characteristic of most food polysaccharides is the presence of various, in
some cases quite bulky, side groups along the polymer chain, unlike the
simple polymers primarily discussed in Chapter 6. The distribution of the
side groups over the chain is quite variable, and in many polysaccharides
‘‘hairy’’ regions—with side groups being near to each other—and
‘‘nonhairy’’ regions—devoid of side groups—are distinguished. The
distribution of such regions can vary, even among molecules of the same
polysaccharide.
One thing that nearly all polysaccharide gels have in common is that
they are relativelystiffgels, the elastic modulus being for a large part due to
change in enthalpy. This is because the length of a statistical chain elementb
is relatively large: see Table 6.1. For most gelling polysaccharidesb¼15–
60 nm, as compared to 2–3 nm for most polypeptide chains. The ‘‘stiffness’’
of the primary chain is often enhanced by the presence of bulky side groups.
This means that the cords between junctions contain but a limited number
of statistical chain elements. Consequently, the linear region tends to be
markedly smaller than that of a gelatin gel: see Table 17.3.
Polysaccharide gels contain various types ofjunctions; generally, these
can only form in the nonhairy stretches of the molecules. The junctions
often involve helices: agar (and its main component agarose), the
carrageenans, gellan, and possibly xanthan. Some anionic polysaccharides
form egg-box junctions (Figure 17.12c) if divalent cations are present:
alginate, and pectin under some conditions. Junctions formed by micro-FIGURE17.14 Effect of the measurement temperature on the shear modulus of
various gels. The arrows indicate the temperature sequence. (a) Gelatin (2.5%). (b)k-
Carrageenan (1%) for two concentrations of CaCl 2 (indicated). (c) Acid casein gels
(2.5%), made and aged at two temperatures (indicated). (d)b-Lactoglobulin (10%)at
two pH values (indicated). The results may greatly depend on heating or cooling rate
and on other conditions.