aggregates are formed, resulting in a coarse gel. Small aggregates lead to a
fine stranded gel, which forms at low ionic strength and further away from
the isoelectric pH.
Upon heating, a weak gel tends to form already when only part of the
protein is denatured, and this gel is of a fractal nature, having relatively
large pores; the fractal dimensionality is about 2.4. Subsequently,
aggregating protein is deposited onto the network, which does not greatly
alter its overall structure; the permeability slightly decreases, but the
modulus strongly increases. Many variables, for instance temperature or
heating rate, affect all of these processes, hence the gel properties. However,
the results greatly vary among proteins, and few general rules can be given.
Mixed Gels. A wide variety of mixed gels is produced, often with
specific (combinations of) properties. A mixture of a gelling and a
nongelling polymer may show phase separation due to incompatibility,
and the mixture can then be made to gel. If the gelling polymer forms the
continuous phase, the presence of the other polymer increases gel stiffness.
Mixtures of polymers that show weak mutual attraction, often of an
electrostatic nature, may give a gel at concentrations at which neither of
them gels by itself.
Adding polysaccharides to a protein solution that is subject to heat
setting may either increase or decrease the modulus at constant concentra-
tion (proteinþpolysaccharide).
A solution of a moderately hydrophobic polysaccharide can often be
made to gel by adding a small-molecule surfactant at a concentration above
the CMC. The surfactant then makes micelles through many of which one
or more polysaccharide chains pass, thereby forming cross-links. A fairly
weak gel results.
The presence of filler particles can markedly affect the properties of
various gels. If the particles are not bonded to the gelling material, their
effect is to decrease the modulus. If they are bonded, generally because the
gelling material also adsorbs onto the particles, the modulus may strongly
increase, the more so as the particles themselves have a higher modulus; the
modulus of a fluid particle equals its Laplace pressure. In protein–emulsion
gels, the gelling material (e.g., gelatin, casein, or heat-setting proteins) is also
used as the emulsifying agent for the oil droplets. Quite high moduli can so
be obtained for low protein concentrations.
Plastic Fats. A plastic fat is a space-filling oil-containing network
of triglyceride crystals. It can thus be considered a gel, although the
mechanical properties are very different from those of most gels. When oil is
cooled, so that part of the material becomes supersaturated, crystal