Microstructure, rheology and demixing in emulsions flocculated by polysaccharides

Abstract

keywords: Emulsion, b-lactoglobulin, polysaccharides, salt, sucrose, depletion, bridging, percolation, microstructure, micro-rheology, rheology, demixing, creaming, network compression. Abstract In this thesis, a study is presented on gravity-induced demixing behaviour of oil-in-water emulsions, stabilised by b-lactoglobulin and flocculated by various polysaccharides. Flocculation by polysaccharides mainly results in formation of emulsion droplet networks and can proceed via depletion and via bridging. Structural and rheological properties of these different networks were investigated and compared on a micro-and macroscopic level. These properties were related to the demixing behaviour of the emulsions. For emulsion droplet networks, gravity-induced compression of the network leads to separation of a serum layer. For depletion-induced networks, the initial rate of demixing by network compression is usually low and at high polysaccharide concentrations, usually a delay-time is observed before substantial demixing occurs. This delay-time scales with the permeability of the network, the viscosity and the density difference between oil and water. Once demixing has started, the network quickly collapses until the emulsion droplets are packed rather closely together. In bridging-flocculated emulsions, the initial demixing rate is higher, but more water was retained at longer times. The effects of protein, sugar and salts on demixing of depletion-flocculated networks were investigated as well. Protein affected the rate of flocculation and counteracted network formation. Sucrose affected the demixing rate via the viscosity and density of the aqueous phase, but it did not affect the droplet-droplet interactions. Salt affected the electrostatic droplet-droplet interactions. As a result, depletion-flocculation by dextran was inhibited at low salt concentrations. Addition of Ca2+ ions led to a decrease in repulsion between the protein layers, resulting in stronger droplet-droplet bonds, reinforcing a droplet network and retarding network compression