Free radical-induced soybean protein isolate -curcumin conjugates: Characterization, antioxidant and in vitro digestion characteristics

Abstract

Natural soybean protein isolate（SPI) is vulnerable to environmental factors that compromise its functionality. One effective method to improve this functionality is through free radical induction. In this study, SPI and curcumin (Cur) were utilized to prepare SPI-Cur conjugates using the free radical induction method. The formation mechanism, structure, and functional properties of these conjugates were subsequently. The results indicated that at pH 9, the SPI-Cur conjugate formed hydroxyquinone dimers, which exhibited higher inhibition of digestive enzymes and increased stability through free radical-induced reactions. Furthermore, the antioxidant properties of SPI-Cur conjugates increased significantly, with increases of 9.17-, 2.3-, and 7.70-fold in DPPH, ABTS, and reducing power, respectively (p < 0.05). Circular dichroism analysis revealed increases in the secondary structure of proteins in SPI-Cur conjugates α-helix from 0.40% to 1.47%, β-sheet from 28.70% to 98.38% (pH 3 to pH 9), β-turns from 70.83% to 0.15%, and Random coils from 0.00% to 81.21%. These results suggest a novel approach for creating SPI-Cur conjugates through free radical-induced reactions, which could aid in the development of a continuous delivery system for polyphenols based on SPI and improve the high-value utilization of SPI.