Food Lipids: Chemistry, Nutrition, and Biotechnology

destabilization of an emulsion. We now turn to a discussion of the origin of the
major destabilization mechanisms, the factors that influence them, and methods of
controlling them. This type of information is useful for food scientists because it
facilitates the selection of the most appropriate ingredients and processing conditions
required to produce a food emulsion with particular properties.
1. Creaming and Sedimentation
The droplets in an emulsion have a density different from that of the liquid that
surrounds them, and so a net gravitational force acts on them [1a,1b]. If the droplets
have lower density than the surrounding liquid, they tend to move up, that is, to
‘‘cream.’’ Conversely, if they have a higher density they tend to move down, resulting
in what is referred to as sedimentation. Most liquid oils have densities lower than
that of water, and so there is a tendency for oil to accumulate at the top of an
emulsion and water at the bottom. Thus droplets in an oil-in-water emulsion tend to
cream, whereas those in a water-in-oil emulsion tend to sediment. The creaming rate
of a single isolated spherical droplet in a viscous liquid is given by the Stokes
equation:
2
2gr (� 2 �� 1)
9� 1
� = � (9)
where � is the creaming rate, g the acceleration due to gravity, � the density, � the
shear viscosity, and the subscripts 1 and 2 refer to the continuous phase and droplet,
respectively. The sign of � determines whether the droplet moves up (�) or down
(�).
Equation (9) can be used to estimate the stability of an emulsion to creaming.
For example, an oil droplet (� 2 = 910 kg/m 3 ) with a radius of 1 �m suspended in
water (� 1 = 1 mPa�s, � 1 = 1000 kg/m 3 ) will cream at a rate of about 5 mm/day.
Thus one would not expect an emulsion containing droplets of this size to have a
particularly long shelf life. As a useful rule of thumb, an emulsion in which the
creaming rate is less than about 1 mm/day can be considered to be stable toward
creaming [3].
In the initial stages of creaming (Fig. 12), the droplets move upward and a
droplet-depleted layer is observed at the bottom of the container. When the droplets
reach the top of the emulsion, they cannot move up any further and so they pack
together to form the ‘‘creamed layer.’’ The thickness of the final creamed layer
depends on the packing of the droplets in it. Droplets may pack very tightly together,
or they may pack loosely, depending on their polydispersity and the magnitude of
the forces between them. Close-packed droplets will tend to form a thin creamed
layer, whereas loosely packed droplets form a thick creamed layer. The same factors
that affect the packing of the droplets in a creamed layer determine the nature of the
flocs formed (see Sec. VI.B.2). If the attractive forces between the droplets are fairly
weak, the creamed emulsion can be redispersed by lightly agitating the system. On
the other hand, if an emulsion is centrifuged, or if the droplets in a creamed layer
are allowed to remain in contact for extended periods, significant coalescence of the
droplets may occur, with the result that the emulsion droplets can no longer be
redispersed by mild agitation.
Creaming of emulsion droplets is usually an undesirable process, which food
manufacturers try to avoid. Equation (9) indicates that creaming can be retarded by
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.