Chemistry Reference
In-Depth Information
induce this effect. Experimental studies of the behaviour of protein-stabi-
lized emulsions have suggested
17
that the interactions between the protein-
stabilized droplets may be responsible for the enhanced rheological properties.
There have been several studies looking at the interactions between pro-
tein-stabilized droplets and surfaces, showing that, although long-range inter-
actions are dominated by electrostatic repulsion,
22-24
short-range interactions
can vary for different proteins.
22
Most globular proteins have a large steric
repulsion term,
22,24
sometimes due to the formation of multilayers,
22
or
adsorbed protein aggregates.
23
Also it has been inferred that non-DLVO
hydration forces may exist,
24,25
but the consequences of these interactions
have not been fully investigated. It has also been shown
26
that interactions
between protein-stabilized emulsion droplets can influence the phase separation
behaviour of the emulsion.
26
As far as we are aware, however, there has been
no systematic experimental study of the role that an adsorbed protein layer has
on the microstructure and bulk rheology of the cream layer of an O/W
emulsion.
The aim of the present study was to investigate more thoroughly how the
interfacial characteristics of proteins can influence the structure and bulk
rheology of O/W emulsions, with a view to understanding how we might be
able to manipulate the sensory properties of food emulsions. Our approach is
to quantify more precisely the role of interfacial composition by creating
emulsions that are as near identical in characteristics as possible, but with
varied interfacial rheological properties. This was achieved by first creating an
emulsion stabilized by protein (whey protein isolate) possessing an immobile,
viscoelastic interface. This interface was then disrupted and displaced by the
addition of surfactant
27,28
to produce a near-identical emulsion stabilized by
the surfactant. Different surfactant mixtures were used to control the surface
charge and zeta potential of the emulsion droplets. The resulting emulsions
were then characterized to determine the effect of interfacial composition on
the dynamic development of structure and rheology.
26.2 Materials and Methods
O/W emulsions (22 wt% oil) were prepared initially using 0.43 wt% whey
protein isolate (WPI) (BiPro, Davisco Foods, MN, USA) with 10 mM citrate
buffer at pH
¼
6.0 as the aqueous phase and 10:90 (by volume) hexade-
cane:heptane as the oil phase. The emulsion was prepared with a Waring
blender using a timed shearing cycle. The particle-size distribution of this
'parent' WPI emulsion was measured. This parent emulsion was then divided
into two equal parts and to each sample was added 9.09 g of sodium citrate
buffer (pH
¼
6) with or without 2.7 wt% surfactant. Displacement studies
showed that this surfactant concentration was sufficient to remove the protein
completely from the oil droplet surface. This produced two separate emulsions
with identical oil phase volumes and droplet-size distributions, one stabilized
by protein and the other by surfactant. The final continuous phase buffer
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