Water-in-oil Pickering emulsions stabilized by an interfacial complex of water-insoluble polyphenol crystals and protein.

Long term stabilization of water-in-oil (W/O) emulsions remains a particularly challenging problem in colloid science. Recent studies have shown that polyphenols act as Pickering stabilizers at the water-oil interface. In this work we propose a novel way to stabilize water droplets via interfacial complex formation. It was observed that polyphenol crystals (curcumin or quercetin) absorb at the interface and provide stabilization of water droplets for several days; however formation of a polyphenol- whey protein (WPI) complex at the water-oil interface revealed a pronounced improvement in the stabilization. The mechanism of complex formation was tested by subjecting the systems to different environmental conditions, such as ionic strength and temperature. The evidence suggests that the complex is probably stabilized by electrostatic attraction between the oppositely-charged polyphenol particles and protein at the interface, although hydrogen bonding between the two components may also contribute. The resulting stable water droplets have a Sauter mean diameter (D _3,2 ) of approximately ∼22 and ∼27 μm for curcumin and quercetin systems, respectively. Emulsions were more stable at pH 3 than at pH 7, due to either weaker complex formation at pH 7 and/or chemical degradation of the polyphenols at this more alkaline pH. Interfacial shear viscosity measurements confirmed that there was strong interfacial complex formation with aqueous WPI concentrations of ∼0.5 wt.%.
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