Conformation,interfacial Characteristics And Emulsion Stability Of Deamidated Wheat Gliadin

Wheat gluten is a kind of cheap and abundant protein that contains many nonpolar amino acids. It has low water solubility which limited its use in food product. Citric acid was used to deamidate wheat gluten. The in vitro digestibility of protein and amino acids composition of deamidated wheat gluten were evaluated. The properties and structural change of wheat gliadin during deamidation were investigated. The interfacial adsorption kinetics and interfacial rheology of deamidated wheat gliadin at the oil-water and air-water surface and their relationship with the emulsifying and foaming properties was evaluated. Polysaccharides were added to emulsion to improve the stability of emulsion against salt ions. The lipid oxidation stability and in vitro digestion of lipid in emulsion was investigated. These results could give useful information on structural characteristics of deamidated wheat gliadin and provide theoretical basis for the development of wheat protein as new additives in food system.The isoelectric point of deamidated wheat gluten was pH 5 and the solubility of protein at neutral pH was significantly improved after deamidation. The hydrolysis degree of deamidated gluten by 0.2 M citric acid was low(< 2.40%). Citric acid deamidation decreased the in vitro digestibility of wheat gluten by pepsin but increased its digestibility by pancreatin(p< 0.05). The Lys and total essential amino acids were increased after deamidation while the antioxidant properties of digests decreased(p< 0.05), which might be related to the lower released amount of amino acids with antioxidant ability.Deamidated gliadin was prepared from wheat gluten with higher purity extracted from Jinan 17 wheat flour with high gluten content. The conformation and properties of citric acid and HCl deamidated gliadin were compared. Citric acid deamidated wheat gliadin had higher content of α-helix while HCl deamided gliadin had lower content of α-helix. Citric acid deamidated gliadin had more compact globular structure and higher denaturation temperature. In addition, deamidated wheat gliadin and its digests had higher content of Lys.Deamidated wheat gliadin could form emulsions which could keep stable during storage. Heating reduced the droplet size especially at lower protein concentration. The maximum wavelength of tryptophan increased after heating, indicating that the protein molecules were rearranged and interacted with each other. The emulsifying properties were related with its surface properties. The adsorption behavior and dilatational properties of protein film at the oil-water and air-water interface were studied. The surface pressure increased with increasing protein concentration and adsorption time. The surface activity of protein was higher at pH 7 compared with pH 3. The interfacial film was mainly elastic. At the oil-water interface, the dilatational modulus at pH 7 and 3 showed no significant difference. While at the air-water surface, E and Ed showed significant difference at pH 7 and 3 with different protein concentration. This might be caused by the different protein structure and interaction pattern of protein molecules at different pH values. The microstructure of protein was in close relationship with its interfacial properties and its emulsifying and foaming properties.Deamidated wheat gliadin could form complexes with polysaccharides in water solution. At neutral pH, gliadin interacted with XG through hydrophobic interaction. Gliadin could form complexes with polysaccharides at acidic pH values through electrostatic interaction. Addition of 0.2 wt%XG or 0.1 wt% pectin to gliadin-stabilized emulsion(2.5 wt% oil) could improve its stability at the pI of protein and its stability against the salt ions.The effect of polysaccharides on the oxidation stability of emulsion and digestibility of fish oil was evaluated. The lipid oxidation extent in deamidated wheat gliadin stabilized emulsion at pH 7 was lower than at pH 3.5 and that in sodium caseinate stabilized emulsion. Adding XG further decreased the lipid oxidation at pH 7 which was mainly related with the ability of XG to bind Fe2+ and the increase of the viscosity of emulsion. The changes of tryptophan intensity in different emulsions proved the correlation between the protein oxidation and lipid oxidation. The released amount of free fatty acids in deamidated wheat gliadin stabilized emulsion was lowest among all emulsions. Addition of polysaccharides in emulsion increased the released amount of free fatty acids. The above results could be due to the flocculation of gliadin stabilized emulsion in the simulated mouth digestion process and the influence of protein molecules on the lipase activity in solution.