Gelatin. Gelatin is a breakdown product of collagen, the main
constituent of tendon and several other connective tissues. According to
source and method of preparation, gelatin properties can vary significantly.
In its native state, collagen exists as a triple helix, strongly resembling a
triple polyproline helix. Gelatin is readily soluble in water at temperatures
above 40 8 C, forming a viscous solution of random-coiled linear polypeptide
chains. On cooling a gelatin solution to, say, 20 8 C, collagenlike helices are
formed, albeit not very long ones and including only part of the material.
This results in a gel.
A peptide chain, as in gelatin, does not allow twisting about the
peptide bond. Hence helix formation as described above can hardly occur, if
at all. The formation of junctions presumably proceeds in the following
manner. At a beta turn in the peptide chain at first a ‘‘hairpin’’ is formed.
Both ends wind around each other and so a double helix of limited length
results, i.e., a kind of stump. This stump can wind itself into another chain,
forming a triple helix. If the third chain in the helix belongs to another
molecule, a junction has formed. If a triple helix is intramolecular, which
appears to occur frequently, it can nevertheless contribute to the gel
properties. This is because also in gelatin stacks of helices, i.e.,
microcrystallites, can be formed, enhancing gel stiffness. Figure 17.12b
illustrates the structure. Junction formation is a slow process, and the
modulus can keep increasing for several days. All this depends on the
cooling regime and on the gelatin concentration, since these variables affect
the junction formation process.
TABLE17.3 Approximate Values
of the Critical Hencky Strainecrafor
Some Materials
Material ecr
Rubber 1.6
Gelatin gel 0.7
Agarose gel 0.25
Alginate gel 0.20
Xanthan gel 0.05
Acid casein gel 0.03
Plastic fat, e.g. 0.0005
aAbove which the stress is no longer
proportional to the strain