A gelatin gel resembles an ideal rubber-type gel to some extent. As
given in Table 17.3, the linear region in a rubber may extend to a strain of
about 1.6, versus, say, 0.7 for gelatin (i.e., extension by a factor of 2). This is
possible because the gelatin chain is highly flexible, the length of a statistical
chain element probably being about 2 nm. On the other hand, Eq. (17.15) is
not obeyed at all, but this is because the amount of material participating in
junctions markedly increases with decreasing temperature.
Figure 17.13a gives an example of the modulus as a function of gelatin
concentration. The minimum concentration for gel formation is about 1%.
As mentioned, several factors affect gel stiffness. Figure 17.14a gives an
example of the temperature dependence, and it is seen that considerable
hysteresisoccurs between cooling and heating curves, although gelation is
completely thermoreversible. The extent of hysteresis greatly depends on
cooling rate. The fact that a gelatin solution gives a gel at low, e.g., room,
temperature, but that the gel melts at body temperature, can of course be
utilized, especially in food systems. The slowness of gelation on cooling
provides the possibility to make a system quite homogeneous by stirring
after its has obtained a high viscosity but not yet a significant modulus; gel
formation can then occur under quiescent conditions.
Polysaccharides. There is considerable variation among gelling
polysaccharides in molecular structure, in type of junctions, and in the
dependence of gel properties on external conditions. Treatment of all of
these systems and their particulars is outside the scope of this book, so the
discussion will be restricted to some general aspects. An important
FIGURE17.13 Effect of concentration of the material making the gel matrix on
the shear modulus of various gels. (a) Agar and gelatin gels. (b) Gels of k-
carrageenan of two molar masses (indicated) in 0.1 molar KCl. (c) Casein gels made
by slow acidification or by renneting. (d) Heat-set gels of bovine serum albumin;
figures near the curves denote pH/added NaCl (molar).