Adhesion and Physicochemical Properties of Soy Protein Modified by Sodium Bisulfite

Soy protein adhesives with a high solid content (28-39 %) were extracted from soy flour slurry modified with sodium bisulfite (NaHSO3) at different concentrations. 11S-dominated soy protein fractions (SP 5.4) and 7S-dominated soy protein fractions (SP 4.5) were precipitated at pH 5.4 and pH 4.5, respectively. The objective of this work was to study the effects of NaHSO3 on adhesion and physicochemical properties of soy protein. The adhesion performance of NaHSO3-modified SP 4.5 was better than SP 5.4; the wet strength of these two fractions was from 2.5 to 3.2 MPa compared with 1.6 MPa of control soy protein isolate. SDS-PAGE results revealed the reducing effects of NaHSO3 on soy protein. The isoelectric pH of soy protein decreased as NaHSO3 increased due to the induced extra negative charges (RS-SO3 (-)) on the protein surface. The rheological properties of soy protein adhesives were improved significantly. Unmodified samples SP 5.4 and SP 4.5 had clay-like properties and extremely high viscosity, respectively; with 2-8 g/L NaHSO3 modification, both SP 5.4 and SP 4.5 had a viscous cohesive phase with good flowability. Overall, NaHSO3-modified soy protein adhesives in our study have many advantages over the traditional soy protein isolate adhesive such as better adhesion performance, higher solid content but with good flowability and longer shelf life.