Chemoselective Oxidation of C6 Primary Hydroxyl Groups of Polysaccharides in Rice Bran for the Application as a Novel Water-Soluble Dietary Fiber

Rice bran contains health-beneficial dietary fiber, but its secondary applications are limited by its poor water solubility. The water solubility of polysaccharides in rice bran was increased through the chemoselective oxidation of primary hydroxyl groups by 2,2,6,6-tetramethyl-1-piperidinyl oxoammonium ion and sodium hypochlorite. Response surface methodology was employed to optimize chemoselective conditions in terms of tetramethyl-1-piperidinyl oxoammonium concentration, sodium hypochlorite concentration, and reaction pH. The maximum degree of oxidation reached 98.77% under the optimum conditions of 0.16 mmol tetramethyl-1-piperidinyl oxoammonium, 8.52 mmol sodium hypochlorite, and pH 10.85. The introduction of C6 carboxyl group was confirmed by 13C-NMR and Fourier transform infrared. Water solubility of the oxidized bran (84.65%) was higher than native rice bran (30.65%). Sensory evaluation indicated that high-quality cookies and bread could be prepared by partially replacing wheat flour with oxidized rice bran, demonstrating that oxidized rice bran could be used as a new water-soluble dietary fiber in the food industry.