Effects of Sodium Bisulfite on the Physicochemical and Adhesion Properties of Canola Protein Fractions

This study focused on investigating the potential of using canola protein fractions as bio-degradable wood adhesives. Native and sodium bisulfite (NaHSO3)-modified canola protein fractions isolated successively at different pH values (7.0, 5.5, and 3.5) was used as adhesives. Wood specimens were assembled with adhesives at a pressure of 2 MPa at 150, 170, or 190 °C for 10 min. The adhesion performance of adhesives were evaluated by wet, soak, and dry shear strength. Their physicochemical properties: extractability, electrophoresis profiles, thermal, rheological and morphological properties were also characterized. Results showed that canola protein had the highest protein yield and purity at pH 5.5. Electrophoresis profile proved that NaHSO3 cleaved the disulfide bonds in canola protein. This could induce extra charges (RS-SO3−) on the protein surface, leading to the reduced apparent viscosity. Thermal analysis implied that the thermal transition temperature of canola protein decreased with modification of NaHSO3. Canola protein adhesives showed excellent dry and soak shear strength with 100 % wood cohesive failure in all curing temperatures. The wet adhesion strength of native and modified canola protein fraction adhesives at pH 5.5 and pH 3.5 (3.9–4.1 MPa) was higher than the fractions at pH 7.0. NaHSO3 had insignificant effects on the adhesion performance of canola protein adhesives but notably improved the handling and flow-ability properties of canola protein adhesives.