Value-added processing of rice bran focusing on dietary fiber modification

Rice bran (RB) is an underutilized byproduct of rice milling industry. RB is rich in insoluble dietary fiber (IDF) but poor in soluble dietary fiber (SDF). Recently, SDF derived from RB has been proven for its superior antioxidant and prebiotic activities which confer human health benefits (digestive, cardiovascular, nerve health, etc.). Moreover, SDF can improve sensory and physicochemical properties (texture, color, uniformity, water binding capacity, hydration, etc.) of SDF-enriched foods. Therefore, conversion of RB-IDF to RB-SDF would be to utilize this affordable byproduct to add value to the rice bran processing industry as a common functional food ingredient. The aim of this study was to maximize soluble pentosan content (a major SDF component) in RB, by investigating the effect of physical (extrusion) and enzymatic (xylanase) technologies in individual and combined ways on water-washed rice bran and its soluble compositions. A water washing procedure was necessary to remove water solubles as a strategy to increase the proportion of total dietary fiber (IDF + SDF) that could be converted to SDF after treatment. Even though water washing wasted native SDF along with starch and other solubles, preparing the sample this way was practical due to the large subsequent increase in IDF. The sequentially combined process of extrusion and enzyme treatments, compared to the individual and simultaneously combined treatments, significantly increased total solubility and soluble pentosan content of the final RB product. The warm-water-soluble pentosan content of treated RB was 6.5% by the sequentially combined process, 4% by either parallel combined process or extrusion alone or xylanase treatment alone. The hot-water-soluble pentosan of treated RB achieved a higher level of 10.5% by the sequentially combined process, 4.8% by extrusion alone, and 6.5% by xylanase treatment alone. A maximum total hot water solubility of 25% was achieved, of which 10.5% was pentosan, when water-washed rice bran was treated with extrusion and enzyme in sequence, representing an approximately four fold increase compared to untreated RB. Overall, washing rice bran with water was shown to be an efficient method to remove non-dietary fiber components. This study will likely represent the first published example for rice bran demonstrating an alternative to enzymatic methods used conventionally (e.g. amylase, protease, lipase) to hydrolyze non-dietary fiber compounds for further fiber processing.