Role of Protein-Stabilized Interfaces on the Microstructure and Rheology of Oil-in- Water Emulsions - Food Colloids: Self-Assembly and Material Science - page 386

The other important properties of emulsions that are relevant to organoleptic
behaviour are microstructure and rheology. 6,7 Apart from the properties of
the continuous phase, the main factors which govern emulsion rheology are
the properties of the dispersed phase droplets. 6,8 These include the droplet
volume fraction, the droplet droplet interactions, and the droplet viscosity
and deformability. Considerable progress has been made in understanding the
factors that influence the physical characteristics of model emulsions. It has
been shown 9 that there is a sharp transition from liquid to solid-like behav-
iour as the droplet volume fraction reaches the random packing limit, partic-
ularly in monodisperse systems. 10 But our knowledge of the structure
and rheology of dispersions containing polydisperse, deformable droplets is
still at an elementary stage. Emulsified oil droplets have a fluid internal phase
and a flexible interface. The application of stress can cause circulation of the
internal phase and may lead to droplet distortion. 11 The deformation of the
droplets is influenced by the viscosities of the dispersed and continuous
phases, and by the strength of the shear field. Taylor 12 predicted that interfacial
tension plays a key role in controlling distortion, and more recently Otsubo and
Prudhomme 8 have demonstrated a scaling relation for the viscosity of an O/W
emulsion in terms of droplet size, the viscosities of the two phases and the
interfacial tension. Even more recently Mason et al. 10,13 showed that the
Laplace pressure (2 g /r), and thus both the interfacial tension g and the
droplet radius r, is an important factor in relation to emulsion rheology, and
so should be used as a normalization factor. Evidence that the interfacial
tension is not the only characteristic of the interface that may be involved in the
deformation of droplets has been supplied recently by the studies of
Pozrikidis 14 and Nadim. 15 Pozrikidis 14 studied the deformation of a liquid
drop with a constant isotropic surface tension and finite surface viscosity. He
found that the surface viscosity acts to suppress the interfacial motion and
hence it reduces the magnitude of the droplet deformation. Nadim 15 provided
results which suggest that the deformation of droplets and the measured
effective viscosity of the emulsion are functions of the surface shear and
dilatational viscosities.
Concentrated emulsions stabilized by proteins tend to have greater elastic
moduli than emulsions stabilized by low-molecular-weight emulsifiers. 7,16-19
We have already reported 20 that the interfacial structure of simple emulsion
systems can have a significant effect on the organoleptic properties of emul-
sions. We showed that the interfacial composition of test emulsions influences
the sensory perception of 'creaminess' and fat content. The main result was that
emulsions stabilized by proteins possess an enhanced sensory response to
fat content. This was attributed to changes in interfacial composition affecting
emulsion rheological behaviour. An enhanced viscosity was found for the
protein-stabilized emulsion when compared with a similar emulsifier-stabilized
emulsion, but the mechanism underlying the phenomenon was not entirely
clear. There have been some theoretical treatments showing how an elastic
interfacial layer can influence droplet deformation 15,21 and emulsion rheo-
logy, 18 and certainly the elasticity of most globular protein interfaces would