Egg white protein microgels as aqueous Pickering foam stabilizers: Bubble stability and interfacial properties

The aim of this study was to design and characterize aqueous foams stabilized by egg white protein microgels (EWPM) and compare their stability with conventional foams stabilized by egg white protein (EWP). Sub-micron sized EWPM (hydrodynamic diameter = 359 +/- 21 nm) were designed using a top-down approach involving the formation of a thermally-crosslinked egg white protein hydrogel (90 degrees C/30 min, pH 7.0) followed by controlled shearing using jet homogenization (300 bar, two passes). Microstructural evaluation at multiple length scales (confocal laser scanning microscopy and cryogenic scanning electron microscopy) indicated that the EWPM stabilized the aqueous foams via a Pickering-type mechanism. Foamability was higher in EWP-stabilized foams compared to EWPM-stabilized foams, irrespective of the protein concentrations tested (0.5-3.0 wt%). However, EWPM-stabilized foams exhibited higher stability to disproportionation over long periods (p < 0.05), even though the initial air bubble size was smaller than with EWP. Bubble coalescence experiments also confirmed that the fraction of the coalescence was much lower in EWPM systems as compared to the EWP counterparts. Changes of surface shear viscosities of EWP at the air-water interface indicated that EWP films were more brittle, exhibiting shear thinning during the measurements, whereas the viscosities of the EWPM films were independent of shear after 24 h of ageing. In summary, our study demonstrates for the first time that microgels of EWP have distinct advantages over EWP itself in terms of generating edible foams with ultra-high stability against disproportionation.