Characterization of hydrolyzed or heat treated wheat gluten by SE-HPLC and 13C NMR: Correlation with wood bonding performance

Abstract

Wheat gluten, being an abundant and relatively inexpensive protein source, is an attractive raw material for sustainable wood adhesives. Mild enzymatic hydrolysis (degree of hydrolysis: 0–5.5%) or heat treatment (50, 70, or 90°C for 15min–24h) was used to improve the bonding performance of wheat gluten. Size exclusion high performance liquid chromatography (SE-HPLC) and nuclear magnetic resonance (13C NMR) were used to correlate the obtained changes in bonding performance with the structural changes of wheat gluten.The protein structures of the samples heated at 50°C or 70°C were mainly unfolded as interpreted from the SE-HPLC results, however, without improvement in bonding performance. The 13C NMR results indicate that the extent of unfolding at 50°C or 70°C is too low to result in improved bond strength. Generally, heat treatment at 90°C or lower levels of hydrolysis (0–0.6%) resulted in similar improvements in bond strength and water resistance. The results indicate that the improvements in bonding performance are due to a combination of unfolding and polymerization for the samples heated at 90°C, while it is due to unfolding of the protein structure for the hydrolyzed samples. Higher levels of hydrolysis (≥1.3%) resulted in impaired bond strength and water resistance, most likely due to the decreased molecular size of the proteins. Carbohydrates, normally strongly associated with the proteins, were liberated during the hydrolysis, possibly contributing to the reduced bond strength for these samples.