Protein network structure and properties of wheat gluten extrudates using a novel solvent-free approach with urea as a combined denaturant and plasticiser

This is, to our knowledge, the first success on solvent-free extrusion of wheat gluten (WG) into high quality films without using NaOH/salicylic acid. It was possible by using urea (concentrations: 10, 15 and 20 wt%) in the single screw-extruder process. Tensile testing, oxygen permeability, water vapor transmission rate, infrared spectroscopy (IR), confocal laser scanning microscopy (CLSM) and protein solubility were used to assess the properties of the extrudates. As the urea concentration increased, the strength and stiffness decreased while the extensibility increased. The oxygen permeability was low and increased, as did the water vapor transmission rate, with increasing urea concentration. The protein solubility of urea-containing films was found to be significantly lower than that of the native gluten and glycerol-plasticized WG extrudate. CLSM, together with the protein solubility, indicated that the urea films were aggregated/polymerized and IR spectroscopy revealed that these films contained a sizeable amount of beta-sheets with a high degree of hydrogen bonds associated with protein aggregation. The aggregation did not change with increasing urea concentration, which suggests that the changes in the mechanical and permeability properties were due to urea-induced plasticisation.