interaction forces between the particles; and (c) the possibility of loss of
substances by evaporation, etc. An oil-in-water emulsion, such as a fairly
concentrated cream, differs greatly in properties from a water-in-oil
emulsion, such as a low fat spread, although both systems may have nearly
the same chemical composition and contain droplets of about the same size.
Table 9.2 gives some important properties of materials that can make up the
continuous phase of foods.
The continuous phase can be asolidor have some characteristics of a
solid. This implies that the structural elements are immobilized, which
considerably enhances physical stability of the system. When making such a
dispersion, the continuous phase is always liquid, but it can solidify
afterwards, e.g., by lowering the temperature or by evaporating the solvent.
The liquid can become crystallized, form a glass (Section 16.1), or turn into
a gel. Especially the last named situation is frequently encountered in foods.
Also the ‘‘solvent,’’ generally an aqueous solution, in the continuous phase
then is more or less immobilized (Section 5.3). If the gel is a classical
polymer gel (Section 17.2.2), the polymer molecules provide a continuous
network, but donotmake up a continuous phase: the polymer strands
cannot be seen as a structureinwhich other molecules can be present and
diffuse.
The structure of food systems can be more complicated. This is
discussed in greater detail in Chapter 17. A system can have two continuous
phases (in theory even a greater number). A good example is bread crumb,
where both the gas phase and the ‘‘solid’’matrixare continuous: one can
blow air through a slice of bread. The matrix is, by and large, a continuous
gluten phase containing partially gelatinized starch granules. Such a
bicontinuousstructure of a solid and a fluid phase is called asponge. Cheese
is a dispersion of oil droplets (the droplets also contain fat crystals) in a
proteinaceous continuous phase. However, a fairly hard and well matured
cheese often exhibits oiling off if the ambient temperature is not too low,
which proves that also the oil phase has become continuous.
TABLE9.1 Various Types of Lyophobic DispersionsDispersed phase Continuous phase Dispersion type
Gas Liquid Foam
Liquid Gas Fog, aerosol
Liquid Liquid Emulsion
Solid Gas Smoke, powder
Solid Liquid Suspension, sol