10
Surface Phenomena
As we have seen in the previous chapter, most foods are dispersed systems,
and many of the structural elements constitute separate phases. This means
that there are phase boundaries or interfaces, and the presence of such
interfaces has several important consequences. Substances can adsorb onto
the interfaces, and if the interfacial area is large, as is often the case, the
adsorbed amounts can be considerable. The adsorption can strongly affect
colloidal interaction forces between structural elements, i.e., forces acting
perpendicular to the interface; this is discussed in Chapter 12. Other forces
act in the direction of the interface, and these are treated in the present
chapter. Altogether, surface phenomena are of considerable importance
during processing and for the physical properties, including stability, of
most foods.
Strictly speaking, the word surface is reserved for an interface between
a condensed phase (solid or liquid) and a gas phase (mostly air). In practice,
the words surface and interface are often used indiscriminately.
10.1 SURFACE TENSION
It is common experience that fluid systems consisting of two phases try to
minimize their interfacial area. For instance, if the one phase is present as a