Extraction and Characteristics of Water-soluble Proteins from Brown Rice (Oryza sativa L. subsp. japonica), Polished Rice and Rice Bran

The brown rice, polished rice and rice bran of Oryza sativa L. subsp. japonica were analyzed for their proximate compositions and protein compositions. Soluble proteins were extracted from the brown rice, polished rice and defatted rice bran (our study indicated that removal of fat was favorable to dissolution of proteins) using deionized water (pH 8.5), respectively, and response surface methodology was adopted to optimize the extraction of water-soluble proteins from the three materials based on one-factor-at-a-time experiments. In addition, the water-soluble proteins from different sources were subjected to analyses of solubility and molecular weight distribution. Protein composition analysis showed that both the brown rice and polished rice contained mainly glutelin, and the glutelin content in the former was 4.78% lower than that in the latter, while the former contained more albumin than latter by 55.93%; the contents of globulin and albumin in the brown rice were lower than those in the rice bran by 77.22% and 44.59%, respectively, while the glutelin content in the former was higher than that in the latter. All extraction duration, temperature, material/liquid ratio influenced yields of water-soluble proteins from these three materials to different extents — among the three factors, material/liquid ratio was the most significant one influencing all the extraction of water-soluble proteins from the three rice materials; temperature was a significant factor that influenced the extraction of polished rice water-soluble proteins (p 0.05), and for the extraction of rice bran water-soluble proteins, it was a extremely significant factor (p 0.05), while no significant influence was observe on the extraction of brown rice water-soluble proteins; extraction duration only presented significant influence on the extraction of polished rice water-soluble proteins (p 0.05). Three quadratic polynomial models for the extraction of water-soluble proteins from the three different materials were set up by response surface regression analysis. Predicted and actual values of extraction yield of water-soluble proteins were well fitted, indicating their validity was good. Solubility and SDS-PAGE analyses revealed that the solubility and molecular weight distri- bution of water-soluble proteins from brown rice were between those from polished rice and rice bran proteins.