Exploring the potential of egg yolk spheres to stabilize foams by pH shift: Structural, interfacial, and capillary...

The poor foaming of egg yolks has long plagued the food industry. In this study, four egg yolk spheres (EYS) were prepared via acid- and alkaline pH-shift methods, and the main factors affecting the variation in their foaming capacity were determined. The tertiary structure of EYS under hydrogen bonding and electrostatic interactions unfolded in acidic shifts, exposing many functional groups, and refolded in basic shifts and exposed hydrophobic side chains. The secondary structure of the acidic shifted EYS becomes disordered, whereas the basic shifts become ordered. Both shifts result in increased flexibility and fewer free sulfhydryl groups in the EYS structure. The particle sizes of the acid-shifted EYS show a bimodal distribution, and their high interfacial diffusion (K_diff × 10² = 63.62), permeation (K_p - 5.49), and recombination (K_r - 110.3) rates prevent interfacial competition for adsorption. The smaller particle size (294.4 nm) of the alkaline-shifted EYS resulted in higher recombination rates. Acidic and basic shifts improved the EYS foamability to 125 % and 142 %. Both shifts had little effect on foam stability because EYS is a capillary foam stabilizer that improves foam stability primarily with oil bridges. This study elucidates the mechanism by which EYS and pH shift improve the foaming properties of egg yolk, which contributes to the refinement of egg yolk for applications in the food industry.