Study On Structure And Rheological Behavior Of Wheat Protein Systems

Nowadays, much more attention has been paid to white pollution caused bynonbiodegradable synthetically polymers attracting worldwide concern on biodegradablepackage films and plastics made from renewable agricultural resource. Wheat proteinsexhibit the advantage for biodegradable materials because of their low cost, abundantresources and good biodegradability.In the first part of this dissertation, thermo-molded wheat gluten plastics, gliadinplastics and solution-casting films were prepared based on glycerol-plasticized wheatproteins. The influences of crosslinking type, molding temperature and glycerol contenton morphology, moisture absorption, mechanical and thermal properties of wheat proteinplastics were studied. In addition, influences of cross-linker concentration and pH of thefilm-forming solution on tensile properties, water absorption and water vaporpermeability of gliadin films were investigated. In the second part, gliadins wereextracted using 70% (v/v) aqueous ethanol and 50% (v/v) aqueous propanol to preparegliadin solutions. Gliadin gels were prepared through heating alkaline solution of 50%(v/v) aqueous propanol at pH = 9.3 and 50℃. Ca²⁺ induced wheat gliadin gels were alsoprepared. Effects of protein concentration (C), temperature, pH and metal ions onrheological behavior of gliadin solutions or gels were examined.Experiment results reveal that morphology, glass transition and mechanical propertiesof thermo-molded wheat protein plastics are related to crosslinking type, moldingtemperature and glycerol content. Crosslinking through disulphide bonding leads to ahigh degree of phase-separation and a high glass transition temperature T_g of thegluten-rich phase. Aldehyde-induced crosslinking reduces the degree of phase- separationand lowers T_g of the protein-rich phase, resulting in higher tensile strength and lowerYoung's modulus and elongation at break in comparison with the disulphide crosslinking.In the absence of additional cross-linker, increasing molding temperature from 25 to 125℃significantly enhances crosslinking density of the three-dimensional proteinnetwork through disulphide bonding, leading to increase of tensile strength, Young'smodulus and relaxation time. For gliadin plastics, increasing glycerol content causesdecrease of both T_g of gliadin-rich and glycerol-rich domains, which lowers tensilestrength and Young's modulus but improves ductility at room temperature. Gliadin filmscasting from acid or alkaline solutions exhibit higher tensile strength than that of thosefrom neutral solution. Meanwhile, acid or alkali treatment of the gliadin solutions slightlyincreases water absorption of the resulted films.pH significantly influences viscosity and characteristic relaxation times of wheatgliadin solutions. As far as gliadins in 50% (v/v) propanol/water solutions are concerned,both zero-rate viscosity and mechanical relaxation time decrease with increasing pH ofthe solution. The hybrid model is applicable to account for the dynamic data, suggestingthat gliadin macromolecules are partially flexible and are highly elongated due toelectrostatic interaction. Wheat gliadin in 50% (v/v) aqueous propanol solutions at 20 gL^-1 to 200 g L^-1 behave as Newtonian fluids with activity energy of flow E_a = 23.5～27.3kJ mol^-1. The intrinsic viscosity ([η]) tends to increase with temperature due to improvedsolvation. The plateau modulus (G_N) of the alkaline gliadin gel is 434.4±13.7 Pa,corresponding to density of elastically effective chains v_e = (1.06±0.03)×10²³ m^-3.Morphological observation of dried gliadin gel reveals a very low degree of structuralheterogeneity involving in protein aggregation during the formation of the gel networkcomposing of crosslinked strands. Additions of glutaraidehyde, n-dodecanethiol or metalion (Na⁺, K⁺, Mg²⁺ or Ca²⁺) bring increase of apparent viscosity of gliadin solutions.Additions of Ca²⁺ in gliadin solutions (C＞200 g L^-1) lead to the formation of fractalweak gels containing abundant filaments as observed by scanning probe microscopy(SPM).