Food Lipids: Chemistry, Nutrition, and Biotechnology

profile of interdroplet pair potential versus separation for an emulsion stabilized by
a charged biopolymer. When the two droplets are separated by a large distance, there
is no effective interaction between them. As they move closer together, the van der
Waals attraction dominates initially and there is a shallow minimum in the profile,
which is referred to as the secondary minimum. If the depth of this minimum is
large compared to the thermal energy (��G(s min2)� > kT), the droplets tend to be
flocculated. However, if it is small compared to the thermal energy, the droplets tend
to remain unaggregated. At closer separations the repulsive electrostatic interactions
dominate, and there is an energy barrier �G(s max) that must be overcome before the
droplets can come any closer. If this energy barrier is sufficiently large compared to
the thermal energy �G(s max) � kT, it will prevent the droplets from falling into the
deep primary minimum at close separations. On the other hand, if it is not large
compared to the thermal energy, the droplets will tend to fall into the primary minimum,
leading to strong flocculation of the droplets. In this situation the droplets
would be prevented from coalescing because of the domination of the strong steric
repulsion at close separations.
Emulsions that are stabilized by repulsive electrostatic interactions are particularly
sensitive to the ionic strength and pH of the aqueous phase [1a,1b]. At low
ion concentrations there may be a sufficiently high energy barrier to prevent the
droplets from getting close enough together to aggregate into the primary minimum.
As the ion concentration is increased, the screening of the electrostatic interactions
becomes more effective, which reduces the height of the energy barrier. Above a
certain ion concentration, the energy barrier is not high enough to prevent the droplets
from falling into the primary minimum, and so the droplets become strongly
flocculated. This phenomenon accounts for the tendency for droplets to flocculate
when salt is added to emulsions stabilized by ionic emulsifiers. The surface charge
density of protein-stabilized emulsions decreases as the pH tends toward the isoelectric
point, which reduces the magnitude of the repulsive electrostatic interactions
between the droplets and also leads to droplet flocculation.
B. Mechanisms of Emulsion Instability
As mentioned earlier, emulsions are thermodynamically unstable systems that tend
with time to revert back to the separate oil and water phases of which they were
made. The rate at which this process occurs, and the route that is taken, depend on
the physicochemical properties of the emulsion and the prevailing environmental
conditions. The most important mechanisms of physical instability are creaming,
flocculation, coalescence, Ostwald ripening, and phase inversion. In practice, all these
mechanisms act in concert and can influence one another. However, one mechanism
often dominates the others, facilitating the identification of the most effective method
of controlling emulsion stability.
The length of time an emulsion must remain stable depends on the nature of
the food product. Some food emulsions (e.g., cake batters, ice cream mix, margarine
premix) are formed as intermediate steps during a manufacturing processes and need
remain stable for only a few seconds, minutes, or hours. Other emulsions (e.g.,
mayonnaise, creme liqueurs) must persist in a stable state for days, months, or even
years prior to sale and consumption. Some food processing operations (e.g., the
production of butter, margarine, whipped cream, and ice cream) rely on controlled
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