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Physico-chemical processes involved in the making of emulsions
For concentrated emulsions containing a significant amount of free hydrocolloid in the aqueous phase, an alternative explanation based on polymer-induced depletion forces is now regarded as more appropriate (Moschakis et al., 2005, Moschakis et al., 2006 and Parker et al., 1995). At very low concentrations, the added hydrocolloid has a destabilizing effect on the emulsion, since the depletion flocculation induced by the non-adsorbing hydrocolloid causes enhanced serum separation of the emulsion. But at higher added hydrocolloid concentrations (still <0.1 wt% for the case of xanthan gum), when the depletion interactions are stronger, creaming is inhibited due to the viscoelastic character of the interconnected regions of emulsion droplets that have become flocculated into a gel-like network. The system becomes kinetically trapped on the microscopic scale in a phase-separated state. For an emulsion containing <0.1 wt% xanthan, the local viscosity of the oil-droplet-rich regions has been estimated to be as much as 103 times larger than that for the neighbouring xanthan-rich regions (Moschakis et al., 2006). Moreover, the oil-droplet-rich microphase viscosity has been found to increase dramatically with xanthan concentration. So, although the xanthan-containing phase does become more viscoelastic with more xanthan present in the system, the main influence of the added hydrocolloid stabilizer on the overall rheology of the emulsion is through its effect on the oil droplet network. In the presence of added hydrocolloid, the kinetics of phase separation (leading in the long-term to enhanced gravity creaming and macroscopic serum separation) is controlled in the short/medium term by the rheological behaviour of the interconnected oil droplet regions. That is, the gravitationally unstable liquid-like emulsion has become transformed into a stable gel-like emulsion containing trapped ‘blobs’ of hydrocolloid-structured water
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