Synergistic heat-moisture-ultrasound modification enhances starch-glutelin interactions: A pathway to tailored physicochemical properties and reduced digestibility in purple rice blends

While physical modifications like heat-moisture treatment (HMT) and ultrasound (UT) improve starch functionality, the role of glutelin-the major rice protein-in modulating starch behavior during such treatments remains unexplored. This study investigates the synergistic effects of HMT-UT co-modification on purple rice starch-glutelin (PRS-G) blends to elucidate structural, thermal, and digestibility changes. PRS-G mixtures (0-20 % glutelin) were subjected to HMT (110 °C, 30 % moisture) followed by UT (600 W, 30 min). SEM revealed fragmented lamellar structures with irregular pores, while rheological analyses demonstrated maximal gel strength at 5 % glutelin. DSC and XRD indicated that HMT-UT increased gelatinization temperatures (ΔT up to 8.2 °C) and short-range molecular order, particularly in HUPRS-10 %G (purple rice starch subjected to HMT-UT with 10 % glutelin), due to enhanced starch-glutelin crosslinking. FTIR confirmed hydrogen bonding between starch hydroxyl groups and glutelin residues, forming compact aggregates. In-vitro digestibility showed that HMT-UT reduced rapidly digestible starch (RDS) by 21.5 % and increased resistant starch (RS) by 172 % in HUPRS-10 %G, outperforming physically mixed PRS-G systems. Kinetic modeling revealed dual-phase digestion inhibition, with glutelin acting as a steric barrier to enzymatic hydrolysis. This work establishes that HMT-UT co-modification promotes starch-protein interactions via structural denaturation and binding-site exposure, yielding thermally stable, slow-digesting starch-protein matrices.