concentration is in the range of thec* values, the modulus will be
higher for a larger molar mass. An example is in Figure 17.13b.
Temperature. For all polysaccharides where gelation depends on helix
formation, there is a critical temperatureTmat which the gel will
melt. The modulus of such gels increases in magnitude with
decreasing temperatures (belowTm). An example is given in Figure
17.14b. The value ofTmmay greatly depend on other variables,
especially those mentioned below. Gels of polysaccharides forming
egg-box junctions generally do not melt below 100 8 C. A few
chemically modified polysaccharides can form reversible gelsabovea
certain temperature. This mainly concerns some cellulose ethers,
especially methylcellulose, which contains 22 OCH 3 groups. It forms
a gel at temperatures above 50–90 8 C, depending on concentration,
degree of methylation, and further structural details. Presumably,
the gel formation is mainly due to hydrophobic bonding.
Ionic composition. For anionic polysaccharides, ion composition and
strength, including pH, can have a large effect on the modulus. Egg-
box junctions need divalent cations, Ca^2 þgenerally being the most
effective. The gelation of the various carrageenans is also enhanced
by cations (see Figure 17.14b). Presumably, the ions screen the
negative charge on the polymer, and the effect greatly depends on
the ion involved. Fork-carrageenan, for example, Kþions are more
effective in inducing gelation than either Naþor Ca^2 þions at the
same molar concentration.
Solvent quality. A good example is provided by the gelation of pectin
in jam. The concentrated sugar solution is a poor solvent for pectin,
markedly increasing its activity coefficient and thereby lowering its
solubility.
Mixed polysaccharide gels are briefly discussed in Section 17.2.5.Fracture. Actually, we have not much to add to what has been
discussed in Section 17.1.2, also because little systematic study has been
made of the fracture of various food polymer gels. These gels show either
yielding (‘‘weak gels’’) or time-dependent fracture (‘‘brittle gels’’) at large
deformation. Breakage of covalent bonds almost never occurs, but
‘‘unzipping’’ of junctionsdoes. Brittle gels are generally notch-sensitive. If
the gel is clear (gelatin, agar, some alginates), the presence of large defects is
unlikely. The inherent defect length then will be about equal to the distance
between junctions and hence quite small. A relation like that given by curve
2 in Figure 17.9 tends to be obeyed. In actual food systems, however, pure
polymer gels almost never occur, and far larger defects are generally present.