Chemistry Reference
In-Depth Information
interesting finding and one which will undoubtedly be developed fur-
ther, as these rheological consequences appear to be attractive in terms
of development of new textures and texture control.
Rheological properties of phase-separated mixtures (known as water-
in-water (w/w) emulsions) at a temperature where neither hydrocolloid
is ordered can be described using an emulsion model (Capron
et al
.,
2001; Stokes
et al
., 2001; Simeone
et al
., 2002). This allows for the de-
duction of the interfacial tension in these systems. Values are in the order
of µN/m, which has been confirmed by direct measurement (Ding
et al
.,
2002; Simeone
et al
., 2004; Spyropoulos
et al
., 2008). In mixtures with a
gelling droplet phase, the shape of the gelled droplets can be controlled
by the flow stresses experienced during gelation (Wolf
et al
., 2000;
Erni
et al
., 2009). The rheological behaviour of the resulting suspen-
sion depends on the particle shape (Wolf
et al
., 2001). The rheological
behaviour of particles of different size and shape suspended in a hydro-
colloid solution, representative of a number of commercially applicable
semi-dilute dispersions, has been described with the Cross model mod-
ified to include yield stress behaviour (Rayment
et al
., 2000). Foster
et al
. (1996) showed that high shear during gelation can induce phase
inversion of w/w emulsions, with the first gelling hydrocolloid form-
ing the dispersed phase and the starting phase volume being retained.
Therefore, high dispersed phase volume systems can be produced. If
the particles are able to interact, a string-of-pearls type of structure can
be created. Firoozmand
et al
. (2007) described a similar particle gel
network for another gelatin:oxidised starch composite formed in the
absence of shear.
Recent work on phase-separating materials has produced new in-
sights. Boyd
et al
. (2009) have shown that mixtures of xanthan with
other polyelectrolytes possess significantly lower viscosities than those
of xanthan solutions of the same concentration alone. This behaviour
appears to be less related to a loss of xanthan viscosity as this forms
the dispersed phase in the mixture, but more to the low-viscosity liquid
crystalline phase that xanthan is driven to form. Upon dilution, the xan-
than forms a conventional polymeric solution and the viscosity increases
dramatically. Lad
et al
. (2010) have indicated that a similar viscosity
reduction can be observed when xanthan is mixed with particulate hy-
drocolloids such as cold water swelling starch granules.
Foster (2007) and more recently Shrinivas
et al
. (2009) have shown
that the spectrum of rheological properties of w/w emulsions can be
enhanced further by entrapping oil or fat in the dispersed phase. Such
systems can be regarded as (oil-in-water)-in-water emulsions. Air may
also be used as the innermost phase, and the opportunity for the cre-
ation of a vast range of products with interesting behaviours is ob-
vious. Recent work by Ganzevles
et al
. (2006a, 2006b, 2008) has
