Effect of extrusion temperature on solubility and molecular weight distribution of wheat flour proteins

To study the molecular mechanism of protein interactions during extrusion processing, the changes in molecular weight distribution and quantity of soluble proteins in wheat extrudates as affected by extrusion temperature were investigated. Wheat flour was extruded at die temperatures of 160, 170, and 185 degrees C. After extrusion, the solubility of wheat proteins decreased dramatically in all tested solvents [water, 0.01 M sodium hydroxide, 0.5 M sodium chloride, 70% ethanol, 0.1 M hydrochloric acid, 0.05 M sodium phosphate buffer (pH 7.0 and 8.0), 6 M urea, 1% SDS, 2% mercaptoethanol] except for an aqueous solvent containing 1% SDS and 2% 2-mercaptoethanol, in which they were almost completely soluble. In the soluble-protein fractions of the extrudates, the content of disulfide bonds decreased dramatically and the content of sulfhydryl groups varied slightly. Both aggregation and fragmentation of wheat proteins occurred during extrusion processing as indicated by SDS-PAGE analysis. The synergistic effect between SDS and 2-mercaptoethanol on solubilizing wheat proteins in extrudates indicated that wheat proteins aggregated primarily through nonspecific hydrophobic interaction and intermolecular disulfide bond formation. This aggregation of proteins might lead to an increase in their molecular weight, which subsequently resulted in a decrease in their solubility. The glutenins and gliadins in wheat flour were mainly responsible for the aggregation of wheat proteins during extrusion processing.