Food Lipids: Chemistry, Nutrition, and Biotechnology

Figure 10 The size of the droplets produced in an emulsion is a balance between droplet
disruption and droplet coalescence.
scattered from the emulsion droplets). With time, the system rapidly reverts back to
its initial state—a layer of oil sitting on top of the water. This is because the droplets
formed during the application of the mechanical agitation are constantly moving
around and frequently collide and coalesce with neighboring droplets. As this process
continues, the large droplets formed rise to the top of the container and merge together
to form a separate layer.
To form a stable emulsion, one must prevent the droplets from merging after
they have been formed. This is achieved by having a sufficiently high concentration
of a surface-active substance, known as an emulsifier, present during the homogenization
process. The emulsifier rapidly adsorbs to the droplet surfaces during homogenization,
forming a protective membrane that prevents the droplets from coming
close enough together to coalesce. One of the major objectives of homogenization
is to produce droplets as small as possible because this usually increases the shelf
life of the final product. It is therefore important for the food scientist to understand
the factors that determine the size of the droplets produced during homogenization.
It should be noted that homogenization is only one step in the formation of a food
emulsion, and many of the other unit operations (e.g., pasteurization, cooking, drying,
freezing, whipping) also affect the final quality of the product.
A. Physical Principles of Emulsion Formation
The size of the emulsion droplets produced by a homogenizer depends on a balance
between two opposing mechanisms: droplet disruption and droplet coalescence (Fig.
10). The tendency for emulsion droplets to break up during homogenization depends
on the strength of the interfacial forces that hold the droplets together, compared to
the strength of the disruptive forces in the homogenizer. In the absence of any applied
external forces, emulsion droplets tend to be spherical because this shape minimizes
the contact area between oil and water phases. Changing the shape of a droplet, or
breaking it into smaller droplets, increases this contact area and therefore requires
the input of energy. The interfacial force holding a droplet together is given by the
Laplace pressure (�P 1):
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