Food Lipids: Chemistry, Nutrition, and Biotechnology

[Eq. (9)], but at high droplet concentrations, it retards creaming because the droplets
are trapped within the three-dimensional network of aggregated emulsion droplets.
In coalescence (Fig. 12), two or more liquid droplets collide and merge into a
single larger droplet. Extensive coalescence eventually leads to oiling off, i.e., formation
of free oil on the top of an emulsion. Because coalescence involves a decrease
in the surface area of oil exposed to the continuous phase, it is one of the principal
mechanisms by which an emulsion reverts to its most thermodynamically stable state
(Fig. 1). Coalescence occurs rapidly between droplets that are not protected by emulsifier
molecules; for example, if one homogenizes oil and water in the absence of
an emulsifier, the droplets readily coalesce. When droplets are stabilized by an emulsifier
membrane, the tendency for coalescence to occur is governed by the droplet–
droplet interaction potential and the stability of the film to rupture. If there is a strong
repulsive force between the droplets at close separations, or if the film is highly
resistant to rupture, the droplets will tend not to coalesce. Most food emulsions are
stable to coalescence, but they become unstable when subjected to high shear forces
that cause the droplets to frequently collide with each other or when the droplets
remain in contact with each other for extended periods (e.g., droplets in flocs,
creamed layers, or highly concentrated emulsions).
3. Partial Coalescence
Normal coalescence involves the aggregation of two or more liquid droplets to form
a single larger spherical droplet, but partial coalescence occurs when two or more
partially crystalline droplets encounter each other and form a single irregularly
shaped aggregate (Fig. 13). The aggregate is irregular in shape because some of the
structure of the fat crystal network contained in the original droplets is maintained
within it. It has been proposed that partial coalescence occurs when two partially
crystalline droplets collide and a crystal from one of them penetrates the intervening
membranes and protrudes into the liquid region of the other droplet [1a]. Normally,
the crystal would stick out into the aqueous phase, thus becoming surrounded by
water; however, when it penetrates another droplet, it is surrounded by oil, and
because this arrangement is energetically favorable the droplets remain aggregated.
With time the droplets slowly fuse more closely together, with the result that the
total surface area of oil exposed to the aqueous phase is reduced. Partial coalescence
occurs only when the droplets have a certain ratio of solid fat and liquid oil. If the
solid fat content of the droplets is either too low or too high, the droplets will tend
not to undergo partial coalescence [5].
Figure 13 Partial coalescence occurs when two partly crystalline emulsion droplets collide
and aggregate because a crystal in one droplet penetrates the other droplet.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.