The Research On The Preparation And Adsorption Properties Of Rice Bran Fiber With High Adsorption Capacities

Rice bran is a byproduct of rice milling, and it accounts for 5⁷% of the whole rice. There is an underestimated potential for high- value rice bran product because of its high content of IDF. RB is composed of 35%⁵0% DF and almost 90% of the DF is IDF. In recent years, some researches demonstrated that rice bran insoluble dietary fiber（RBIDF） has good hydration properties, oil binding capacity and some functional properties such as high adsorption capacities for cholesterol and heavy metals. However, the mechanism for RBIDF adsorbing some specific functional materials is still not clear. The relationship between microstructure and physicochemical properties of RBIDF was not studied yet. Therefore, the simple acid-alkaline method was used to extract the insoluble fiber from rice bran and the effect of inorganic acid concentration on the composition and microstructure was investigated in order to establish the relationship between composition-microstructure and physicochemical properties of RBIDF. Moreover, based on the oil binding capacity, water holding capacity and cation-exchange capacity of RBIDF, the adsorption capacities of RBIDF to functional compounds in oil system, water system and toxic cations were evaluated and the adsorption mechanism was discussed, so as to improve the potential applications of RBIDF. The main research contents and results are as follows:First, the composition and structural modification of RBIDF by sequential regimes of sulphuric acid（H2SO4） and their effects on physicochemical attributes were studied. The results indicated that acid treatment mainly induced the hydrolysis of starch while protein was mainly dissolved by KOH for all samples and its residual amout was effectively controlled within 2.0%. The increment of H2SO4 concentration（within 1.25%） resulted in decreased bulk density（from 0.32 to 0.14 g/mL） and increment of specific surface area（from 0.18 to 0.45 m2/g）, further induced decreased water holding capacity, swelling capacity, cation-exchange capacity and bulk density that ultimately enhanced oil binding capacity due to the partial removal of starch, protein and hemicelluloses. Moreover, higher H2SO4 concentration improved the porosity and crystallinity that led to higher thermal stability of fiber as evident from XRD and TGA analysis.Based on the results obtained in the first section, three cellulosic RBIDFs（RBIDF-0.2, RBIDF-1.25, RBIDF-2.0） with different structure and physicochemical properties were selected for evaluating their adsorption capacity and activity inhibitory capacity against PL. The changes in secondary structure of PL and the interaction force between PL and RBIDF was analyzed by fluorescence spectroscopy and circular dichroism（CD）, respectively. The results showed that the PL adsorption rate was greatest for RBIDF-2.0, the time to achieve its saturation level was the shortest, only 0.75 h. However, the RBIDF-1.25 had the highest PL activity inhibitory capacity which confirmed that the mechanism for RBIDF adsorbing PL different from it for RBIDF inhibiting PL activity. The increment of unordered structure of PL after interacting with RBIDF confirmed by CD suggested that the activity of PL was altered. Free energy calculations using data from fluorescence spectroscopy revealed that binding of PL to fibre depends primarily on electrostatic interac tions and the binding process is spontaneous and exothermic, the binding site is only one between PL and RBIDF. The fluorescence quenching of PL by RBIDF is from the static quenching.After evaluating the adsorption capacity of RBIDF to PL in oil system, the in vitro hypoglycemic properties of RBIDF-0.2, RBIDF-1.25 and RBIDF-2.0 that in terms of glucose and amylase adsorption capacity, glucose retardation index and starch digestability in water system, were investigated. The results revealed that all the three RBIDFs have strong glucose adsorption capacity and α-amylase inhibition activity, with RBIDF-1.25 being the most effective one（about 3-fold to original rice bran）. The glucose retardation index was the largest for RBIDF-0.2 that mainly depends on its large particle size. The α-amylase inhibition activity of RBIDF was associated with the formation of amylase-RBIDF complex as confirmed by fluorescence spectroscopy, with the highest amylase activity inhibitory（24.72%） for RBIDF-1.25. The interaction force between PL and fibre was major electrostatic interactions and the binding process is spontaneous and exothermic, the binding sites are two between amylase and RBIDF-1.25 at 37℃. The results of CD showed that β-sheet, and β-turn of second structure decreased by 50.15⁵7.23%, 14.55²7.23%, respectively while unordered structure increased 0.65².41%. The conformational deformation of amylase after interacting with RBIDFs suggested that the activity of amylase was altered.In order to evaluate the effects of different functional groups on the adsorption capacities of RBIDFs to lipase and amylase, the selected RBIDFs were further modified by carboxymethylation and hydroxypropylation. The adsorption capacities of RBIDF to lipase and amylase before and after modification were compared. The results suggested that the lipase adsorption capacity and lipase activity inhibiting capacity of carboxymethylated RBIDF were1.25².5 folds and 7 folds higher than unmodified RBIDF, respectively. The main reason is ascribed to the increment of electrostatic interaction between RBIDF and lipase with introducing carboxy group. Whereas, the lipase adsorption capacity and lipase activity inhibiting capacity of hydroxypropylated RBIDF decreaed by 17.28%²5.80% and 16.88%²7.28% compared to unmodified RBIDF. This mainly resulted from the formation of stable hydrogen bond between hydroxypropyl groups and water that depressed the protein adsorption. Moreover, the acitivity inhibitory capacities against amylase of both carboxymethylated and hydroxypropylated RBIDF decreased, which was ascribed to the drcrease of porous structure.In view of the cation-exchange property of RBIDF, the adsorption capacities of RBIDF-0.2,RBIDF-1.25,RBIDF-2.0 to Pb²⁺ were investigated and compared. The binding capacities of all the tested RBIDFs to Pb²⁺ at pH 7.0 were higher than that at p H 2.0. The experimental data suggested that the adsorption rate for Pb²⁺ followed the second-order kinetic model. The Langmuir model fits well for the adsorption equilibrium data in the examined concentration range, which indicated that the uptake of Pb²⁺ occurred on a homogenous surface by monolayer sorption without any interaction. The data of thermodynamic constants indicated that adsorption of Pb²⁺ was endothermic and spontaneous. The positive values of ΔS suggest that there are some structural changes in the sorbate(hydrated Pb²⁺) and sorbent（RBIDF）. Under all the temperatures studied, the binding property of RBIDF for Pb²⁺ mainly depends on physical adsorption according to the average free energy E < 8 kJ mol^-1.