Production of non-digestible oligosaccharides as value-added by-products from rice straw

The efficacy of rice straw as a raw material for the production of non-digestible oligosaccharides (NDOs) was evaluated using water and acid extraction. Fructooligosaccharides (FOS) were extracted with water at different times and temperatures. Other NDOs, consisting of xylooligosaccharides (XOS), cellooligosaccharides (COS), galactooligosaccharides (GOS) and arabinooligosaccharides plus mannooligosaccharides (AOS + MOS), were investigated with different sulfuric acid concentrations, residence times and temperatures. FOS was found only when the straw was treated at 60 °C and 70 °C for all extraction periods (2 h, 3 h, and 5 h). The FOS production (1.55-1.61% weight per weight (w/w) dry biomass at 60 °C and 1.44-1.67% w/w dry biomass at 70 °C) did not differ with either temperature or extraction period. All the other NDOs were detected, with XOS being the predominant one found in the acidic extracts. The highest yield of XOS was 6.53% w/w dry biomass (equivalent to 27.82% by weight of xylan in the rice straw). The optimum conditions for XOS production were: 2% (w/w) sulfuric acid, 100 °C and 0.5 h. Extraction time, temperature and acid concentration had effects on the production of XOS, COS, GOS, AOS, and MOS. These factors determined whether the products were oligosaccharides or the monosaccharide constituents.