Crosslinks in wheat gluten films with hexagonal close-packed protein structures

Wheat gluten/glycerol (WGG) films were extruded with aqueous ammonia/salicylic acid or urea to investigate the reactions contributing to their hexagonal close-packed protein structures and material properties. The addition of aqueous ammonia and salicylic acid increased the pH, which, in turn, increased the level of intermolecular disulfide and lanthionine cross-links in the WGG films. Increased protein cross-linking reactions resulted in higher material strength and tensile modulus. These cross-linking reactions and the resulting material properties were similar for WGG films with 7.5% and 10% aqueous ammonia. Added urea into WGG film partially degraded into cyanate and ammonium. Cyanate subsequently reacted with lysine and cysteine to ɛ-carbamyllysine and S-carbamylcysteine, respectively. Even though these reactions resulted in a more alkaline reaction environment, hereby favoring disulfide bond formation and decreasing protein extractability, they also prevented the involvement of cysteine and lysine in protein cross-linking. The alkylation of these reactive amino acids, together with the plasticizing effect of urea, led to lower material strength and elastic modulus with increasing levels of urea.