Food Lipids: Chemistry, Nutrition, and Biotechnology

sion is known as the phase inversion temperature (PIT). Consider what happens when
an emulsion that is stabilized by a lipid-based emulsifier is heated (Fig. 4). At temperatures
well below the PIT (�20�C), the packing parameter is significantly less
than unity, and so a system that consists of oil-in-water emulsion in equilibrium with
a swollen micellar solution is favored. As the temperature is raised, the hydrophilic
head groups of the emulsifier molecules become increasingly dehydrated, which
causes p to increase toward unity. Thus the emulsion droplets become more prone
to coalescence and the swollen micelles grow in size. At the phase inversion temperature,
p � 1, and the emulsion breaks down because the droplets have an ultralow
interfacial tension and therefore readily coalesce with each other. The resulting system
consists of excess oil and excess water (containing some emulsifier monomers),
separated by a third phase that contains emulsifier molecules aggregated into bilayer
structures. At temperatures sufficiently greater than the PIT, the packing parameter
is much larger than unity, and the formation of a system that consists of a water-inoil
emulsion in equilibrium with swollen reversed micelles is favored. A further
increase in temperature leads to a decrease in the size of the reversed micelles and
in the amount of water solubilized within them. The method of categorizing emulsifier
molecules according to their molecular geometry is now widely accepted as
the most useful means of determining the types of emulsion they tend to stabilize
[17].
4. Other Factors
The classification schemes mentioned above provide information about the type of
emulsion an emulsifier tends to stabilize (i.e., O/W or W/O), but they do not provide
much insight into the size of the droplets that form during homogenization or the
stability of the emulsion droplets once formed [1]. In choosing a suitable emulsifier
for a particular application, these factors must also be considered. The speed at which
an emulsifier adsorbs to the surface of the emulsion droplets produced during ho-
Figure 4 Phase inversion temperature in emulsions.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.