Effects Of Oxidative Modification On Structure Functional Properties And Digestibility Of Rice Bran Protein By Peroxyl Radicals And Malondialdehyde

The rice bran resource is very abundant in China,the annual output reached 14 million tons in recent years.Rice bran protein with 12%?18%protein which has the characteristics of hypoallergenicity,reasonable amino acid composition,high biological efficiency and digestibility,has been brougt to the forefront.But rice bran containing 14%-24%oil will cause lipid hydrolysis and oxidation rancidity,the formation of rice bran oxidative rancidity can induce lipid free radicals and reactive oxidation products which resulted in rice bran protein oxidation.2,2'-azobis(2-amidinopropane)(AAPH)derived peroxyl radicals,and 1,1,3,3-tetramethoxypropane(TMP)derived malondialdehyde were used to represent the lipid free radicals and active aldehydes during lipid peroxidation reactions.The changes of structure,functional properties and in vitro digestibility of rice bran protein caused by peroxide radicals and malondialdehyde in lipid peroxyl products were studied in this paper.The concentrations of AAPH and malondialdehyde were determined by the range of carbonyl content of rice bran protein at different storage time:0,0.1,1,3,6,10 mmol/L and 0,0.01,0.1,1,10 mmol/L.The effect of oxidation structure of rice bran protein induced by peroxyl free radical and malondialdehyde were investigated by protein carbonyl content,free sulphydryl content,disulfide bond content,fourier transform infrared spectrometer,intrinsic fluorescence,surface hydrophobicity,sodium dodecyl sulphate-polyacrylamide gel electrophoresis(SDS-PAGE),exclusion chromatogram size,particle size,Zeta potential.The results indicated that as AAPH and malondialdehyde concentration increased,protein carbonyl increased from 2.0 nmol/mg to 7.09 nmol/mg,10.34 nmol/mg respectively,free sulphydryl content decreased from 23.97 nmol/m to 15.29 nmol/mg,10.12 nmol/mg,disulfide bond content increased from 12.04 nmol/mg to 14.69 nmol/mg,16.51 nmol/mg,a-helical content decreased from 22.54%to 19.41%,19.59%respectively,intrinsic fluorescence intensity dropped from 610.1 to 506.5,307.7,surface hydrophobicity decreased from 913.4 to 668.3,163.0,size of rice bran protein increased from 127.9 to 179.9,223.1.Peroxyl radical and malondialdehyde oxidation both caused blue shift of wavelength of the maximum emission.Comparing peroxyl radical and malondialdehyde oxidation,it's obvious that under the same degree of oxidation,malondialdehyde had more significant oxidation effects on intrinsic fluorescence,surface hydrophobicity,Zeta potential and particle size of rice bran protein,and accompanied by formation of oxidation aggregates and non-disulphide covalent.The changes of functional properties of rice bran protein affected by peroxyl free radical and malondialdehyde were investigated by solubility,water holding capacity,oil holding capacity,foaming capacity,foaming stability,emulsion activity,emulsion stability.The results showed that as AAPH and malondialdehyde concentration increased,solubility capacity unchanged before decreased,water holding capacity,oil holding capacity,foaming capacity,foaming stability,emulsion activity,emulsion stability of oxidized rice bran protein by peroxyl free radical firstly increased and then decreased,reaching the maximum when the AAPH concentration was 3 mmol/L.The maximum were:484.62%,302.12%,73.5%,56.0%,24.97 m2/g,165.46 min.Water holding capacity,oil holding capacity,foaming stability unchanged before decreased,foaming capacity,emulsion activity,emulsion stability of oxidized rice bran protein by malondialdehyde still decreased.Peroxyl free radical and malondialdehyde changed the functional properties of rice bran protein significantly.The effects of heating and non-heating on in vitro pepsin digestibility of oxidized rice bran protein by peroxyl free radical and malondialdehyde were studied by treating oxidized rice bran protein with heating at 95? and without heating.The results indicated that as AAPH concentration increased,pepsin digestibility,initial digestion rate and the content of peptides with molecular weight distribution of 500?1500 u in the pepsin digestion products of rice bran protein oxidized by peroxide free radicals without heat treatment reached the maximum when the AAPH concentration was 1 mmol/L,the maximum were 35.01%,0.48 OD280/h and 26.54%respectively.The antioxidant capacity of the digested products firstly increased and then decreased.The pepsin digestibility and initial digestion rate,the content of peptides with molecular weight distribution of 500?1500 u in the pepsin digestion products firstly unchanged,and then decreased,ABTS free radical(ABTS+·)scavenging activity and reduction ability firstly unchanged,and then decreased,DPPH· scavenging activity,superoxide(O2-·)scavenging activity,radicals hydroxyl(·OH)scavenging activity and metal chelating ability of enzymatic product of heat-treated oxidized rice bran protein by peroxyl free radical firstly increased,and then decreased.As malondialdehyde concentration increased,pepsin digestibility and initial digestion rate,the content of peptides with molecular weight distribution of 500?1500 u in the pepsin digestion products,hydrolysates' antioxidant capacity of oxidized rice bran protein by malondialdehyde without heating still decreased.The pepsin digestibility,the content of peptides with molecular weight distribution of 500?1500 u in the pepsin digestion products,and the reduction ability of hydrolysates gradually decreased,the initial digestion rate unchanged before decreased.In this study,the effects of peroxy free radical and malondialdehyde oxidation on the in vitro pepsin digestion of rice bran protein were significantly different:when the AAPH concentration was low,the digestion of rice bran protein pepsin in vitro was promoted,while malondialdehyde oxidation continuously inhibited the digestion of rice bran protein pepsin in vitro.After heat treatment of rice bran protein oxidized by peroxide free radical and malondialdehyde,the digestion properties and the antioxidant properties of the digested products of rice bran protein with the same degree of oxidation in the same oxidation system,were significantly improved.Peroxyl free radical and malondialdehyde had significantly changed in vitro pepsin digestion properties of rice bran protein.The results indicated that as AAPH concentration increased,trypsin digestibility and initial digestion rate firstly increased,and then decreased,ABTS+·,O2-·,·OH scavenging activity and reduction ability of hydrolysates of trypsin firstly increased and then decreased,DPPH· scavenging activity,and metal chelating ability of enzymatic product unchanged before decreased.As malondialdehyde concentration increased,trypsin digestibility and initial digestion rate still decreased,ABTS+·,DPPH·,O2-·,·OH scavenging activity and reduction ability of hydrolysates of trypsin still decreased.The comparison of peroxyl radicals and malondialdehyde to the same degree of oxidation of rice bran protein showed that the malondialdehyde oxidation inhibited the digestive properties of rice bran protein in vitro,and when the AAPH concentration is low,it could promote the digestion of trypsin in vitro.