Preparation And Function Of Protein From Rice Bran And Broken Rice Using Hydrothermal Cooking

The production of rice by-products is huge, but the value is low. Rice protein has beenfound to be of nutritional, hypoallergenic and having anti-cancer activity for food applicationsand rice protein products has been in demand in recent years. However, due to the complexityof protein composition and difficulties in the processing, low protein extraction and purityhave been recognized, which greatly limit its application in food industry. In this work,hydrothermal cooking (HTC) was firstly used to extraction and modification of protein fromheat-stabilized rice bran and broken rice kernels. Protein extraction, structure and functionproperties were described in the HTC process, and the mechanism induced by HTC wasinvestigated. The broken rice protein with low denaturation and high digestibility wasprepared by enzyme-assisted microfluidization, and the reason of high digestibility was alsoinvestigated. Main results are as follows:1. Hydrothermal cooking (HTC) combined with amylase pretreatment (AP) was used toimprove protein extraction from heat-stabilized rice bran. The physicochemical andemulsifying properties of rice bran protein isolate (RPI) were evaluated. HTC alonesignificantly increased extraction yield, while protein purity was decreased. In contrast, HTCcombined with AP significantly improved both extraction yield and protein purity (about45-50%and72-74%, respectively). Electrophoresis and size exclusion chromatographyprofiles indicated that HTC led to the dissociation of insoluble protein aggregates in rice bran,with subsequent increase of soluble aggregates in RPI, linked by non-covalent (e.g.,hydrophobic interaction) and covalent bonds (disulfide bond). This result was evidenced bythe increased disulfide bond contents and surface hydrophobicity of RPI. In addition, the dateof DG, fluorescence spectrum and electrophoresis stained by PSA indicated that HTC led tothe conjugations of protein and reducing sugar in rice bran, which led to the change inphysical and chemical characteristics of the protein.2. HTC-prepared RPI exhibited excellent functional properties and in vitro digestibilityproperty. All protein samples exhibited typical U-shaped solubility-pH profiles, with theminimum at pH3.5–4.5and gradually increasing at pH values below3.0and above6.0. Foamcapacity and stability, the water holding capacity (WHC) and the oil holding capacity (OHC)in these proteins were superior to previous products. In addition, HTC-prepared RPI exhibitedexcellent emulsifying property. The emulsion is stable under neutral pH after heat treatment,but is unstable after freeze-thaw processing. In vitro digestion model, heat-stabilized rice bran protein showed similar digestibility with un-stabilized rice bran protein after HTC treatmentand the soluble nitrogen release was77.9%.3. In order to verify the glycosylation reaction occurred in the HTC process, reducingsugar was added in heat-stabilized rice bran. HTC treatment is an effective technique in theenhancement of the conjugation of protein and reducing sugar in heat-stabilized rice bran, andglycosylation reaction extent of lactose is higher than that of the glucose. Glycosylationreaction products exhibited excellent solubility emulsifying property, but have no change invitro digestion. Therefore, HTC can be applied in extraction andmodification of the insoluble protein as a new liquid heat glycosylation reaction technique.4. High protein recovery (81.87%) and purity (87.89%) were obtained from broken riceby enzyme-assisted microfluidization. The protein composition, solubility, structuralproperties, and in vitro digestibility of rice proteins prepared by enzyme-assistedmicrofluidization (EM-RP) and alkaline extraction (AE-RP) were compared. EM-RP wasmainly composed of glutelin with low solubility and native structure, whereas large quantitiesof prolamin and globulin appeared in the AE-RP except glutelin. Glutamic acid/glutamine,leucine, aromatic and charged amino acids were rich in the AE-RP because of differentprotein composition. Compared to AE-RP, EM-RP showed higher digestibility due to therichness of glutelin (an easy-to-digest protein), as evidenced by higher nitrogen release duringpepsin-trypsin digestion. The presence of prolamin (an indigestible protein) in AE-RPdecreased protein digestibility although alkaline extraction improved its hydrolysis.5. The interaction of between rice protein and rice starch and the protein effect onstarch pasting properties were studied systemically and acidic deamidation of rice protein wasprepared by HTC.The gelatinization, peak viscosity, breakdown and setback of rice starchwere reduced after the addition of EM-RP protein, but the gelatinization temperature and peaktemperature have significantly different. The rheological property of rice starch showedapparent viscosity and shear force increased with increasing the protein content under lowerprotein content (<10%). However, these values will decrease with further increasing theprotein content. HTC treatment was a feasible acidic deamidation of EM-RP rice protein,but had no significant effect on the protein structure. The degree of protein deamidation wascorresponding increasing with acid concentration increasing, but the degree of that wasdecreased with protein content increasing under the same acid concentration. The optimaldeamidation conditions were0.4mol/L HCl,1%rice protein,90s for reaction time and140℃under HTC treatment with the25%deamidation and low degree of hydrolysis levels.