2009_Book_FoodChemistry

12.3 Muscle Tissue: Composition and Function 583
Collagenase enzymes which are serine proteinases
are also known.
Denatured collagen, as formed post-mortem by
the action of lactic acid, can also be cleaved by
lysosomal enzymes, e. g., lysosomal collagenase
and cysteine proteinase cathepsin B 1 . Thermally
denatured collagen is attacked by pepsin and
trypsin.
One characteristic of the intact collagen fiber is
that it shrinks when heated (cooking or roasting).
The shrinkage temperature (T s )isdifferent
for different species. For fish collagen, the
T s is 45 ◦ C and for mammals, 60–65 ◦ C. When
native or intact collagen is heated to T > T s ,
the triple-stranded helix is destroyed to a great
extent, depending on the cross-links. The disrupted
structure now exists as random coils which
are soluble in water and are called gelatin. Depending
on the concentration of the gelatin solution
and of the temperature gradient, a transition
into organized structures occurs during cooling.
Figure 12.21 schematically summarizes these
transitions. At low concentrations, intramolecular
back-pleating occurs preferentially with singlestrands.
At higher concentrations and slow rates
of cooling, a structure is rebuilt which resembles
the original native structure. At even higher
concentrations and rapid cooling, structures are
obtained in which the helical segments alternate
with randomly coiled portions of the strand. All
these structures can immobilize a large amount of
water and form gelatin gels.
(12.20)
a series of collagenases from different sources
and with different specificities. A vertebrate animal
collagenase, which is a metal proteinase,
splits a special bond in native collagen while the
collagenase from Clostridium histolyticum, also
a metal proteinase, cleaves collagen preferentially
at glycine residues, forming tripeptides:
↓
↓
−Pro − X − Gly − Pro − X − Gly − Pro−
(12.21)
Fig. 12.21. Collagen conversion into gelatin. (according
to Traub and Piez, 1971). T s : shrinkage temperature,
T: temperature, c: concentration; (see text)