11
Formation of Emulsions and
Foams
There are two main methods of making dispersions. One issupersaturation
of the continuous phase with the component that is to become the disperse
phase. This involves the processes of nucleation (Chapter 14) and growth
(Chapter 15 for solids). The particles can be obtained in a wide range of
sizes. The method is applied in food processing, for instance to obtain sugar
crystals that are subsequently separated from the liquid, or to make a plastic
fat—a dispersion of aggregated fat crystals in oil. The other method involves
dividing a materialinto small particles that are then suspended in a liquid.
Making suspensions of solid materials is done by dry or wet grinding, which
tends to be a very difficult operation if small particles are desired. The
theory involves fracture mechanics and shows that it is very difficult to
break up small particles into still smaller ones, which is also observed in
practice (see Section 17.1.2). An example of the process is the lengthy milling
of the (liquid) cocoa mass in chocolate manufacture to obtain very small
sugar particles (about 10mm).
Emulsions and foams are dispersions of two fluids, which implies that
the interface between the phases is deformable. This makes the breakup of
one of the materials into small particles far easier, which does not imply that
quantitative understanding of the phenomena involved is easy. Some aspects
are discussed in this chapter.