Food Lipids: Chemistry, Nutrition, and Biotechnology

0.1–50 �m [2,3]. A system that consists of oil droplets dispersed in an aqueous
phase is called an oil-in-water (O/W) emulsion. A system that consists of water
droplets dispersed in an oil phase is called a water-in-oil (W/O) emulsion. The material
that makes up the droplets in an emulsion is referred to as the dispersed or
internal phase, whereas the material that makes up the surrounding liquid is called
the continuous or external phase. Multiple emulsions can be prepared that consist
of oil droplets contained in larger water droplets, which are themselves dispersed in
an oil phase (O/W/O), or vice versa (W/O/W). Multiple emulsions can be used for
protecting certain ingredients, for controlling the release of ingredients, or for creating
low-fat products [9].
Emulsions are thermodynamically unstable systems because of the positive free
energy required to increase the surface area between the oil and water phases [3].
The origin of this energy is the unfavorable interaction between oil and water, which
exists because water molecules are capable of forming strong hydrogen bonds with
other water molecules but not with oil molecules [8,9]. Thus emulsions tend to reduce
the surface area between the two immiscible liquids by separating into a system that
consists of a layer of oil (lower density) on top of a layer of water (higher density).
This is clearly seen if one tries to homogenize pure oil and pure water together:
initially an emulsion is formed, but after a few minutes phase separation occurs
(Fig. 1).
Emulsion instability can manifest itself through a variety of physicochemical
mechanisms, including creaming, flocculation, coalescence, and phase inversion
(Sec. VI). To form emulsions that are kinetically stable for a reasonable period (a
few weeks, months, or even years), chemical substances known as emulsifiers must
be added prior to homogenization. Emulsifiers are surface-active molecules that adsorb
to the surface of freshly formed droplets during homogenization, forming a
protective membrane that prevents the droplets from coming close enough together
to aggregate [3]. Most food emulsifiers are amphiphilic molecules, i.e., they have
both polar and nonpolar regions on the same molecule. The most common types
used in the food industry are lipid-based emulsifiers (small molecule surfactants and
phospholipids) and amphiphilic biopolymers (proteins and polysaccharides) [2,3].
Most food emulsions are more complex than the simple three-component (oil,
water, and emulsifier) system described above [3,5,9]. The aqueous phase may contain
water-soluble ingredients of many different kinds, including sugars, salts, acids,
bases, surfactants, proteins, and polysaccharides [1]. The oil phase may contain a
variety of lipid-soluble components, such as triacylglycerols, diacylglycerols, monoacylglycerols,
fatty acids, vitamins, and cholesterol [1]. The interfacial membrane
Figure 1 Emulsions are thermodynamically unstable systems that tend to revert back to
the individual oil and water phases with time. To produce an emulsion, energy must be
supplied.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.