Cold gelation of water in water emulsions stabilized by protein particles

Model water in water (W/W) emulsions were formed by mixing polyethylene oxide (PEO) and dextran in aqueous solution. The emulsions were stabilized by addition of 5 g/L fractal aggregates or microgels of proteins that accumulated at the interface between the two polymer phases. Excess protein particles partitioned to the dextran phase for pH > 4.0. Aggregation of the protein particles was induced at room temperature by reducing the net charge density of the proteins or by increasing the ionic strength. This so-called cold gelation process led to densification and gelation of the interfacial layer and to bridging of the emulsion droplets by the protein aggregates. Creaming of PEO droplets could be avoided by the formation of a system spanning network of excess protein particles in the continuous dextran phase. The behavior was different if gelation was induced by varying the pH or by addition of NaCl and depended on the morphology of the protein particles. Time-dependent measurement of the shear moduli showed that the emulsions formed weak gels with an elastic modulus that increased slowly with time. The results are compared with cold gelation in water and in equivalent dextran solutions. Cold gelation of excess protein particles in the dispersed phase is also discussed.