Improving Emulsibility Properties Of Rice Bran Protein With Ultrasonic And Papain Hydrolysis Methods

Rice bran protein is a valuable source of plant protein with high-quality.However,due to the strong polymerization of the proteins with other substances?such as phytic acid,cellulose?and the intermolecular cross-linking through a large number of disulfide bonds,rice bran protein showed poor solubility which in turn affected other functional properties such as emulsification.In this research,ultrasonic,papain and ultrasonic assisted papain were used to improve the emulsifying properties of rice bran protein.The mechanism of emulsification improvement was preliminarily investigated.The modified rice bran protein was applied to the powder oil.The following results were obtained.Under the condition with ultrasonic power density 5W/mL,ultrasonic time 30min and protein mass concentration 4%,the solubility,emulsifying activity and emulsion stability of U-RBP were increased by 171.06%,60.4%and 107.79%,respectively.The average particle size and CI of U-RBP WPI emulsion droplets was lower than that of RBP.The zeta-potential and F_ads were higher than that of RBP.After 24 hours of storage,TEM showed that the U-RBP had small droplet size and uniform distribution characteristics.This indicated that U-RBP had good emulsifying properties.The emulsifiability of U-RBP was enhanced further by adding NaCl and CaCl₂ at pH 3 and 7.During the ultrasonic time 0³0min,the disulfide bond content was decreased,the surface hydrophobicity was increased,and the intrinsic fluorescence intensity was increased first and then decreased.This indicated that protein structure partially expanded.The?-sheet and random coil were increased,indicating that the protein structure was stretched and recombined,which results in the enhancement of the molecular flexibility and the further improvement of the emulsifying properties.When the amount of papain was 2478U/g,incubation time was 4h at pH 7 and 50°C,the solubility and emulsifying activity of Pap-RBP were increased by 118%and 42.33%,respectively,and the emulsion stability was decreased by 36.61%.During the incubation time0⁴h,the disulfide bond was broken,the polypeptide chain was stretched,the hydrophobic amino acid inside the molecule was exposed,and the intrinsic fluorescence intensity was first increased and then decreased.?-sheet was increased,indicating that the protein was more loose and stretched,thereby increasing the emulsifying properties.The rice bran protein was pretreated by ultrasound,then enzymatic reaction was performed.When the amount of papain was 1770U/g,and incubation time was 3h at pH 7 and50°C,the solubility and emulsifying activity of U-Pap-RBP were increased by 207.3%and54.94%,respectively,and the emulsion stability was decreased by 38.25%.Compared with Pap-RBP,the solubility and emulsifiability of U-Pap-RBP were improved and meanwhile the incubation time and enzyme substrate ratio could be decreased.The average particle size of U-Pap-RBP emulsion droplets was significantly lower than that of RBP,U-RBP and Pap-RBP,indicating that U-Pap-RBP had good emulsifying activity.The zeta-potential was significantly higher than that of RBP and Pap-RBP.The CI was lower than Pap-RBP.After 24 hours of storage,TEM showed that the droplet size of U-Pap-RBP was smaller than that of Pap-RBP.This indicated that U-Pap-RBP emulsion droplets had higher deflocculation resistance than that of Pap-RBP and RBP.The emulsifiability of U-Pap-RBP was enhanced by adding carrageenan or xanthan gum at pH 3 and 7.Especially with the addition of xanthan gum??0.01%?at pH 7,the emulsion droplets was not broken after 24 hours.Compared with Pap-RBP,the disulfide bond content of U-Pap-RBP was decreased,the surface hydrophobicity was increased,and the intrinsic fluorescence intensity was increased,indicating that ultrasound pretreatment promoted the enzymatic hydrolysed proteins to become more stretched and increase?-sheet and random coil.This could partly explain the improvement of the protein functionalities.Powdered-oil prepared with U-RBP addition had higher encapsulation efficiency,and the wettability,dispersibility,solubility and zeta-potential were significantly higher than others.After reconstitution,its particle size was smaller and the distribution was more concentrated.SEM showed that the wall material was intact with smooth and continuous appearance,and the particles were small.Compared with powdered-oil with RBP addition,its amide I band was red shifted 1.93cm^-1,indicating a decrease of intramolecular hydrogen bonds.Maillard reaction were occurred between rice bran protein and polysaccharide.Its amide II was blue shifted 3.86cm^-1,indicating an overall increase of hydrogen bonds.The infrared spectra of-OH and-CH were red shifted 30.87cm^-1 and 3.86cm^-1,respectively.The red shift might indicate that the carboxyl group of the protein interacted with the hydroxyl group of the maltodextrin through hydrogen bonds.Both formed a thick and viscoelastic protective film at the oil-water interface together.Its?-turn and?-helix were higher.The increase of?-turn content results in enhancement of molecular flexibility,and the protein conformation could undergo a large transition at the interface.The increase of?-helix content might indicate that the cross-linking reaction between the protein and the maltodextrin producing hydrogen bonds.Since?-helix had amphiphilicity,it further promoted rapid adsorption and orientation of the protein at the interface.This explained the reason why the protein had better emulsifying properties after reconstitution.