Polysaccharide molecules tend to have much stiffer chains; conse-
quently, the linear region of a polysaccharide gel tends to be much smaller,
and its modulus is for a considerable part enthalpy determined (as in an
elastic solid). The gelling polysaccharides vary widely in properties, and
gelation often involves quite specific aspects. Factors determining gel
stiffness, deformability, and strength generally are concentration of gelling
material, gel time, temperature of gelation and of measurement, and solvent
quality. For ionic polysaccharides, pH and ionic strength are important
variables, and for some ionic composition is important. Often weak and
brittle (or strong) gels are distinguished. Ideally, the former type has a low
modulus and shows yielding at a relatively small strain; the latter type has a
higher modulus and does not yield but fractures, at a relatively large strain.
Particle Gels. These gels generally form by fractal aggregation.
Two types of casein gels, made by acidification and by renneting, make
about ideal fractal gels. The fractal dimensionality mostly is 2.3–2.4.
Considerable short-term rearrangement can occur, which means that the
small fractal aggregates initially formed change into more compact particles
(containing, say, 4–40 primary particles), which then form the building
blocks of the larger fractal aggregates, which ultimately form the gel.
The gel structure is determined by the volume fraction of particle
material, the size of the building blocks, and the fractal dimensionality.
Simple scaling laws are derived for the permeability and for rheological
properties as functions of particle concentration. The rheological para-
meters also depend on those of the particles, especially the extent of the
linear range.
Long term rearrangement, i.e., after gel formation, can occur under
some conditions. In first instance, it leads to straightening of strands of
particles in the gel, which causes an increase in modulus, a weaker
dependence of the modulus on particle concentration, and a decrease in
fracture strain; the fracture stress and the permeability are hardly affected.
Stronger rearrangement does lead to an increase in permeability, and
syneresis can readily occur. All these changes depend on gel type, formation
temperature, storage temperature, pH, etc.
Heat-Set Protein Gels. When a solution of a globular protein is
heated, the protein molecules denature and subsequently aggregate to form
a gel. The gelation is irreversible: upon cooling the modulus increases. The
aggregation appears to proceed in two steps. At first roughly spherical
aggregates form, size 20 nm to 4mm, and these then aggregate to form a gel.
At a pH close to the isoelectric point and a high ionic strength, large