Food Lipids: Chemistry, Nutrition, and Biotechnology

divide homogenization into two categories. The creation of an emulsion directly from
two separate liquids will be referred to as primary homogenization, whereas the
reduction in size of droplets in an existing emulsion will be referred to as secondary
homogenization (Fig. 9). The creation of a food emulsion may involve the use of
one or the other form of homogenization, or a combination of both. For example,
salad dressing is formed by direct homogenization of the aqueous and oil phases
and is therefore an example of primary homogenization, whereas homogenized milk
is manufactured by reducing the size of the fat globules in natural milk and hence
is an example of secondary homogenization.
In many food processing operations and laboratory studies it is more efficient
to prepare an emulsion using two steps. The separate oil and water phases are converted
to a coarse emulsion, with fairly large droplets, using one type of homogenizer
(e.g., high speed blender). Then the droplet size is reduced by means of another type
of homogenizer (e.g., colloid mill, high pressure valve homogenizer). In reality, many
of the same physical processes that occur during primary homogenization also occur
during secondary homogenization, and there is no clear distinction between them.
Emulsions that have undergone secondary homogenization usually contain smaller
droplets than those that have undergone primary homogenization, although this is
not always the case. Some homogenizers (e.g., ultrasound, microfluidizers, membrane
homogenizers) are capable of producing emulsions with small droplet sizes directly
from separate oil and water phases (see Sec. V.C).
To highlight the important physical mechanisms that occur during homogenization,
it is useful to consider the formation of an emulsion from pure oil and pure
water. When the two liquids are placed in a container, they tend to adopt their
thermodynamically most stable state, which consists of a layer of oil on top of the
water (Fig. 1). This arrangement is adopted because it minimizes the contact area
between the two immiscible liquids and because the oil has a lower density than the
water. To create an emulsion, it is necessary to mechanically agitate the system, to
disrupt and intermingle the oil and water phases. The type of emulsion formed in
the absence of an emulsifier depends primarily on the initial concentration of the
two liquids. At high oil concentrations a water-in-oil emulsion tends to form, but at
low oil concentrations an oil-in-water emulsion tends to form. In this example, it is
assumed that the oil concentration is so low that an oil-in-water emulsion is formed.
Mechanical agitation can be applied in a variety of ways, the simplest being to
vigorously shake the oil and water together in a sealed container. An emulsion is
formed immediately after shaking, and it appears optically opaque (because light is
Figure 9 The homogenization process can be divided into two steps: primary homogenization
(creating an emulsion from two separate phases) and secondary homogenization (reducing
the size of the droplets in a preexisting emulsion).
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.