Stability of water-in-oil emulsions co-stabilized by polyphenol crystal-protein complexes as a function of shear rate and temperature

The process stability of water-in-oil (W/O) Pickering emulsions (10 or 20 wt% water), co-stabilized by 0.14 wt% of polyphenol crystals (curcumin or quercetin) dispersed in a soybean oil phase, plus 2.0 wt% whey protein isolate (WPI) or 0.1–2.0 wt% whey protein microgel (WPM) particles present in the inner aqueous phase, was assessed by measuring the apparent viscosity (η), water droplet size (via light scattering) and microstructural changes (via confocal laser scanning microscopy, CLSM). Stability was measured as a function of temperature (25–50 °C), using a shear rate cycle between 0.1 and 100 s−1 to highlight shear- and time-dependent hysteresis of η. All the emulsions showed shear thinning to some extent, but those without added WPI or WPM particles in the aqueous phase exhibited coalescence at increasing shear rate, that was more pronounced at higher temperatures. Emulsions containing WPI in the dispersed phase were stable, whilst those containing WPM particles showed a decrease in mean droplet size (D4,3) on shearing due to the disruption of the aggregates of droplets, polyphenol crystals and/or WPM particles in the continuous oil phase, but with no droplet coalescence. The low shear rate (0.1 s−1) viscosity showed an increase with increasing WPM particle concentration. This increase, plus CLSM of the emulsions, suggested that the WPM particles increased W/O emulsion stability not only via their adsorption to the inner surface of the water droplets, but possibly also due to them promoting the formation of mixed weak flocs of polyphenol crystals + WPM particles + small water droplets in the oil phase attached to the surface of the main population of water droplets.