hydrolysis
Sarcoplasma µ-Calpain a 110,000 6.5–7.5 Z line proteins
m-Calpain a 110,000 6.5–7.5 Z line proteins
Lysosomes Cathepsin B a 25,000 3.5–6.0
Cathepsin L a 28,000 3.0–6.0 Myosin, actin,
Cathepsin D a 42,000 3.0–6.0 troponin, collagen
a Cysteine endopeptidase.
b Aspartic acid endopeptidase.
Maturation or aging is accompanied by morphological
changes which primarily affect the
cytoskeleton. Microexaminations show that the
Z lines, which as cross structures (cf. 12.2.1)
separate the individual sarcomeres in the muscle
fibril, are broken up during aging. In addition, the
fibrillar proteins titin and desmin are degraded.
In comparison, the contractile proteins myosin
and actin are stable. They are attacked only at
temperatures above 25 ◦ C. The connective tissue
present outside the muscle cells also remains
intact.
The degradation of the myofibrillar proteins is
catalyzed by endopeptidases. The participation
of the enzymes listed in Table 12.15 is under discussion.
Special attention is directed to µ-calpain,
which, like m-calpain, is activated by the Ca ions
liberated during the rigor phase. Both calpains
are cysteine endopeptidases, which consist of
a large (80 kdal) and a small (28 kdal) subunit.
The large subunit contains the active center.
These two calpains can be distinguished by the
Ca concentration required for their activation. µ-
Calpain requires about 30 µmol/l and m-calpain
250–270 µmol/l. The activity of the calpains is
regulated, among other factors, by their endogenous
inhibitor calpastatin. It has been proposed
that the calpains synergistically cooperate in
the aging of meat with the cathepsins shown
in Table 12.15. Most of the cathepsins are also
cysteine endopeptidases, which are similar to
papain. Their endogenously occurring inhibitors
are the cystatins. On the whole, the processes
involved in the aging of meat are so unclear
that it has not been possible to define markers
which can predict the development of tenderness
in meat.
12.5 Water Holding Capacity
of Meat
Muscle tissue contains 20–25% protein and
approx. 74–76% water, i. e. 350–360 g water
per 100 g protein. Of this total water not more
than 5% is bound directly to hydrophilic groups
on the proteins. The rest of the water in the
muscle tissue, i. e. 95%, is held by capillary
forces between the thick and thin filaments.
When a larger amount of water is bound to the
network, the muscle is more swollen and the
meat is softer and juicier. Hence, water holding
capacity, protein swelling and meat consistency
are intimately interrelated. The extent of water
holding by the protein gel network depends on
the abundance of cross-links among the peptide
chains. These links may be hydrophobic bonds,
hydrogen and ionic bonds and may involve
divalent metal ions. A decrease in the number of
these cross-linkages results in swelling, whereas
an increase in the number of cross-linkages
results in shrinkage (syneresis) of the protein gel.
The transversal swelling of the myofibrils caused
by NaCl has been visualized by phase-contrast
microscopy. On washing with 40.6–1.0mol/L
of NaCl, first the centers of the myosin polymer
A bands (thick filaments) (cf. 12.3.1
and 12.3.2.1.1) are extracted, and, with increasing
concentration, the entire bands are extracted.
There is a 2.5 fold increase in the diameter of the
myofibrils, corresponding to a 6 fold increase in
volume. The cause of these changes is attributed
to the depolymerization of the thick filaments to
give soluble myosin molecules and the weakening
of the bonding of myosin heads to actin. Furthermore,
weakening of transversal structural ele-