Adhesive properties of soy proteins modified by urea and guanidine hydrochloride

An investigation was conducted on the adhesive and water-resistance properties of soy protein isolates that were modified by varying solutions of urea (1, 3, 5, and 8 M) or guanidine hydrochloride (GH) (0.5, 1, and 3 M) and applied on walnut, cherry, and pine plywoods. Soy proteins modified by 1 and 3 M urea showed greater shear strengths than did unmodified protein. The 3 M urea modification gave soy protein the highest shear strength. Soy proteins modified with 0.5 and 1 M GH gave greater shear strengths than did the unmodified protein. The 1 M GH-modified soy protein gave the highest shear strength. Compared to the unmodified protein, the modified proteins also exhibited higher shear strengths after incubating with two cycles of alternating relative humidity, zero delamination, and higher remaining shear strengths after three cycles water soaking and drying. These results indicate that soy proteins modified with urea and GH enhance water resistance as well as adhesive strength. Secondary structures of globule proteins may enhance adhesion strength, and the exposure of hydrophobic amino acids may enhance water resistance. Proteins modified by 3 M urea or 1 M GH may have higher content of secondary structure and more exposed hydrophobic amino acids, compared with other modifications or unmodified proteins.