Structural Changes of Gluten/Glycerol Plastics under Dry and Moist Conditions and during Tensile Tests

The structures of wheat gluten-based materials are greatly influenced by plasticizer content, moisture content, and external mechanical loading. In this study, the effects of moisture on the structure of wheat gluten (WG) plasticized by glycerol were investigated by using in situ small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS), mechanical tensile testing, and thermal analyses. The materials were processed with additives of ammonium hydroxide/salicylic acid or urea and conditioned at 0, 50, and 100% relative humidity (RH). In general, water showed similar effects on the WG structure and mechanical properties regardless of the type of additive. It was observed that the known hexagonal close-packed (HCP) structure in WG was present mainly under moist conditions and swelled with an increase in water content. The absorbed water molecules hydrated the protein chains at 50% RH and further led to the formation of a separate water/glycerol phase at 100% RH. An interesting feature was observed by in situ SAXS during tensile deformation; both the HCP structure and other protein aggregates packed more densely in both the tensile and transverse directions. It is interpreted as follows: "randomly oriented" chains were drawn out and stretched in the tensile direction, which squeezes the self-assembled structures together, similar to "tightening a knot".