Physical and chemical properties of soy protein isolates treated with sodium sulphite under low temperature extrusion

Summary Soy protein isolates (SPIs) treated with sodium sulphite (Na 2 SO 3 ) under low temperature (50 °C) extrusion were investigated to create a system where the disulphide bonds were destroyed. The physicochemical properties of the extrudates were characterised after the addition of 0.0%, 1.5% and 3.0% (w/w) Na 2 SO 3 . Under these conditions, free sulfhydryl content significantly increased (from 98.46 to 449.64 μmol g −1 ), specific mechanical energy during extrusion significantly increased (from 426.91 to 593.39 kJ kg −1 ), degradation temperature reduced (from 315.7 to 302.0 °C), protein solubility of the water extraction increased incrementally (from 5.41% to 20.17%) and water absorption capacity decreased (from 3.51% to 2.69%). Moreover, the addition of Na 2 SO 3 caused a blue shift in the tryptophan fluorescence spectra of soluble SPI. These results suggest that Na 2 SO 3 breaks disulphide bonds in SPI, which accelerates protein molecule denaturation to promote peptide disentanglement; consequently, SPI become more hydrophobic and less thermally stable.