Two-step hydrolysis of rice (Oryza sativa) husk as treated by semi-flow hot-compressed water

Abstract

Two-step hydrolysis of husk obtained from rice (Oryza sativa) was investigated as one of the monocotyledonous angiosperms under the semi-flow hot-compressed water treatment at 230 °C/10 MPa/15 min (1st stage) and 270 °C/10 MPa/30 min (2nd stage). Prior to the hot-compressed water treatment, cold-water extraction at 20 °C/10 MPa/30 min was performed. It was found that some inorganic constituents and free neutral sugars not being chemically bonded with the plant cell wall were recovered in the cold-water extracts. In the 1st stage, hemicelluloses and pectin were selectively hydrolyzed, as well as lignin being partially decomposed. In addition, protein was found to some extent to be hydrolyzed by the hot-compressed water treatment and various amino acids to form the protein of rice husk were identified. Hydrolysis of cellulose was, however, observed in the 2nd stage. Some additional decomposition of lignin and protein was revealed at this stage as well. In total, 96.1% of oven-dried extractives-free rice husk sample could be solubilized into cold and hot-compressed water. Various products in the water-soluble portion were primarily recovered as saccharides, which were partially isomerized and then dehydrated and fragmented. The 3.9% of residue after the treatment was composed mainly of lignin and a trace of silica.