Chemistry Reference
In-Depth Information
of oil droplets and gel droplets has provided a yield stress and flow be-
haviour which are similar (although still not an exact rheological match)
to the full-fat emulsion and with a close match in sensory properties
when tested by a consumer panel. The tribology data discussed above
suggests that at 5% polymer concentration, the thin-film behaviour will
be significantly affected and thus give an improved lubrication score in
sensory scores, although no such data has yet been reported in the open
literature.
10.3.5
Water-in-water emulsions
The next level of complexity is to have mixed biopolymers in which
the two or more molecular structures lead to phase separation. The
simplest way to consider these is to regard them as composites where
one of the components forms a continuous network across the entire
system and the other serves as a gel filler, i.e. resembling an oil/water
or water/oil system. A typical photomicrograph is shown in Fig. 10.16.
This figure shows different water-in-water emulsion structures and how
they are influenced by the amount of the phases. So Fig. 10.16a shows
the single phase observed for the protein phase with LBG at a level that
is miscible. Fig. 10.16b then shows the microstructure obtained when
25% of the LBG phase is present. In this picture, the droplets of LBG
can be seen as a dark phase included in the protein continuous phase.
Fig. 10.16c shows the bi-continuous structure, which occurs when 50:50
mixtures are used. This is the condition when both phases are capable
of being continuous and the confused system is obtained. This type of
structure persists for quite a concentration range around the 50:50 mix
as discussed by Norton and Frith (2001). Once a dominant phase volume
of the LBG phase inversion of the structure has occurred, we can see
protein phase included in a continuous LBG phase.
As can be seen from this set of photomicrographs, the mixed biopoly-
mer systems behave like a water-in-oil mixture in which the phase with
the highest phase volume dominates. When a 50:50 mixture is present,
the structure attempts to be bi-continuous. In mixed biopolymer sys-
tems, this bi-continuous region can and does persist over a far wider
concentration range. The design principles for these types of system
have been described before (Norton and Frith, 2001). The way that
they behave in flow has been described and from this came the name
of water-in-water emulsions (Spyropoulos
et al
., 2007, 2008a, 2008b).
So if these systems behave as water-in-water emulsions, can they be
induced to phase invert in shear? Phase inversion was originally talked
about some time ago (Kasapis
et al
., 1993a, 1993b, 1993c, 1993d) for
the gelatin/maltodextrin mixture in which both polymers were gelling.
It was argued that inversion was a consequence of the molecular weight
