Dispersion behaviour of insoluble particles with different surface properties in non-aqueous media – biopolymer based oleogels

Abstract

Particles with some degree of hydrophilicity are known to aggregate when directly dispersed in non-aqueous media. Proteins are generally insoluble in oil and have complex surface properties, but they may form networks in oil like more simple colloidal particles, depending on particle size and surface hydrophilicity. Here, the particle size of pea protein isolate (PPI) particles in oil was reduced to submicron sizes by stirred media milling. The rheology of milled PPI oil suspensions was compared to dispersions prepared with two types of colloidal silica particles – hydrophobic and hydrophilic. PPI particles form structured aggregates in oil which break down under shear that, similarly to silica, can form an elastic network like an oleogel system. As PPI size decreased, aggregation increased, shown by higher apparent viscosities and gel strength. PPI particles with an average size of 1 μm exhibited elastic behaviour (G′ > G′′) at 11.2 wt%. Rheological scaling models obtained two fractal dimensions: a higher intra-floc dimension and a lower network backbone dimension, suggesting that colloidal PPI and silica particles have an inhomogeneous microstructure with denser particle flocs compared to a relatively sparse backbone. For smaller PPI particles the inter- and intra-floc fractal dimensions become like that of hydrophobic silica, suggesting that the average ‘surface’ character of the PPI may be close to that of the silica. Therefore despite the complexity of the protein surface, parallels can be drawn with simpler colloidal systems. Pre-wetting the particles with ethanol tuned this behaviour, highlighting the role of surface chemistry in gel formation.