Differential Effects of pH-Shift and Ultrasound Intervention Sequences on Physicochemical, Structural and Gelling Properties of Potato–Egg White Composite Proteins

Composited proteins from animal and plant sources can address the limitations of single proteins and expand their applications in food processing. In this study, potato protein and egg white protein (PP/EWP) composites were modified through pH-shift and ultrasound treatments. A systematic comparison was conducted, for the first time, among three ultrasound application protocols (US-A-pH, US-P-pH, and US-B-pH, corresponding to ultrasound after, during, and before pH-shift treatment, respectively) over a range of six pH conditions (3, 4, 5, 8, 9, 10) in a composite system. It was found that PP/EWP treated with alkaline pH shifts (especially pH 9 and 10) exhibited better physicochemical, structural, and gelling properties. The complexes pretreated with US-P-pH (pH = 9) possessed higher solubility, smaller particle size, enhanced hydrophobicity, and higher number of disulfide bonds. Furthermore, this treatment yielded gels with superior hardness, chewiness, adhesion, water-holding capacity, and finer network structure. US-B-pH treatment increased the random coil content of PP/EWP, producing flexible and disordered protein states and gels with higher elasticity. In contrast, US-A-pH induced significant structural changes in the protein particles, but did not yield the required gel quality. These findings were intended to offer practical guidance for designing green and efficient protein modification strategies in food processing, with promising applications in composite gel products.