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BACKGROUND In this paper, the effects of different succinic anhydride (SA) additions on the flexibility of soy protein isolate (SPI) were investigated, and changes in protein conformation and interfacial functional properties were measured. The structure-effect relationship between conformation, flexibility, and interfacial functional properties was established. RESULTS SPI was bound to SA through disulfide bonds, the zeta potential was reduced. The β-sheet content decreased, the disordered structure increased, and changes in tertiary and microstructure. The surface hydrophobicity, disulfide bond content, and solution turbidity were reduced to 5063, 1.0967 μmol/g and 0.0036, respectively. When the mass ratio of SA/SPI was 15%, the best flexibility of SPI (0.3977) and interfacial functional properties were obtained. Correlation analysis showed a highly significant positive correlation (p < 0.01) between flexibility and emulsification and foaming properties, with correlation coefficients of 0.960 and 0.942 for flexibility with emulsifying activity and emulsion stability, respectively, and 0.972 and 0.929 for flexibility with foaming capacity and foaming stability, respectively. CONCLUSION It suggested that succinylation-induced conformational changes of SPI improved its interfacial functional properties by changing its flexibility. These results provided theoretical guidelines for the development and application of highly emulsifiable and stable soy protein products utilizing succinylation. This article is protected by copyright. All rights reserved.