Enthalpic Relaxation of Vital and Protease-Treated Wheat Gluten

Abstract

Commercially available vital wheat gluten was purified with α-amylase and treated with protease in a one-step process. The supernatant of the protease-treated gluten was freeze dried, quench cooled, and aged for 2 weeks and analyzed using differential scanning calorimetry, sodium dodecyl sulfate poly-acrylamide gel electrophoresis, reverse phase high performance liquid chromatography, size exclusion high performance liquid chromatography, capillary zone electrophoresis, and dynamic mechanical analysis. The differential scanning calorimetry profile of the aged (un-purified or un-modified) gluten sample exhibited enthalpic relaxation beneath its glass transition (Tg) indicating molecular relaxation. Samples treated with 0.006 g protease/g gluten showed Tg at 49.11°C with ΔCp = 0.12 (J/°C g). Higher protease levels and longer aging time caused high Tg temperature, ΔCp, and ΔH of enthalpic relaxation. The size exclusion high performance liquid chromatography profile of the protease-treated gluten clearly showed differences in molecular size between the supernatant and the precipitate, while reverse phase high performance liquid chromatography profiles signified more hydrophilic gluten molecules than the control. The small difference in gluten molecular size after protease treatments (0.024 g protease versus 0.032 g) appeared to have a significant effect on the enthalpic relaxation, where the two treatments showed different degrees of enthalpic relation. The capillary zone electrophoresis data confirmed the insignificant difference between 0.024 and 0.032 g protease/g gluten regarding the size to charge ratio. The molecular interactions for protease-treated gluten were much weaker than vital gluten, as indicated by much weaker network and sensitivity to temperature, especially above Tg. The weaker network was evidenced by higher G″ versus G′, which explained the fluid like behavior of the protease-treated gluten.