Food Lipids: Chemistry, Nutrition, and Biotechnology

2
4kTn0 N = (10)
3�
16 3 2
0
N = Gr n (11)
3
where n 0 is the initial number of particles per unit volume and G is the shear rate.
The collision efficiency, E, is the fraction of encounters between droplets that lead
to aggregation. Its value ranges from 0 (no floccuation) to 1 (fast flocculation) and
depends on the interaction potential. The equations for the collision frequency must
therefore be modified to take into account droplet–droplet interactions:
where
2
4kTn0 N = E (12)
3�
�� � � �
� �1
�G(x) �2
E = exp x dx
kT
2r
with x the distance between the centers of the droplets (x =2r � s) and �G(x) the
droplet–droplet interaction potential (Sec. VI.A). Emulsion droplets may remain unaggregated,
or they may aggregate into the primary or secondary minima depending
on �G(x).
The equations above are applicable only to the initial stages of aggregation in
dilute emulsions containing identical spherical particles. In practice, most food emulsions
are fairly concentrated systems, and interactions between flocs as well as between
individual droplets are important. The equations above must therefore be modified
to take into account the interactions and properties of flocculated droplets.
The nature of the droplet–droplet interaction potential also determines the
structure of the flocs formed, and the rheology and stability of the resulting emulsion
[1a]. When the attractive force between them is relatively strong, two droplets tend
to become ‘‘locked’’ together as soon as they encounter each other. This leads to the
formation of flocs that have quite open structures [3]. When the attractive forces are
not particularly strong, the droplets may ‘‘roll around’’ each other after a collision,
which allows them to pack more efficiently to form denser flocs. These two extremes
of floc structure are similar to those formed by filamentous and particulate gels,
respectively (Fig. 8).
The structure of the flocs formed in an emulsion has a pronounced influence
on its bulk physicochemical properties. An emulsion containing flocculated droplets
has a higher viscosity than one containing unflocculated droplets, since the water
trapped between the flocculated droplets increases the effective diameter (and therefore
volume fraction) of the particles (Eq. 3). Flocculated particles also exhibit strong
shear thinning behavior: as the shear rate is increased, the viscosity of the emulsion
decreases because the flocs are disrupted and so their effective volume fraction decreases.
If flocculation is extensive, a three-dimensional network of aggregated particles
extends throughout the system and the emulsion has a yield stress that must
be overcome before the system will flow. The creaming rate of droplets is also
strongly dependent on flocculation. At low droplet concentrations, flocculation increases
the creaming rate because the effective size of the particles is increased
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.