Preparation Of Feruloyl Oligosaccharides From Rice Bran And Its Effect On Rice Protein Properties

Rice protein,a kind of high-quality plant protein,has high nutritional value and low allergenicity.However,the low solubility limits its application in food industry.In recent years,enzymatic oxidation and free radical grafting have been used to catalyze the covalent conjugation between feruloyl polysaccharides or phenolic compounds and proteins to improve the properties of proteins.In theory,feruloyl oligosaccharides(FOs)had better solubility.Thus,it was speculated that the grafting efficiency between FOs and rice protein was higher and could effectively improve the solubility of rice protein.Rice bran is a good raw material for preparing FOs,but the traditional preparing method was time-consuming and had a low yield.Therefore,the effect of extrusion on the yield of FOs was studied to improve the yield and efficiency of preparing FOs,and the structure of FOs from rice bran was identified.Subsequently,the conjugation between rice protein and FOs was performed by laccase catalyzed oxidation and free radical oxidation which was induced by the ascorbic acid/hydrogen peroxide system.Moreover,the effect of grafting with FOs on the structure and properties of rice protein was explored.The main results are as follows.The effects of traditional hydrolysis,a combination of traditional extrusion and hydrolysis and enzymatic extrusion on the contents of starch,protein,ferulic acid and pentose in rice bran and the yield of FOs were investigated and compared.All three treatments reduced the contents of starch and protein in rice bran,and accordingly increased the contents of ferulic acid and pentose,which was beneficial to prepare FOs.Compared with traditional hydrolysis,traditional and enzymatic extrusion damaged the cell wall of rice bran and resulted in a looser and more wrinkled surface,which improved the yield of FOs prepared from rice bran.However,extrusion did not change the structure of FOs.Moreover,enzymatic extrusion reduced the time required for removal of starch and protein to 12 min.Therefore,enzymatic extrusion was used to prepare FOs in this study.The highest yield of FOs from enzymatic extrusion was5.78%,when the moisture content was 30% and the screw speed was 50 rpm.However,the yield of FOs prepared by traditional hydrolysis was only 4.23%.High performance anion exchange chromatography analysis found that FOs from rice bran(RB-FOs)had more kinds of monosaccharide units but a lower ratio of arabinose to xylose than FOs from wheat bran(WB-FOs),which suggested that RB-FOs had a more complex side chain but a smaller degree of branching than WB-FOs.Ultra performance liquid chromatography-mass spectrometry analysis showed that RB-FOs were mainly composed of O-β-D-xylopyranosyl-(1→4)-O-[5-O-(trans-feruloyl)-α-Larabinofuranosyl-(1→3)]-O-β-D-xylopyranosyl-(1→4)-D-xylose and O-β-Dxylopyranosyl-(1→4)-O-[5-O-(trans-feruloyl)-α-L-arabinofuranosyl-(1→3)]-Dxylose.In addition,RB-FOs had excellent antioxidant activity which was positively correlated with its concentration in the range of 0～2.0 mg/m L(r = 0.950～0.998).Our pre-experiments found that grafting efficiency between rice protein and FOs was quite low.Thus,rice protein was hydrolyzed by chymotrypsin to obtain rice protein hydrolysate(RPH)which exposed tyrosine and other amino acid residues.Then,FOsRPH conjugates were prepared by laccase-catalyzed oxidation and ascorbic acid/hydrogen peroxide oxidation(the FOs-RPH conjugate induced by laccase was called the laccase conjugate,and the FOs-RPH conjugate induced by free radical was the radical conjugate.)and the effect of grafting with FOs on structure and properties of RPH was investigated.The formation of two kinds of FOs-RPH conjugates was confirmed by Tricine-SDS-PAGE gel electrophoresis analysis.The grafting efficiency of the laccase conjugate and the radical conjugate was 60.45% and 22.70%,respectively.Under the catalysis of laccase,covalent conjugation with FOs reduced the tyrosine content in RPH but did not affect the free amino group content.However,under the catalysis of free radicals,covalent conjugation with FOs reduced the free amino group and tyrosine content in RPH.These results suggested that in the laccase conjugate,the tyrosine residue in RPH and the ferulic acid moiety in FOs were conjugated,and the C-O or O-O bond was formed.However,in the radical conjugate,the free amino group or tyrosine residue in RPH were conjugated to the ferulic acid moiety in FOs,and the C-N or C-O bond was formed.Compared to RPH,both conjugates had a higher proportion of β-fold and a lower proportion of α-helix and random coil.Covalent conjugation with FOs increased the size of RPH molecules.Moreover,the radical conjugate molecules were more rounded,while the laccase conjugate molecules were flatter.In addition,covalent conjugation with FOs increased the solubility and antioxidant activity of RPH.Particularly,the radical conjugate had greater solubility and antioxidant activity than the laccase conjugate.However,covalent conjugation with FOs reduced the emulsifying properties of RPH,and the radical conjugate had lower emulsifying activity and emulsifying stability than the laccase conjugate.The above results indicated that laccase-catalyzed oxidation and free radical oxidation had different conjugation mechanisms,resulting in different conjugation sites and efficiencies between RPH and FOs,which led to different structures and physicochemical properties of the two conjugates.