Mechanism underlying the effects of ultrasound-assisted alkaline treatment on the structure and vitamin D₃ encapsulation-release performance of heat-induced egg yolk granule/whey protein composite gel

Alkali induction promotes the formation of fibrous gel network structure. • Increasing pH weakens the hydrogen bond and hydrophobic interactions of the gel. • To reveal the link between gel structure formed by different pH and VD 3 encapsulation stability. • Composite gel structural compactness inhibits VD 3 release during digestion. This study explored the effects of ultrasound-assisted alkaline treatment between the structure and vitamin D₃ (VD₃) encapsulation efficiency, together with in vitro release behavior of heat-induced egg yolk granule/whey protein composite gels. Results showed that the alkaline environment remarkably modulated the intermolecular conformation and aggregation behavior of composite proteins. With increasing pH, protein unfolding extent augmented, while surface hydrophobicity first decreased and then increased, peaking at pH 11. Enhanced electrostatic repulsion improved system dispersibility, accompanied by reduced particle size and elevated absolute zeta potential. Gel microstructure evolved from a spongy homogeneous porous matrix at pH 7 to a fibrous network at pH 11. Moreover, higher pH resulted in finer fibers, increased pore density, and enlarged average pore size. These changes were accompanied by the strengthening of disulfide bonds and hydrophobic interactions, which contributed to the formation of a robust elastic-dominated gel network. The highest VD 3 encapsulation efficiency (95.74%) was achieved at pH 9, where gels also exhibited the optimal VD₃ retention rate (26.18%) after 28 days of storage at 4 °C. In vitro digestion experiments indicated that pH 11 gels displayed the fastest release kinetics during gastric and intestinal phases, with a final cumulative release rate of 89.82%. In contrast, pH 7 and 9 gels showed sustained-release features, and their release behaviors were well fitted to kinetic models, respectively. In conclusion, ultrasound-assisted alkaline treatment optimizes composite gel network structure and functional attributes via precise pH modulation, providing a theoretical foundation for stable storage and controlled release of VD₃ in food matrixes.