present. This illustrates the other point made: structure and properties
depend on the history (process steps applied, storage conditions, tempera-
ture history) of the product.
Besides those mentioned, some other properties may greatly depend
on structure. Several kinds of structure give the food a certain consistency,
and this may greatly reduce transport rates in the system; these are discussed
in Chapter 5. In many foods, the various chemical components are fully or
partly compartmentalized: in cells, tissue fragments, emulsion droplets, etc.
This implies that reactions between those components may be greatly
hindered; this is mentioned in Section 4.4. The compartmentalization may
also apply toflavorsubstances, and it will slow down their release during
eating. If the compartments are fairly large it can lead to fluctuation in
flavor release during eating, thereby enhancing flavor, because fluctuation
can offset adaptation of the senses to flavor stimuli.
Note the response of the senses to a continuous stimulus generally
decreases in time; this is called adaptation.Thus a compartmentalized food generally tastes quite different from the
same food that has been homogenized.
If a system is physically heterogeneous it can also bephysically unstable.
Several kinds of change may occur during storage, which may be perceived as
a change in consistency or color, or as a separation into layers. Moreover,
during processing or usage, changes in the dispersed state may occur. These
may be desirable—as in the whipping of cream—or undesirable—as in
overwhipping of cream, where butter granules are formed.
Most of the ensuing part of this book deals with dispersed systems.
These generally have one or more kinds of interface, often making up a
considerable surface area. This means that surface phenomena are of
paramount importance, and they are discussed in Chapter 10.Colloidal
interactionforces between structural elements are also essential, as they
determine rheological properties and physical stability; these forces are the
subject of Chapter 12. The various kinds of physical instability are treated in
Chapter 13, and the nucleation phenomena involved in phase transitions in
Chapter 14. Specific dispersed systems are discussed in Chapters 11 and 17.
The present chapter explains important concepts and discusses geometrical
aspects.
9.1 STRUCTURE
Structurecan be defined as the distribution over space of the components in
a system. This is a purely geometrical concept, as it just concerns angles and