Functional Properties Of Modified Wheat Gluten By Enzymatic Hydrolysis-Membrane Ultrafiltration

Wheat gluten is a byproduct of the wheat starch industry. The use in food is limited by its water-insoluble characteristic. The aim of the present study was to modify wheat gluten by enzymatic hydrolysis-membrane ultrafiltration fractionation (UF) for improving its functional properties and extending utilization. Optimal conditions and some intrinsic mechanisms of enzymatic hydrolysis were studied. Inhibition of gluten hydrolysate for enzymatic hydrolysis was investigated. The Enhanced functional properties of the modified glutens were evaluated. Graft reaction of gluten hydrolysate and polysaccharide subjected to ultrasound treatment was also investigated. Moreover, relation of composition/structure of the modified glutens to functional properties was analyzed.Five proteases (protamex, PTN6.0S, alcalase, neutrase and papain) were used to hydrolyze gluten protein and hydrolytic conditions were optimized. The contents of soluble nitrogen, amino nitrogen and peptide-based nitrogen in gluten hydrolysates were compared. Protamex was suitable for preparing the peptides with various molecular weight followed by Papain. According to SDS-PAGE and SE-HPLC analysis, gliadin and soluble glutenin were prone to degradation during enzymatic hydrolysis. Some of HMW-GS had resistance to hydrolysis.Wheat gluten subjected to thermal, extrusion, ultrasound and reducing agent led to change in its structure such as SH/S-S, hydrophicity / hydrophobicity. These pretreatment resulted in increase in degree of hydrolysis and protein recovery compared to control.Inhibition rate (IR) of gluten hydrolysates for enzymatic hydrolysis showed two distinct dynamic regions, DH-dependent and DH-independent region. The IR of gluten hydrolysate fractions obtained after UF had significant (P<0.05) difference. The peptides with below 5 kDa played dominant role in inhibition of enzymatic hydrolysis. Removal of these peptides was favor for enzymatic hydrolysis and maintenance of proteinase activity.The gluten hydrolysates were fractionated into 50-K, 30-K, 10-K and P fractions by membranes with different molecular weight cut-off. These modified glutens had different structure based on SDS-PAGE and FT-IR analysis. The enhanced (P<0.05) functional properties (solubility, water-holding capacity, emulsifying and foaming properties) of the gluten hydrolysates were found compared to original gluten. The enhanced functional properties were associated with DH of hydrolysates. Moreover, viscosity of the hydrolysates was influenced by its DH and concentration. Significant (P<0.05) increase in nitrogen soluble index at range of pH3-10 was found in the modified glutens compare with control. The pI of modified glutens shifted into acid or alkalin region. Low concentration of KCl and Cys can improve solubilization of the modified glutens. In addition, the thermal stability greatly improved. The modified glutens had better emulsion activity index, foaming capacity and water-holding capacity than the control sample, 30-K fraction had the highest value among these fractions. However, emulsion and foam stability decreased compared to control. Viscosity of the modified glutens decreased and increased with concentration of these proteins. Surface hydrophobicity of the modified glutens increased. The initial gel temperature of the modified glutens increased and decreased as ion strength increases. 50-K fraction had the highest storage modulus.The conjugate with the highest degree of graft using DH8.8% of Protamex hydrolysate and arabian gum could obtained. Optimal conditions of gluten hydrolysate and arabian gum for graft reaction were obtained by surface response design. Solubility, thermal stability, emulsifying and foaming properties of the resultant conjugate were significantly (P<0.05) improved compared the hydrolysate. Solubility curve kept relatively plateau at pH3-10. No obvious pI was noticed.The ratio of free SH/S-S of the modified glutens increased compared to control. Microstructure of the modified glutens had great change based on SEM observation. In secondary structure, modification led to decrease inα-helix/β-sheet ratio. The conjugate had the lowestα-helix/β-sheet ratio. Change in molecular weight distribution and structure of the modified gluten proteins resulted in improvement of flexibility and surface properties. These changes play an important role in improvement of functional properties.