In this research, the protein participation that occurs with the flocculation and sedimentation phenomena caused by adding enzymes to solid particles derived from rice washing drainage and the electrostatic properties of the solid particle surfaces was examined. As seen in the results, adding protease to rice washing drainage decreased the protein content, strengthened the negative surface electron charge, and decreased the isoelectric point (IEP) value of the solid particles. In addition, about 25% of the proteins in the rice washing drainage still remained after enzyme treatment. Moreover, due to the enzyme treatment, specific and gradual degradations of 10, 13, and 16 kDa of prolamin and 20-22 kDa of gluterin basic subunit, both categorized in the basic protein group, were observed in an SDS-PACE analysis. These results suggest that the flocculation and sedimentation of solid particles in rice washing drainage occur as a result of the formation of the IEP-dependent strengthening of the negative surface electron charge on the solid particle surface. (C) 2011, The Society for Biotechnology, Japan. All rights reserved.