Particle Tracking as a Probe of Microrheology in Food Colloids - Food Colloids: Self-Assembly and Material Science - page 317

time of onset of network collapse depends on the effective yield stress of the oil-
droplet network and on the relative densities of the microphases. Such a system
typically exhibits a characteristic delay time before the emulsion gel finally
collapses completely. 11,25 While the stabilizing emulsion gel structure is inher-
ently metastable, it can be very long-lasting for a sufficiently high xanthan
concentration.
Hydrocolloid thickeners like xanthan gum are commonly added to pourable
salad dressings and other food emulsions to enhance the shelf life during
extended storage. The traditional explanation for the stabilizing effect of the
hydrocolloid is via control of the rheology of the aqueous continuous phase.
This explanation appears quite sound at low oil volume fractions, where
individual droplets can be separately immobilized in an entangled biopolymer
network, and the small buoyancy force acting on each droplet is considered
insufficient to overcome the effective yield stress of the surrounding weak gel-
like biopolymer matrix. But, a different explanation is required for concen-
trated emulsions because the evolving microstructure is highly heterogeneous, 14
and it has long been recognized 15,26,27 that the emulsion (in)stability correlates
more with the rheology of the emulsion than with that of the hydrocolloid
aqueous phase.
On the basis of experimental evidence presented here, we therefore can
confirm the stabilization mechanism proposed by Parker and co-workers. 25
That is, the concentrated emulsion is prevented from creaming by the visco-
elastic character of interconnected regions of droplets that have become
flocculated (and locally further concentrated) under the influence of attractive
depletion interactions induced by the non-adsorbed polysaccharide. In essence,
the incipiently unstable salad-dressing-like emulsion is transformed, through
changes in microrheology, into a stable 'thick' mayonnaise-like emulsion
containing embedded blobs of low-viscosity biopolymer-structured water.
Acknowledgements
This research was supported by an EPSRC Studentship with ICI plc (UK). TM
thanks the State Scholarships Foundation of Greece for financial support.
References
1. F.C. MacKintosh and C.F. Schmidt, Curr. Opin. Colloid Interface Sci.,
1999, 4, 300.
2. A. Mukhopadhyay and S. Granick, Curr. Opin. Colloid Interface Sci.,
2001, 6, 423.
3. H. Freundlich and W. Seifriz, Z. Phys. Chem., 1922, 104, 233.
4. A.R. Bauch, W. Moller and E. Sackmann, Biophys. J., 1999, 76, 573.
5. T. Gisler and D.A. Weitz, Curr. Opin. Colloid Interface Sci., 1998, 3, 586.
6. T.G. Mason and D.A. Weitz, Phys. Rev. Lett., 1995, 74, 1250.