Studies On The Rheological Behaviors And Mechanical Properties Of Wheat Gluten

The increasing pollution from nondegradable plastic wastes have directly threatened human being's survival, health and development. Biodegradable materials thus are desired and their study and application grow rapidly in this century. Wheat proteins show the advantage for usage as biodegradable materials because of their abundant resource and good biodegradability.In this dissertation, wheat gluten (storage proteins) is used as matrix to prepare glycerol plasticized gluten bioplastic, and gluten/starch, gluten/glutenin-rich fraction and gluten/methylcellulose biocomposites. Influences of plasticizer content, compression temperature, acid or alkali treatment and filler type and content on rheological behaviors and mechanical properties are investigated. Rheological behaviors of alkaline ethanol soluble glutenin solution are also investigated.Glycerol plasticized wheat gluten bioplastics were prepared through compression molding. The results show that the glycerol content significantly affects the mechanical properties of the bioplastics. Young's modulus and tensile strength decrease while elongation at break increases with increasing glycerol content. The moisture absorption of the bioplastics strongly depends on the glycerol content.Acid and alkali effects on rheological behaviors and biaxial/uniaxial extensional deformation of wheat gluten bioplastics were studied. The results show that addition of both HCl and NaOH facilitate cross-linking of proteins through thiol-disulfide interchange reaction, resulting in higher dynamic storage modulus (G') and dynamic loss modulus (G″) at room temperature. At the earlier stage, HCl promotes the crosslinking reaction more than NaOH. Loss factor (tanδ) decreases with increasing HCl content while it increases with increasing NaOH content. The characteristic relaxation time from biaxial deformation decreases with increasing HCl content while it increases with increasing NaOH content, indicating that HCl improves elasticity of composites and NaOH mainly improves viscosity. HCl does not show catalysis function for crosslinking proteins at 80℃and even hinders the crosslinking reaction at 110℃. On the other hand, NaOH generally accelerates the crosslinking reaction during compression-molding thus improves Young's modulus, tensile strength and strain at break significantly. However, NaOH with a content above 0.5 wt % could cause an adverse effect at molding temperature as high as 110℃. E_U,σ_br andε_br of the bioplastics are significantly increased as increasing molding temperature from 80℃to 100℃.Glycerol plasticized gluten/starch biocomposites were prepared using thermal and cold molding process. For the samples with 10 % moisture, G' increases while tensile strength and elongation at break decrease with increasing starch content. Temperature scan reveals a "rubber plateau" above 100℃. The plasticized gluten exhibits gel-like behavior at 30℃while it forms a network at 80℃. For the samples with 20 % and 25 % moisture, their tensile strength and elongation at break decrease with increasing starch content, which is ascribed to the dilution and lubrication between starch granule and gluten network by moisture.Glutenin-rich fraction was extracted from wheat gluten using a modified method. Dynamic rheological behaviors and mechanical properties of glycerol plasticized gluten/glutennin-rich fraction biocomposites were studied. The results show that the crosslinking reaction is accelerated with increasing glutenin-rich fration content. The composites exhibited "second plateau" due to the formation of crosslinking network. Samples containing gliadin exhibit strain softening and strain hardening. However, upon removing gliadins, the samples do not exhibit strain softening but fractures during strain hardening. Increasing glutenin-rich fraction content improves Young's modulus and tensile strength but lowers extensibility, indicating that gliadins are highly important for large deformation properties.Glycerol plasticized wheat gluten/methylcellulose biocomposites were prepared by thermal compression molding at 110℃. The results show that G', G″, E_U,σ_brr and T_gincrease while tanδandε_br decrease with increasing methylcellulose content. Appearance of "second plateau" in the low frequency region reflects the formation of physical crosslinking between methylcellulose and proteins. Rheological behaviors of glutenins in 50 % (v/v) alcohol / water and 0.1 NaOH solutions were investigated in relation to glutenin concentration, thermal treatment temperature, sodium (Na⁺) and calcium (Ca²⁺) ionic strength. Glutenin solutions of 48 mg ml^-1 behave as weak nonNewton liquids with slight shear thinning, which has a zero shear viscosityη₀=0.0237 Pa·s, infinite shear viscosityη_∞=0.0125 Pa·s and shear thinning index n=0.931. The solutions approach newtonian behavior concentration is decreased. The solutions preheated at 30℃-90℃show low viscosity and weakened shear thinning. Addition of Na⁺ also lowers viscosity and weakens the shear thinning. Addition of Ca²⁺ has an obvious effect on rheological behavior of gliutenin solutions. The solution with 0.2 M Ca²⁺ shows the highest viscosity, yielding stress and low-frequency plateau modulus.