The interfacial and foam properties of proteins can be enhanced by altering the interactions between polyphenols and proteins. The aim of this study was to determine the influence of gallic acid (GA) on the structural properties of whey protein isolate (WPI), specifically focusing on particle size, potential, and surface hydrophobicity, as well as the subsequent alterations in its interfacial and foam properties when utilized as a foaming agent. An increase in turbidity and a decrease in particle size suggested the formation of a soluble complex between GA and WPI at a pH of 6. The results from fluorescence spectroscopy and surface hydrophobicity analyses indicated that the primary interactions between GA and WPI are characterized by hydrogen bonding and hydrophobic interactions. The reduction in particle size enhances the capacity of WPI/GA complexes to lower the surface pressure, thereby demonstrating significant efficacy at the macroscopic scale. Furthermore, the structural connectivity of GA facilitates the formation of a stable interfacial film at the air–water interface by WPI/GA, resulting in high foam stability at a macroscopic level. This research contributes to a deeper understanding of the application of protein–polyphenol complexes as surfactants and provides theoretical support for their use in food applications.

To address the low solubility and inferior interfacial activity of rice protein isolate (RPI), RPI/sodium alginate (SA) composite colloidal particles were fabricated via a facile heat-induced co-assembly. Incorporating SA stabilized the flexible, unfolded conformation of RPI while forming an outer hydrophilic shell, as evidenced by a reduction in surface hydrophobicity and an increased negative zeta potential. Consequently, both foamability and foam stability followed a bell-shaped trend with increasing SA proportion, peaking at RPI/SA mass ratios of 1:0.3 and 1:0.4. At these optimal ratios, the composites exhibited a minimum polydispersity index (0.20), a highly negative zeta potential (−43 mV), moderate apparent particle size (~1500 nm), and the highest interfacial dilatational modulus (~18–24 mN/m). Mechanistically, this balanced architecture facilitated rapid interfacial adsorption and subsequent molecular rearrangement at the air-water interface. Once adsorbed, the particles synergistically enhanced foam stability through robust electrostatic repulsion and steric hindrance—imparted by their surface charges and the hydrophilic SA shell—effectively suppressing bubble coalescence. Conversely, an excess of SA rendered the particles overly hydrophilic, hindering interfacial adsorption and impairing foaming performance. This work clarifies the composition–structure–foaming property relationships of RPI/SA composites and offers fundamental support for utilizing rice protein as a plant-derived foaming agent.