17
Soft Solids
Many foods can be considered soft solids: bread, cheese, margarine, peanut
butter, meat, several fruits, jam, puddings, mousse, aspic, boiled potatoes,
etc. The term soft solid is ill-defined, as is the word semisolid, which is also
used. The (implicit) meaning of the word solid does not comply with the
definition given in Section 16.1 (a material with a viscosity over a given
value, e.g., 10^12 or 10^14 Pa?s). A ‘‘solid’’ as meant in this chapter is a
material that primarily exhibits elastic deformation upon applying a stress.
The word soft then signifies that a relatively small stress is needed to obtain
a substantial deformation; this may be due to the elastic modulus of the
material being low (bread, soft fruits), or the yield stress being small
(margarine, jam). The examples mentioned represent a wide variety of
physical properties.
Virtually all soft solids arecomposite materials, which implies that they
are inhomogeneous on a mesoscopic or even macroscopic scale. Their
properties depend on this physical structure, and a structural classification
can be useful. Main types are
Gels. These are systems that consist mainly of solvent (mostly water),
with the solid character being provided by a space-filling network.
Some idealized types of network are provided by (a) long and