Subcritical Water Preparation Of Feruloylated Arabinoxylan From Wheat Bran And Its Oxidative Gelation Properties

Arabinoxylan(AX)is the most abundant non-starch polysaccharide in cereals,and its feruloyl esters act as important functional groups and are closely related to its oxidative gelation property.However,conventional extraction methods(water extraction,alkaline extraction)limit the preparation and performance of feruloylated AX.Recently,subcritical water has shown significant potential in the extraction of feruloylated AX,but its effect on the oxidative gelation performance of feruloylated AX is not clear,which limits the preparation and utilization of AX gels.Therefore,it is significant to understand the mechanism of subcritical water extraction of feruloylated AX and to clarify the inherent relations between subcritical water conditions,AX structure,and its oxidative crosslinking property.In this study,wheat bran feruloylated AX was extracted by subcritical water,the migration and structural transition of AX in subcritical water medium were clarified,the regulation mechanism of subcritical water on the AX structure and oxidative crosslinking behavior was systematically analyzed,on this basis,different preparation strategies and properties of feruloylated AX hydrogel were investigated.The establishment of subcritical water extraction of feruloylated AX and the migration of wheat bran components.The effects of subcritical water temperature(120-180?),pH(4-10),extraction time(10-180 min),and cycle times(1-3 times)on extraction were investigated,the process was compared with water extraction and alkaline extractions.The results showed that the selective order of subcritical water to different components of wheat bran is protein,AX,cellulose,and lignin.Higher extraction temperature(180?),acidic condition(pH 4),or longer duration(120 min)led to higher AX contents(50%-60%).The A/X value(0.5?0.6)of subcritical water-extracted AX were between water-extracted AX(WEAX)(0.4)and alkali-extracted AX(0.7).Compared to alkaline extractions,the subcritical water extraction can effectively preserve the feruloylated esters of AX(13?15 mg/g AX).Compared to WEAX,subcritical water-extracted AX contained higher levels of diferulic acids(DFA),which indicated its origination from the cell wall cross-linked fractions.The structural characterization of the residues from subcritical water extraction showed that long-term subcritical water treatment(>120 min)significantly destroyed the microstructure of the residues,and the extensive removal of hemicellulose(68%)promoted the enzymatic hydrolysis of its cellulose,and the hydrolysis rate reached to 90%.The extraction rate of AX by subcritical water under different conditions ranged from 1%to 35%,which is generally higher than the extraction rate of WEAX(9%).The mass balance of the extraction process showed that the subcritical water extraction resulted in a certain loss of xylose(<20%),arabinose(<55%),and phenolic acid(30%?40%).In conclusion,the mechanism of action of subcritical water involved multi-targeted effects,including the hydrolysis of the branch?-glycosidic bond and the backbone?-glycosidic bond,the removal of 8-5-DFA,and the cleavage of the hydrogen bond,which released the crosslinked AX from cell wall matrix.The structural characteristics and oxidative cross-linking behavior of subcritical water-extracted AXs.Subcritical water can cause AX depolymerization to various extents,produced AX with different molecular weights(2-60×10⁴ g/mol).The molecular weight of AX(4-5×10⁵ g/mol)obtained under mild neutral or weakly alkaline subcritical water conditions(160?,10 min,pH 7-10)exceeded the molecular weight of WEAX(2×10⁵ g/mol),only slightly lower than the molecular weight of alkali-extracted AX(>7.0×10⁵ g/mol).The glycosidic linkage patterns of subcritical water-extracted AXs were similar to that of WEAX.The backbone was mainly substituted at O-3 and O-2,3 positions,and the O-2,3 substituents were more sensitive to subcritical water hydrolysis.Oxidative crosslinking of AX was induced by laccase/O₂ and was analyzed through dynamic rheology,showing a wide range of gelling capacities under different subcritical water conditions.The AXs obtained under mild conditions(120-160?,10 min,pH 7-10)exhibited the highest gelling capacity,and its 2%gel strength?G'(80-180 Pa)was much higher than WEAX(8 Pa).While the AXs obtained under severe conditions(160?