Abstract: In this paper, soybean protein isolate (SPI) and oligosaccharides (raffinose and stachyose) were used to prepare a mixed aqueous solution. The phase behavior and microstructure of the composite system were studied at varying pH from 3.0 to 10.0 in order to determine the generation conditions of soluble electrostatic complex and its influence on the emulsifying properties of SPI. The results of zeta potential, confocal laser scanning microscope and turbidity measurement showed that SPI and oligosaccharides could form an electrostatic complex under acidic conditions. Moreover, the isoelectric point of the complex shifted toward acidity compared to SPI. The intrinsic fluorescence spectra illustrated that the fluorescence intensity of the complex was lower than that of SPI, and it showed a more significant decrease with stronger electrostatic interaction. This research also indicated that SPI and oligosaccharides could form soluble electrostatic complexes at pH 6.0, whose functional properties were improved compared to SPI. The emulsifying capacity of SPI-stachyose and SPI-raffinose complexes were increased by 50.66% and 39.69% compared to that of SPI.

Key words: soybean protein isolate, oligosaccharides, electrostatic interaction, solubility, emulsifiying properties