net result always depends on the distance over which they are acting. This is
discussed in Chapter 3 and is further worked out in Chapter 12. Net
attractive interaction between particles can cause their aggregation into a
floc (Fig. 9.2, G). Such a floc is also a structural element and the interaction
forces determine (part of) the structure.
Note This is a very common situation, and some authors include
these forces in the description of structure.Geometrical structure can, in principle, be seen: by eye or by means of a
microscope. Interaction forces can only be derived from mechanical
experiments. It is essential to know about the forces to understand the
structure and most properties of the system.
Structure should not be confused with attributes like consistency or
texture.Consistencyis defined by rheologists as the resistance of a material
against permanent deformation, characterized by the relation between
deformation and external force working on it. Its magnitude is determined
by structure and interaction forces of the material. The original meaning of
textureis the perceivable (visible or tactile) inhomogeneity of a surface, be it
an outer surface or a cut one. It is a direct consequence of structure.
Currently, food scientists tend to use the word texture as also including
consistency (or, more precisely, rheological and fracture properties),
especially if it concerns sensory perception. It is indeed often difficult to
distinguish between the two in sensory testing.
Aphaseis defined as a part of a system that is homogeneous and
bounded by closed surfaces, at which surfaces at least some of the intensive
parameters change abruptly (see Section 2.1). This looks much like the
definition of a structural element, but there are differences. First, a
structural element need not be physically homogeneous, whereas a phase is.
Note This does not imply that a phase consists of one chemical
component: most phases in foods contain many components, like
all aqueous solutions encountered.Second, the distance over which intensive parameters change is always on
the order of a few molecules, and the criterion of abruptness of the phase
boundary implies that the smallest dimension of a structural element must
be several times that of a molecule, for the element to constitute a phase. A
phase boundary is a true interface, which contains an amount of interfacial
free energy, and onto which other, surface active, molecules can become
adsorbed.
Whether structural elements constitute true phases is an important
issue, as it marks the difference between what colloid scientists calllyophilic
(¼ solvent loving) or reversible, andlyophobic (¼ solvent hating) or