Polyhedral foams and emulsions: if they are not very polydisperse,
theory for the modulus and the yield stress is available; the only variables
are particle volume fraction and Laplace pressure.
Concentrated starch gels consist of partly swollen starch granules that
fill virtually the whole volume. The modulus roughly equals that of the
starch granules, and thereby greatly increases with starch retrogradation.
Relations for other mechanical properties are more complex. Wheat flour
doughs are somewhat similar, but the particle volume fraction is much
lower, and the continuous liquid, which contains gluten, is highly
viscoelastic.
‘‘Fluid gels’’ consist of anisometric gel particles in a continuous liquid,
made, e.g., by cooling a suitable polysaccharide solution while agitating it.
The modulus increases with increasing volume fraction, anisometry, and
modulus of the particles. The systems generally have a yield stress. Several
kinds of pastes are similar to fluid gels.
Cellular Systems. Superficially, these closely packed systems are
much like some of those just discussed, but the stiffness now derives from
the rigidity of the continuous mass rather than the particles. Three main
types are distinguished: solid foams; gas-filled sponges (where the cells are in
open contact with each other); and plant tissues (where the cells are largely
filled with liquid).
For gas-filled systems, theory has been developed for various
rheological properties. The main variable is the relative density, about
equal to one minus the volume fraction of gas. In most systems, the modulus
roughly scales with the density squared. What occurs at large deformation
depends on the properties of the cell walls: buckling if they are rubberlike,
yielding if they are plastic solids, and fracturing if they are rigid solids.
Bread and related products, or expanded products formed by
extrusion, reasonably fit the theory. This is not so for plant tissues. Here
the modulus depends on three main factors: the rigidity of the cell walls, the
turgor of the cells, and the strength of the material cementing the cells. All of
these parameters greatly decrease upon cooking.
BIBLIOGRAPHY
A recent book covering many aspects discussed in this chapter is
A. J. Rosenthal, ed. Food Texture: Measurement and Perception. Aspen,
Gaithersburg, 1999.
Chapters 4 and 5 discuss rheological and other mechanical properties; Chapters 6–10
various types of products.