Alkali-modified soy proteins: Effect of salts and disulfide bond cleavage on adhesion and viscosity

Soy protein isolates were treated with NaCl, Na2SO4, or Na2SO3 (disulfide bond-cleaving agent) at a pH of 10.0 and 50 degrees C, and the effects of these salts on viscosity, adhesive strength on woods, and water resistance of the treated isolates were investigated. Viscosity and adhesive strength decreased with increasing concentrations of these salts. At a concentration of 0.1 M, these three salts reduced the viscosity of soy proteins with no significant adverse effects on adhesive strength and water resistance. Addition of 0.1 M NaCl, Na2SO4, or Na2SO3 reduced adhesive strength insignificantly from 1230 N to 1120, 1060, or 1013 N, respectively. The viscosity of protein isolate modified at pH 10.0 and 50 degrees C in the absence of salts was >30,000 cP. Treatment with NaCl or Na2SO4 resulted in viscosity reductions to 6000 or 1050 cP, respectively. The Na2SO3 treatment yielded an isolate with the lowest viscosity of 110 cP and which retained adhesive and water-resistive properties. The water resistance of modified soy proteins with and without 0.1 M Na2SO3 treatment was not significantly different with 3.3 and 6.6% cumulative delaminations occurring after four water soaking cycles. Treatment with 0.1 M Na2SO3 resulted in an isolate with a 28% decrease in disulfide linkages.