Structural transitions and functional outcomes of soy protein isolate modified with carbohydrate conjugation via spray drying and ultrasonication

Soy protein isolate (SPI) exhibits promising nutritional and techno-functional properties; however, its practical application is often restricted by limited solubility, structural instability, and poor interfacial performance under processing conditions. To address these limitations, this study investigated the structural and functional modification of SPI through conjugation with glucose, sucrose, and pectin using spray drying and ultrasound-assisted Maillard reaction pathways. The study comparatively evaluated how carbohydrate class and processing route influence grafting behavior, conformational transitions, thermal stability, and techno-functional outcomes. Results showed that glucose conjugates achieved the highest degree of grafting (≈49.5%), whereas pectin conjugates exhibited the most negative zeta potential (−50.4 mV) and superior emulsifying capacity (6.86 m 2 /g). FTIR analysis revealed helix-to-coil transitions in ultrasound-treated systems, while spray drying promoted β-sheet aggregation and enhanced thermal stability. Functionally, spray-dried glucose conjugates demonstrated the highest foaming capacity (50.5%), whereas ultrasound-assisted pectin conjugates showed the best emulsifying performance. Solubility was maximized in spray-dried pectin conjugates (86.3%), and antioxidant activity increased significantly across all modified systems (DPPH up to 176 μM TE/g DM). Overall, the findings demonstrate that carefully tailored protein–carbohydrate conjugation pathways can overcome intrinsic SPI limitations and enable the development of structurally tunable and functionally enhanced protein ingredients for food applications.