Effect of Charge Density Modification of Soy Protein Isolate by Succinylation on Its Conformation and Emulsifying Properties

In this paper, we explored the relationship between the conformational changes of soybean protein isolate (SPI) caused by charge density modification through succinylation and the improvement in emulsifying properties. Succinic anhydride was used to modify SPI. The conformational changes of SPI with different degrees of succinylation were analyzed by fluorescence spectroscopy, ultraviolet spectroscopy and Fourier transform infrared (FTIR) spectroscopy. The effect of succinylation on the physiochemical properties of SPI was characterized by scanning electron microscopy, zeta potential, surface hydrophobicity, molecular flexibility and emulsification. The results showed through succinylation reaction, succinyl groups were successfully grafted onto SPI, so that the isoelectric point of SPI decreased and the electronegativity increased. Additionally, the microstructure was changed with small holes on the surface of the smooth and irregular sheet structure, and the the molecular mass was increased. The increase in charge density led to unfolding of the tertiary structure of SPI, exposure of tryptophan residues burying and tyrosine residues, and a red shift in the fluorescence and ultraviolet absorption spectra, which proved that succinylated SPI is located in a more hydrophilic environment. The reaction between free amino groups in SPI and succinyl groups transformed the N–H bond into C–N bond, resulting in changes in the amide Ⅲ band. The surface hydrophobicity of succinylated SPI was decreased, the molecular flexibility was increased, and the emulsification activity and emulsifon stability were improved compared to SPI. This study confirmed that the increase in the charge density of succinylated SPI can affect the spatial conformation of SPI and consequently improve the emulsifying properties significantly.