9
Dispersed Systems
Most foods are dispersed systems; in other words, they are physically
heterogeneous. This means that their properties are not fully given by their
chemical composition. For a homogeneous liquid, like apple juice or
cooking oil, it is often reasonable to assume that thermodynamic
equilibrium exists. In such a case, full knowledge of chemical composition
will, in principle, give all properties, including reactivity, of the system, for
given external conditions (temperature, pressure, etc.). A heterogeneous
system has structural elements, and making these mostly costs energy. The
system is therefore not in thermodynamic equilibrium; it can be
manufactured in various ways, leading to a variety of structures. The
properties of the system are determined by its structure and by the chemical
composition of its structural elements.
Difference in structure generally implies differences in properties.
Take, for example, ice cream. It is made (by freezing and agitation) of ice
cream mix and air, and the two systems are very different, as is illustrated in
Figure 9.1. Nevertheless, they have exactly the same chemical composition.
Also the properties are very different, as we all know: when we let ice cream
melt, we obtain a product of very different appearance, consistency, and
eating qualities. However, melted ice cream is not quite the same as ice
cream mix as, for instance, larger fat globules or clumps of them will be