Gels. Gels consist for the greater part of liquid; the solid character is
provided by a space-filing structure (the gelling material). Nearly all food
gels are man-made; the main purpose is to provide one or more specific
physical properties. Desirable mechanical properties, be they for storage,
handling, or eating, mostly involve the yield stress. In some cases fracture
properties are more important. Gels are also made to provide physical
stability to a product; again, the yield stress often comes into play, but also
the permeability can be an important characteristic.
Gels can be of various types. Properties of some of the main types in
foods are summarized in Table 17.5.
Polymer Gels. These are very common. They are made of long
polymer molecules that are cross-linked at some places. In food gels, these
are rarely chemical bonds; rather they are junctions, each of which involves
a number of weak physical bonds. They are often microcrystallites, i.e.,
stacks of helices or other straightened polymer sections. Most of these gels
are thermoreversible: they melt upon heating and (re)form upon cooling.
Some gels of anionic polymers can form egg-box junctions in the presence of
calcium ions; these gels generally do not melt at high temperatures.
A gelatin solution gels on cooling by forming triple helices and stacks
of these; it may take a long time before it is fully set. Because of the high
flexibility of the gelatin chains, the gel is more or less rubberlike, with a large
linear region. The modulus is largely entropy determined.
TABLE17.5 Comparison of Some Properties of a Typical
Polymer Gel, a Typical Fractal Particle Gel, and a
Heat-Set Protein Gel.
Heat-set
Gel type Polymer Particle protein
Appearance Clear Turbid Clear–turbid
PermeabilityB/m^210 ^1610 ^1210 ^15 –10^12
Modulus at 3%aG^0 /Pa 10^3 –10^6102 0–10^2
Fracture strainefr > 1 b < 1 c < 1
Stability of the geld Good Variable Good
aPercent (w/w) of gelling material.
bWidely variable.
cFor rigid particles<0.1.
dAt constant conditions.