Arabinoxylan is the main polysaccharide in fiber from rice coproducts, and increased concentration of fiber decreases in vitro digestibility of dry matter

The first objective of this experiment was to determine carbohydrate composition and coefficients of in vitro dry matter digestibility (IVDMD) and in vitro organic matter digestibility (IVOMD) in brown rice, broken rice, full fat rice bran (FFRB), defatted rice bran (DFRB), and rice mill feed. The second objective was to test the hypothesis that IVDMD and IVOMD in rice coproducts are negatively correlated with the concentration of fiber in the ingredients. Three samples of broken rice, brown rice, FFRB, DFRB, and rice mill feed were used. Each sample was analyzed in duplicate for gross energy (GE), dry matter (DM), ash, neutral detergent fiber (NDF), acid detergent fiber (ADF), acid detergent lignin (ADL), crude protein (CP), and acid hydrolyzed ether extract (AEE). The 5 rice coproducts were also analyzed for content of starch, and the fiber concentration was quantified as total non-starch polysaccharides (T-NSP), cellulose, soluble and insoluble non-cellulosic polysaccharides (NCP), and Klason lignin. In vitro total tract digestibility of DM and IVOMD were determined in 3 samples of each ingredient using a 3 step procedure. Correlation coefficients between IVDMD or IVOMD and fiber fractions were determined. Results indicated that broken rice and brown rice contained more than 80.0% starch, FFRB and DFRB both contained 26.7% starch, whereas rice mill feed only contained 11.2% starch. The concentration of soluble-non cellulosic polysaccharides (S-NCP) was in the range from 0.1% in brown rice to 1.9% in rice mill feed, whereas insoluble dietary fiber (IDF) was 1.5% in broken rice and 52.9% in rice mill feed. The insoluble non-cellulosic non-starch polysaccharide (I-NCP) fraction in all rice coproducts consisted mainly of arbinoxylan and the arabinose to xylose ratio varied from 1.37 in broken rice to 0.27 in rice mill feed. Broken rice and brown rice had greater (P < 0.05) IVDMD and IVOMD than the other rice coproducts. Concentrations of ADF, NDF, ADL, I-NCP, cellulose, T-NSP, SDF, IDF, TDF were all negatively correlated (P < 0.05) with IVDMD and IVOMD. In conclusion, arabinoxylan is the main polysaccharide in the NSP fraction of rice coproducts. Arabinoxylan in broken rice, brown rice, FFRB, and DFRB has a different structure and is more soluble, than arabinoxylan in rice mill feed, but IVDMD and IVOMD are negatively correlated with the concentration of fiber in rice coproducts.