Composition, structural, physicochemical and functional properties of dietary fiber from different milling fractions of black rice bran

Black rice bran is a superior dietary fiber (DF) resource. However, the lack of specific distribution and functionality of DF in black rice bran has constrained its refined processing. This study divided black rice bran into five equal bran fractions (BF) by stepwise milling to obtain BF1(outermost layer) to BF5 (the innermost layer). The contents, structure, physicochemical and functional properties of DF in each bran fractions were investigated. The results revealed that the insoluble, soluble and total dietary fiber (IDF, SDF and TDF) contents were 17.52 +/- 0.48 to 31.14 +/- 0.96, 2.03 +/- 0.20 to 3.86 +/- 0.01 and 19.57 +/- 0.28 to 34.61 +/- 1.10 g/100 g dry basis, respectively. Compared to BF1, the TDF content of BF2-BF5 was reduced by 3.90, 3.20, 21.74 and 43.46%, respectively. The main monosaccharides in both IDF and SDF were xylose, glucose, arabinose and galactose. The molecular weight of SDF samples ranged from 17.19 +/- 1.95 to 36.32 +/- 0.80 kDa. The outer bran fractions (BF1-BF3) IDF exhibited higher water swelling, water retention, oil adsorption capacity, together with cholesterol and sodium nitrite adsorption capacity than those of inner bran fractions (BF4-BF5), owing to their lower crystallinity and well -developed porous structure. These results provide essential data for the development of black rice DF-based food processing with enhanced functional benefits.