Pea protein microgel particles as Pickering stabilisers of oil-in-water emulsions: Responsiveness to pH and ionic strength

Abstract

The aim of this study was to design plant protein-based microgel particles to create Pickering emulsions (20 wt% sunflower oil, 0.05–1.0 wt% protein) and investigate the role of electrostatic interactions on colloidal behaviour of such emulsions. Pea protein microgels (PPM) were designed using a facile top-down approach of heat-set protein gel formation followed by controlled shearing. The aqueous dispersion of PPM had hydrodynamic diameters ranging from 200 to 400 nm at pH 7.0 to pH 9.0 with high negative charge (−30 to −35 mV) and pI was at pH 5.0. With increasing ionic concentration from 1 to 250 mM NaCl, the ζ-potential of PPM changed to −8 mV due to charge screening effects, in line with theoretical calculations of the electrostatic potential. The Pickering emulsions with smallest droplet sizes (d43) ~25 μm exhibited excellent coalescence stability and high adsorption efficiency of PPM at the oil-water interface (>98%) at pH 7.0, with the latter being supported by confocal microscopy showing effective adsorption of the PPMs at the droplet surface. Adjusting the pH of the emulsions to pI demonstrated aggregation of adsorbed PPM at the particle-laden interface providing a higher degree of adsorption as well as enhancing inter-droplet flocculation and the shear-thinning character as compared to those at pH 7.0 or pH 3.0. Charge screening effects in presence of 100 mM NaCl resulted in PPM-PPM aggregation and enhanced viscosity of the emulsions. Findings from this study on pea protein microgels would open avenues for rational designing of sustainable Pickering emulsions in the future.