Food Lipids: Chemistry, Nutrition, and Biotechnology

of these mechanisms, the relationship between them, and the factors that affect them,
so that effective means of controlling the properties of food emulsions can be
established.
A. Droplet–Droplet Interactions
The bulk properties of food emulsions are largely determined by the interaction of
the droplets with each other. If the droplets exert a strong mutual attraction, they
tend to aggregate, but if they are strongly repelled they tend to remain as separate
entities. The overall interaction between droplets depends on the magnitude and
range of a number of different types of attractive and repulsive interaction. A knowledge
of the origin and nature of these interactions is important because it enables
food scientists to predict and control the stability and physicochemical properties of
food emulsions.
Droplet–droplet interactions are characterized by an interaction potential �G
(s), which describes the variation of the free energy with droplet separation. The
overall interaction potential between emulsion droplets is the sum of various attractive
and repulsive contributions [3]:
�G(s) =�G VDW(s) � �G electrostatic (s) � �G hydrophobic(s)
� �G short range(s)
(5)
where �G VDW, �G electrostatic, �G hydrophobic, and �G short range refer to the free energies
associated with van der Waals, electrostatic, hydrophobic, and various short-range
forces, respectively. In certain systems, there are additional contributions to the overall
interaction potential from other types of mechanism, such as depletion or bridging
[1a,1b]. The stability of food emulsions to aggregation depends on the shape of the
free energy versus separation curve, which is governed by the relative contributions
of the different types of interaction [1–3].
1. van der Waals Interactions
The van der Waals interactions act between emulsion droplets of all types and are
always attractive. At close separations, the van der Waals interaction potential between
two emulsion droplets of equal radius r separated by a distance s is given by
the following equation [12]:
Ar
�G VDW(s)
=� (6)
12s
where A is the Hamaker parameter, which depends on the physical properties of the
oil and water phases. This equation provides a useful insight into the nature of the
van der Waals interaction. The strength of the interaction decreases with the reciprocal
of droplet separation, and so van der Waals interactions are fairly long range
compared to other types of interaction. In addition, the strength of the interaction
increases as the size of the emulsion droplets increases. In practice, Eq. (6) tends to
overestimate the attractive forces because it ignores the effects of electrostatic screening,
radiation, and the presence of the droplet membrane on the Hamaker parameter
[11].
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.