Characterization of complexes of soy protein and chitosan heated at low pH

The objective of this research is to investigate the complex of soy protein isolate (SPI) and chitosan (CS) heated (121 °C, 15 min) at low pH. The formation and functional properties of heated SPI-CS complex were investigated by electrophoresis, Fourier transform infrared spectroscopy (FTIR), solubility, ζ-potential and emulsifying property. The results indicated that the heated SPI-CS complex is probably more like a soluble SPI-CS aggregate which is driven by electrostatic interaction. Heated SPI-CS complex exhibited improved solubility than SPI (less than 10%) and unheated SPI-CS mixture (about 50%) at pH 4.0 because of the increased electrostatic repulsion (ζ-potential) after heat treatment. The effects of mixing ratios, molecular weights (MW) and charge densities (CD) of chitosan on the stability of heated SPI-CS complex showed the influence was mainly dependent on mixing ratios and CD of chitosan. Concomitantly, the emulsifying activity index of SPI at pH 4.0 was significantly improved by heated complexation and the emulsion stability index was not affected. Additionally, the heated SPI and/or SPI-CS complex exhibited slightly decreased pepsin and trypsin digestibility. The results suggested that the physicochemical and functional properties of SPI could be modulated by heated complexing with chitosan, using appropriate mixing ratio, MW and CD of chitosan.

► The solubility of soy protein at acid pH improved by complexing with chitosan. ► Soy protein and chitosan interactions were mainly electrostatic in nature. ► Mixing ratios and charges of chitosan affect functional properties of the complex. ► Heating and adding chitosan reduce the in vitro digestibility of soy protein.