Food Lipids: Chemistry, Nutrition, and Biotechnology

2�
�P 1 = (4)
r
where � is the interfacial tension between oil and water, and r is the droplet radius.
This equation indicates that it is easier to disrupt large droplets than small ones and
that the lower the interfacial tension, the easier it is to disrupt a droplet. The nature
of the disruptive forces that act on a droplet during homogenization depends on the
flow conditions (i.e., laminar, turbulent, or cavitational) the droplet experiences and
therefore on the type of homogenizer used to create the emulsion. To deform and
disrupt a droplet during homogenization, it is necessary to generate a stress that is
greater than the Laplace pressure and to ensure that this stress is applied to the
droplet long enough to enable it to become disrupted [21–23].
Emulsions are highly dynamic systems in which the droplets continuously
move around and frequently collide with each other. Droplet–droplet collisions are
particularly rapid during homogenization because of the intense mechanical agitation
of the emulsion. If droplets are not protected by a sufficiently strong emulsifier
membrane, they tend to coalesce during collision. Immediately after the disruption
of an emulsion droplet during homogenization, there is insufficient emulsifier present
to completely cover the newly formed surface, and therefore the new droplets are
more likely to coalesce with their neighbors. To prevent coalescence from occurring,
it is necessary to form a sufficiently concentrated emulsifier membrane around a
droplet before it has time to collide with its neighbors. The size of droplets produced
during homogenization therefore depends on the time taken for the emulsifier to be
adsorbed to the surface of the droplets (� adsorption) compared to the time between
droplet–droplet collisions (� collision). If � adsorption � � collision, the droplets are rapidly
coated with emulsifier as soon as they are formed and are stable; but if � adsorption �
� collision, the droplets tend to rapidly coalesce because they are not completely coated
with emulsifier before colliding with one of their neighbors. The values of these two
times depend on the flow profile the droplets experience during homogenization, as
well as the physicochemical properties of the bulk phases and the emulsifier [1a,23].
B. Role of Emulsifiers
The preceding discussion has highlighted two of the most important roles of emulsifiers
during the homogenization process:
1. Their ability to decrease the interfacial tension between oil and water
phases and thus reduce the amount of energy required to deform and disrupt
a droplet [Eq. (4)]. It has been demonstrated experimentally that when
the movement of an emulsifier to the surface of a droplet is not ratelimiting
(� adsorption � � collision), there is a decrease in the droplet size produced
during homogenization with a decrease in the equilibrium interfacial tension
[24].
2. Their ability to form a protective membrane that prevents droplets from
coalescing with their neighbors during a collision.
The effectiveness of emulsifiers at creating emulsions containing small droplets
depends on a number of factors: (a) the concentration of emulsifier present relative
to the dispersed phase; (b) the time required for the emulsifier to move from the
bulk phase to the droplet surface; (c) the probability that an emulsifier molecule will
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