| The rice bran resource is abundant in our country with an average annual output of about 16 million tons.The protein content of rice bran is as high as 12%-18%.Rice bran protein has attracted extensive attention due to its excellent digestibility,hypoallergenicity and reasonable amino acid composition.Rice bran is also rich in phenolics,which can reach 9.60-81.85 mg GAE/g.During the process of rice bran processing,storage and transportation,the interaction between rice bran phenolics and rice bran protein will inevitably lead to changes in the structure and digestive properties of rice bran protein.Rice bran is prone to rancidity.Oxidative stress caused by rancidity changed the structure of rice bran phenolics and rice bran protein,which affected the interaction between rice bran phenolics and rice bran protein,and made the influence of rice bran phenolics on the structure and digestive properties of rice bran protein more complicated.At present,the mechanism by which rice bran phenolics affect the structure and digestibility of rice bran protein under the condition of rancidity induced oxidation remains unclear.In this thesis,the influence mechanism of rice bran phenolics on the structure and digestibility of rice bran protein under the oxidative stress caused by rice bran rancidity was investigated by studying the changes of the structure and digestibility of rice bran protein with non-dephenolized and dephenolized at different degrees of rancidity.After 0,1,3,5 and 10 days of storage,fresh rice bran was defatted to obtain the defatted rice bran with different degrees of rancidity.Then,the defatted rice bran with different degrees of rancidity was dephenolized by methanol and ethanol,respectively.Then,the bound and free phenolics were extracted from the non-dephenolized and dephenolized rice bran,and the antioxidant capacity was determined.The results showed that,compared with non-dephenolized rice bran,the free phenolics content of dephenolized rice bran decreased significantly after dephenolization(free phenolics removal rate of methanol:83.83%-86.03%;free phenolics removal rate of ethanol was 81.56%-83.52%),and the content of bound phenolics decreased slightly(bound phenolics removal rate of methanol:6.58%-9.86%;bound phenolics removal rate of ethanol:12.47%-16.63%).The antioxidant capacity of free phenolics decreased significantly and that of bound phenolics decreased slightly.With the increase of rice bran storage time,the content of free phenolics in defatted rice bran first increased and then decreased,while the content of bound phenolics showed a trend of first decreasing and then increasing.In addition,the antioxidant capacity of free and bound phenolics in three kinds of defatted rice bran was consistent with the corresponding contents.Rice bran protein was extracted from non-dephenolized rice bran,methanol dephenolized rice bran and ethanol dephenolized rice bran.The results showed that compared with non-dephenolized rice bran protein,the free phenolics content of rice bran protein dephenolized by methanol and ethanol decreased by 18.29%-23.55%and 19.8225.36%,respectively.Bound phenolics content decreased by 28.87%-59.22%and 22.51%-69.71%,respectively.The contents of free and bound phenolics in nondephenolized rice bran protein,methanol dephenolized rice bran protein and ethanol dephenolized rice bran were consistent with the contents of free and bound phenolics in non-dephenolized rice bran,methanol dephenolized rice bran and ethanoldephenolized rice bran.The results showed that dephenolization by methanol and ethanol removed part of phenolics in defatted rice bran and rice bran protein,which reduced the antioxidant capacity of defatted rice bran,and ethanol had better dephenolization effect than methanol.With the increase of storage time of rice bran,oxidative stress caused by rancidity of rice bran affected the content of phenolics in defatted rice bran,and lead to the change of dephenolization rate of defatted rice bran and rice bran protein,as well as the antioxidant capacity of defatted rice bran.By analyzing the structural characteristics of non-dephenolized and dephenolized rice bran protein with different rancidity degrees.It was found that compared with nondephenolized rice bran protein,the contents of free sulfhydryl groups of methanol dephenolized and ethanol dephenolized rice bran protein(non-dephenolized rice bran protein:24.41-32.33 μmol/g;methanol dephenolized rice bran protein:32.30-43.74μmol/g;ethanol dephenolized rice bran protein:32.64-45.04 μmol/g)and carbonyl content(non-dephenolized rice bran protein:5.32-24.59 μmol/g;methanol dephenolized rice bran protein:5.90-25.94 μmol/g;ethanol dephenolized rice bran protein:6.11-26.52 μmol/g)was decreased,and the disulfide bond content(nondephenolized rice bran protein:24.26-26.61 μmol/g;methanol dephenolized rice bran protein:22.43-25.60 μmol/g;ethanol dephenolized rice bran protein:21.78-25.42μmol/g)was increased,the content of α-helix structure increased,the content of β-sheet structure decreased,the maximum fluorescence peak blue shifted,the surface hydrophobicity decreased,the particle size increased,the absolute value of Zeta potential increased,and the color of SDS-PAGE high molecular weight band became lighter.In addition,compared with methanol dephenolized rice bran protein,the structure difference between ethanol dephenolized rice bran protein and nondephenolized rice bran protein was more obvious.With the increase of rice bran storage time,the content of free sulfhydryl group decreased gradually,the content of carbonyl group and disulfide bond increased gradually;the content of α-helix structure first decreased and then increased,the content of β-sheet structure first increased and then decreased;the maximum fluorescence peak first red shifted and then blue shifted,the surface hydrophobicity first increased and then decreased,the Zeta potential first increased and then decreased,the particle size first decreased and then increased,and the color SDS-PAGE high molecular weight bands were shallower and then deepened.The absolute value of Zeta potential of rice bran protein was increased by introducing charged groups.As the rancidity of rice bran increased,the oxidation degree of rice bran protein increased gradually,the flexibility of rice bran protein secondary structure increased first and then decreased,and the spatial structure unfolded first and then aggregated.By analyzing the in vitro digestibility of non-dephenolized and ethanol dephenolized rice bran protein at different rancidity degrees.It was found that compared with non-dephenolized rice bran protein,ethanol dephenolized rice bran protein had a higher digestibility(non-dephenolized rice bran protein:57.76%-64.45%;ethanol dephenolized rice bran protein:64.23%-74.20%),the inhibition rate of pepsin(non-dephenolized rice bran protein:19.05%-33.12%;ethanol dephenolized rice bran protein:15.06%-29.48%)and the inhibition rate of trypsin(non-dephenolized rice bran protein:37.97%-62.31%;ethanol dephenolized rice bran protein:35.88%-46.18%),smaller particle size,lower antioxidant capacity and lighter color of SDS-PAGE high molecular weight subunit bands.With the increase of rice bran storage time,the digestibility of rice bran protein firstly increased and then decreased,and reached the maximum value at 1 day of rice bran storage.The inhibition of pepsin and trypsin increased first and then decreased,and reached the maximum value when rice bran was stored for 5 days.The particle size of digestion products increased first and then decreased,and reached the maximum value when rice bran was stored for 5 days.The color of the high molecular weight subunit bands on SDS-PAGE was deepened first and then lightened,and the color was darkest when the bran was stored for 5 days.During rice bran storage time,the contents of free phenolics in non-dephenolized rice bran protein and ethanol dephenolized rice bran protein were significant positive correlation with the inhibition rate of pepsin,the inhibition rate of trypsin and the particle size of digestion products of rice bran protein(P<0.01).However,with the increase of rancidity of rice bran,the antioxidant capacity of rice bran protein digestion products changed irregularly.The results showed that rice bran phenolics affected the digestibility of rice bran protein by inhibiting the activities of pepsin and trypsin,changed the particle size of rice bran protein digestion products through crosslinking,and affected the antioxidant capacity of rice bran protein digestion products.With the deepening of rancidity,oxidation caused by rancidity changed the structure and phenolics content of rice bran protein,and affected the digestibility of rice bran protein. |
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