Ultrafiltration of partially hydrolyzed rice bran protein to recover value-added products

Rice brans were treated with a protease together with a disulfide bond-breaking agent (Na2SO3) to achieve a 2-4% peptide bond hydrolysis (DH). Ultrafiltration (UF) with 3 kDa molecular weight cut-off (MWCO) membrane led to substantial loss of feed protein due to permeation. Using 1 kDa MWCO membrane increased protein yields, but it was not effective in purifying the protein hydrolysates despite the increase in membrane area and operating time. The efficiency of this UF process can be improved using a larger MWCO membrane (e.g., 2 kDa MWCO), which may facilitate complete removal of phytate. Based on disparity of molecular sizes, use of phytase may also increase purity of protein retentates and allow the recovery of functional inositol phosphates in permeates. The presence of Na2SO3 during proteolysis to 2% DH of preheated bran (100 degrees C, 10 min) repaired the damage caused by preheat treatment by increasing protein recovery but increased the concentration of small peptides in hydrolysates, i.e., <1 kDa, particularly for highly aggregated proteins. Heat treatment is necessary to stabilize rice bran, but the sulfite treatment may be avoided to increase UF yield and purity of protein retentates and allow higher DH values for hydrolysis of stabilized brans. Accordingly, this UF process can be an efficient method for recovering high-value components from rice bran, an underutilized rice milling co-product, for many industrial applications.