two or more kinds of structural elements can be distinguished. Most of the
aspects covered in the following chapters concern dispersed systems, and
this chapter defines concepts and introduces some general aspects.
A wide range of highly involved structures occurs, especially in natural
foods, though manufactured foods can also have an intricate structure. In
the simplest case, we have a dispersion of particles, say emulsion droplets, in
a continuous liquid. Another category is that of ‘‘soft solids,’’ in which
many structural elements are bonded to each other. Interaction forces
between structural elements have an often overriding effect on the properties
of dispersed systems, and these forces could even be considered as being part
of the structure. Geometrical structure can in principle be seen through a
microscope, but not forces, of course.
The existence of a physical structure has several important con-
sequences. It determines a number ofphysical properties,notably mechan-
ical ones: viscosity, elasticity, consistency, fracture properties. Because of
the physical inhomogeneity, some kinds ofphysical instabilitycan occur,
often manifest as a kind of segregation, and these are generally undesired.
Chemical components are to some extent compartmentalized, which may
affect their mutual reactivity, hence thechemical stabilityof a food. The
compartmentalization also tends to affectflavor. The result of some process
operations can greatly depend on the physical structure of the system, and
several processes are intended to produce or to alter structure.
It is important whether the various structural elements can be said to
constitutephasesor not. If not, we generally have a so-calledlyophilic
system, which is in principle in equilibrium. In most foods, structural
elements do constitute phases, which implies that they have phase
boundaries in which free energy is accumulated. This means an excess of
free energy, hence alyophobicsystem; it costs energy to make it, and it is
inherently unstable. The properties of such foods thus depend on the
manufacturing and storage history and, for natural foods, on growth
conditions.
Scale. Besides knowledge of the composition and the material
properties of the structural elements, theirsizeis important. The length
scales involved span a wide range, from molecular to visible, i.e., about six
orders of magnitude. Especially natural foods may show a hierarchy of
scales, all of which would need study for a full understanding of properties.
Several static properties, such as visual appearance and consistency, greatly
depend on the size of the structural elements, and so may the rates of change
occurring in foods. In general, length scale and time scale—the time needed
for a change in structure or composition to occur—are correlated.
Dispersions with small particles are in most instances far more stable than