Effect Of Deamidation On The Proteolysis Susceptibility And Heat-induced Gel Properties Of Wheat Gluten

Wheat gluten is a natural plant protein with rich nutrition,but due to the lack of ionized groups and its large molecular mass,resulting in the low interactions of protein-water molecules,and the poor water-solubility further influence its other functional properties such as foaming and emulsifying capacities,which finally limit the application of wheat gluten in many food fields.In addition,wheat gluten can also cause allergic reactions in patients with celiac disease.Therefore,it is an important research to improve the solubility,allergenicity and other functional properties and promote its high value application through appropriate modification.In this paper,wheat gluten was deamidated using protein-glutaminase(PGase),the effects of PGase on the functional and structural properties of wheat gluten were studied.The wheat gluten samples deamidated using PGase were digested in vitro,the effect of PGase on the proteolysis susceptibility of wheat gluten were studied,and the antioxidant activity of the hydrolysates were analyzed.The study also prepared heat-induced gel using the PGase-deamidated wheat gluten,and variations of the conformation and its gel properties were investigated.The results were as follows:(1)The optimum reaction conditions for the PGase-induced deamidation of wheat gluten by single factor method: the substrate protein concentration was 7%,the reaction temperature was 40?,the reaction p H was 7.0,the PGase addition level was 20 U/g,the reaction time was 24 h.Under this condition,the degree of deamidation of wheat gluten reached 41.84%.The results of deamidated wheat gluten samples using PGase indicated that PGase can significantly improve the solubility of wheat gluten in weak acidic and neutral conditions,the solubility of them reached 60.87% and 61.67%,which were 3.52-fold and 4.05-fold higher than the untreated wheat gluten,respectively.The PGase-induced deamidation of wheat gluten was accompanied with a low degree of hydrolysis reaction.With the deamidation time increasing,the particle size of deamidated wheat gluten samples decreased and the number of negative charges increased.The effect of different PGase deamidation time on the structure/conformation of wheat gluten were studied,and the results showed that moderate deamidation can significantly improve the surface hydrophobicity and endogenous fluorescence intensity of wheat gluten,among them,the fluorescence intensity of DWG-18 was 3.6 times higher than that of the control.Moreover,the maximum fluorescence emission wavelength was red shifted.The results of FTIR showed that the content of hydrogen bonding ?-sheet structure in wheat gluten decreased with the modification of PGase,while the structure of more flexible ?-turn and ?-helix were corresponding increased.The result of free sulfhydryl showed that PGase can promote partial cleavage of disulfide bonds in wheat gluten molecules,resulting in an increase of free sulfhydryl content.The analysis of the thermal characteristics by Differential Scanning Calorimetry(DSC)showed that the PGase-induced deamidation could extend the protein structure of wheat gluten,but with the degree of deamidation increased,the protein structure of wheat gluten did not continue to stretch.(2)The effects of PGase deamidation on the proteolysis sensitivity of wheat gluten and the changes of antioxidant activity of wheat gluten hydrolysates were investigated,the results showed that: PGase-induced deamidation increased remarkably the degree of hydrolysis,titratable acid content and short peptide content of wheat gluten after the PGase and protease composite modification,indicating that PGase-induced deamidation can improve the proteolytic susceptibility of wheat gluten to trypsin and alkaline protease.At first,alkaline protease had higher hydrolysis efficiency on wheat gluten,but enzymatic hydrolysis of wheat gluten by trypsin showed a significant increase after pretreatment with PGase.In the curve of Size exclusion high performance liquid chromatography(SE-HPLC)of wheat gluten hydrolysates,with the deamidation time prolonged,the chromatographic peaks values of the lower elution time decreased,while the values of the chromatographic peak of the higher elution time increased,which indicated that wheat gluten was hydrolyzed into smaller peptides.Judging from the distribution of three components in wheat gluten hydrolysates,the main components in composite modified wheat gluten samples are small molecule with molecular weight <5 KDa.With deamidation time increased,the content of small molecule components increased,while that of middle molecular components and macromolecular components decreased.The antioxidant activity of wheat gluten hydrolysates was analyzed,the results showed that the 1,1-diphenyl-2-picrylhydrazyl(DPPH)radical scavenging activity and the reducing power of composite modified wheat gluten samples were all increased after PGase-induced deamidation,but compared with PGase and alcalase modified wheat gluten samples(DWGA),DPPH radical scavenging activity of PGase and trypsin composite modified wheat gluten(DWGT)were higher.The surface hydrophobicity and free sulfhydryl content of wheat gluten hydrolysates increased significantly after PGase pretreatment,indicating that PGase affects the proteolysis susceptibility of wheat gluten by changing its structure.(3)The effects of PGase modification on the conformation and its gel properties of heat-induced wheat gluten were studied.The results of SE-HPLC indicated that PGase-induced deamidation of wheat gluten for 1 h can cause a low degree of hydrolysis reaction,while heat-induced treatment easy to form macromolecular protein aggregates.Fourier transform infrared spectroscopy and endogenous fluorescence spectroscopy showed that PGase deamidation could inhibit the heat-induced aggregation of wheat gluten to a certain extent.PGase deamidation can improve the gel strength and water holding capacity.Under the enzyme addition level of 3 U/g,the values of gel strength and water holding capacity reached the maximum of 903.27 N and 78.70 %,respectively.In addition,the non-freezable water content in heat-induced wheat gluten gel samples after deamidation.Scanning electron microscope(SEM)confirmed that the gel network structure of PGase-treated wheat gluten followed by heat-induced treatment were more orderly.