Role of Protein-Stabilized Interfaces on the Microstructure and Rheology of Oil-in- Water Emulsions - Food Colloids: Self-Assembly and Material Science - page 395

the local interaction potentials for the two systems may be very different,
particularly in the light of the differences in surface mobility.
It is known 17 that the rheological properties of dispersions are related to their
droplet-size distributions through the Laplace pressure. Because of this, we
have kept the droplet size distribution as constant as possible for all the
emulsions, and have measured the equilibrium interfacial tension of the systems
used to make the emulsions. The surface tensions differ by a factor of two for
the protein and surfactant systems, so indicating a predicted twofold variation
in the bulk elastic modulus of the emulsions. 9,17 The bulk rheology data of the
emulsions (Figure 2) shows that the elasticity is about a factor of 10 larger for
the protein-stabilized system than for the surfactant system. As this clearly
cannot be explained solely through changes in the Laplace pressure, it suggests
that other factors are also involved.
There would seem to be two possible explanations for the difference in the
bulk rheological data. First, there is the variability in interfacial rheology as
shown in Table 1. These data indicate that, in the concentrated region of the
cream, the increased interfacial shear elasticity of the protein system would tend
to slow down, and possibly to arrest, the rearrangement of the droplets. This
could explain the disparity in the maximum packing fractions for the two
systems seen in Figures 3 and 4. Nevertheless, there is clearly no simple linear
relationship between interfacial and bulk rheology, as the interfacial shear
rheology shows a difference of more than 100-fold between the systems.
Further to this, there is also substantial variation in the interfacial dilatational
rheology (Table 1). A higher surface elasticity would tend to reduce droplet
deformation 15,21 and thus increase bulk elasticity. 18
The second point of contrast between the protein- and surfactant-coated
interfaces is in the close-range interaction. While surfactant-stabilized systems
tend to have a repulsive interaction even at close range, it has been shown,
using several different methods, such as thin films, 31 magnetic chaining, 23,32
etc., that protein-stabilized interfaces can show a significant degree of adhesion.
Thus, when two protein-covered interfaces are brought into close proximity
they tend to stick together. This was also suggested 16 by experiments on
concentrated emulsion systems which do not spontaneously redisperse on
dilution. The cream layer contains droplet interfaces that are forced together
by gravity, and the lack of Brownian diffusion observed by confocal micros-
copy in the cream layer of the protein-stabilized system suggests that the
droplets were attracted to each other at close approach, at least more so than
for the surfactant-stabilized droplets.
Each of these hypothetical explanations seems equally plausible, and there
has been insufficient experimental evidence obtained so far to suggest which is
the more important. Thus, for future work, we are designing systems which
possess a homogeneous surfactant-like interfacial layer, but also display similar
interfacial rheological properties to a protein-stabilized interface. The bulk
rheological behaviour of the concentrated dispersion stabilized by this type of
system should then provide powerful evidence for determining whether either