Combination of egg white protein and microgels to stabilize foams: impact of processing treatments

The aim of this study was to compare the properties of foams stabilized by egg white protein (EWP) and egg white protein microgels (EWPM), and combinations thereof (EWP + EWPM; 1.0 + 0, 0.7 + 0.3, 0.5 + 0.5, 0.3 + 0.7 or 0 + 1.0, wt%) without or with processing treatments (freezing at −20 °C/30 min, oven-heating at 80 °C/30 min, or microwaving at 700 W/15 s). In order to provide a twofold benefit of high initial foam volume (dictated by rapid adsorption of EWP) and high foam stability (governed by Pickering stabilization by EWPM), various ratios of EWP + EWPM on foam stabilization were investigated. The EWP + EWPM systems generated similar initial foam volumes as compared to those prepared solely with EWP (p > 0.05), and foams generated with increasing ratios of EWPM + EWP showed higher stability to bubble shrinkage and coalescence at longer time scales (9 h). Confocal images revealed that EWPM were preferentially located at the air/water interface with the increasing EWPM + EWP ratio, suggesting pure Pickering stabilization at 1.0 wt% EWPM. Plateauing of bubble size at ca. 75 μm occurred only at EWPM> 0.5 wt% for EWP + EWPM = 1.0 wt% total protein, when stability became EWPM-dominated. Frozen foams showed the most stable bubbles, irrespective of the systems (p > 0.05). The combination of EWP + EWPM significantly improved the stability of the bubbles during oven-heating as compared to the EWP-stabilized counterparts (p < 0.05), whilst the EWPM system was the most stable during microwaving. These results suggest that microgels could be used to formulate food foams with enhanced stability to processing conditions, whereas combination with non-microgel protein can improve the initial foamability.