-180?,10-120 min,pH 4-7)showed poor capacity(2%gel?G'is 0.5-7 Pa).Besides,no crosslinking occurred to alkali-extracted AXs.After oxidative crosslinking,the ferulic acid content of AX reduced by 50%-80%,accompanied by the formation of a large amount of 8-5-DFA.The two necessary conditions for AX oxidative crosslinking were the ferulic acid content and molecular weight.The gelling capacity showed strong positive correlations with ferulic acid content and molecular weight(r>0.8).An ideal gelling capacity(2%gel?G'>10 Pa)could be obtained when the ferulic acid reaches 8 mg/g AX and the weight average molecular weight exceeds 10⁵ g/mol.The subcritical water extraction was proved to be able to produce AXs that meet the above conditions and possess an excellent gelling capacity.In addition,subcritical water showed good controllability over AX molecular weight and can be used for regulating gel structure.The effect of blending AX with different molecular weights on oxidative crosslinking.High-molecular-weight HAX and low-molecular-weight LAX were extracted from wheat bran at 160?,10 min,pH 7 and 4.The extraction rates were 7.65%and 16.22%,respectively,and the total extraction rate reached 24%.The molecular weights of HAX and LAX were 2.5×10⁵and 6.2×10⁴ g/mol,respectively.Both AXs exhibited spherical coil conformation in solution,but the HAX coil was looser and more stretched when lacking ions.Rheological analysis of their solutions showed that the critical overlapping concentration ranges of HAX and LAX were1%-2%and 4%-6%,respectively,and both exhibited shear thinning behavior.Analysis of oxidative crosslinking,it can be seen that the minimum gelling concentration of HAX and LAX are 0.5%and 4%,respectively.The gel performance of HAX was much stronger than that of LAX,but the high-concentration LAX gels had better thermal stability(<75?).For their blend gels,the combination of low concentration of HAX(0.5%-1%)and high concentration of LAX(4%-8%)showed a synergistic effect and enhanced the gel strength.After crosslinking,soluble crosslinked fractions of close molecular weights(10⁶-10⁸ g/mol)were observed in HAX,LAX,and their blends.Increasing the gel concentration and strength was prone to inhibit the crosslinking of HAX,which was manifested as a decrease in DFA content.During the crosslinking process,the AX coils quickly aggregated to form soluble crosslinked fractions.As the solution viscosity increases,gel networks were formed through the bridging between crosslinked fractions.In the blend gels,HAX exhibited a synergistic effect with LAX by enhancing the strength of the blend crosslinked fractions.Four methods were used to analyze the antioxidant properties of HAX,LAX,and their gels.The results showed that the antioxidant capacities of HAX and LAX(ORAC values of 45 and 60?M TE)were better than WEAX and alkaline extraction AX(ORAC values about 27?M TE).However,the antioxidant capacity decreased after oxidative crosslinking.The antioxidant capacity of HAX-LAX blend gels(23-47?M TE)was better than that of single HAX gels(8-15?M TE).Preparation and performance of AX-lysozyme blended gel.At pH 5.5(the crosslinking pH),HAX was negatively charged(-10.35 m V)while lysozyme was positively charged(4.29m V),thus they could be combined by electrostatic attraction.The oxidative crosslinking of HAX-lysozyme blends showed that the presence of a certain proportion of lysozyme(0.1%to1%)in HAX solutions with high concentrations(3%-4%)significantly improved the strength of the blended gel(<65%).The blend crosslinked fractions showed a slightly higher molecular weight(10⁷ g/mol)than the HAX crosslinked fractions(5×10⁶ g/mol)and a looser conformation.The blend gels with synergistic effects showed a higher 8-5-DFA content than the corresponding HAX single gels,indicating that lysozyme promoted the crosslinking of HAX.It is inferred that lysozyme was adsorbed in the HAX coil,shielding its local charge and making adjacent segments closer,which facilitated crosslinking.Lysozyme showed strong oxygen radical absorption capacity(ORAC value 46?M TE)and ferrous ion chelating capacity(chelating rate 98%).After being combined with HAX,some of the antioxidant groups were masked,resulting in slightly lower ORAC value(13-20?M TE)and ferrous ion chelation rate(40%-50%)in blend gels than HAX single gels.Lysozyme could give full play to its activity in the blend gels.