Physical Chemistry of Foods

An important point to be made is that processes occurring during
formation and changes in the system at rest (‘‘instability’’) should be
distinguished. Conditions or ingredients—such as the surfactant type—that
are suitable for making an emulsion or a foam are not necessarily suitable
for obtaining a stable system. Several different kinds of instability exist, and
they are discussed in Chapter 13. For most emulsions, the formation and the
changes occurring in the system made can be studied separately, but this is
generally not so for a foam, because a foam is far less stable and it takes
more time to make it: formation and subsequent changes may in fact occur
simultaneously. This is because of differences in particle size, volume
fraction, density of the disperse phase, etc. between foams and emulsions.
Some typical values of internal variables are given in Table 11.1. The
quantitative differences between an emulsion and a foam can be large
enough to obtain systems with very different properties.
Finally, making an emulsion or a foam is a highly complex process,
involving several different, though mutually dependent, phenomena in the
realm of hydrodynamics and dynamic surface properties. The quantitative
relations depend on the composition of the system, the construction of the
apparatus used, and the energy input level. We will not consider all these
aspects but merely outline the most important principles involved.

TABLE11.1 Order of Magnitude of Some Quantities in Foams and Emulsions at
Room Temperaturea

Property Foam Foam

Emuls.

W–O

Emuls.

O–W Units

Drop/bubble diameter 103 100 3 0.5 mm
Volume fraction 0.9 0.9 0.1 0.1 —
Drop/bubble number 109 1012 1016 1018 m
^3
Interfacial area 0.005 0.05 0.2 1.2 m^2 ?ml
^1
Interfacial tension 40 40 6 10 mN?m
^1
Laplace pressure 102 103 104 105 Pa
Solubility D in C 2.1b 2.1b 0.15 0 c %v/v
Density difference D
C
103
103 102
102 kg?m
^3
Viscosity ratio D/C 10
^410
^410
^2102 —
Typical time scaled 10
^310
^410
^510
^6 s

aOil phase is a triacylglycerol oil, gas phase is air, D¼disperse phase, C¼continuous phase.
bIf the gas phase is CO 2 , the solubility is about 100%v/v, but strongly dependent on

composition of the aqueous phase, especially pH and salt composition.
cOils often contain minor components that are somewhat soluble in water, but the solubility
of the triacylglycerols is generally negligible.
dTime needed for separate events during formation, e.g., the deformation time of a drop or
bubble.