title: Precipitate obtained following membrane separation of hydrothermally pretreated rice straw liquid revealed by 2D NMR to have high lignin content.
abstract: BACKGROUND
  Hydrothermal pretreatment of lignocellulosic biomass such as rice straw can dissolve part of the lignin and hemicellulose into a liquid fraction, thus facilitating enzyme accessibility to cellulose in bioethanol production process. Lignin is awaited to be recovered after hydrothermal pretreatment for utilization as value-added chemical, and lignin recovery also means removal of fermentation inhibitors. To recover lignin with high content from the liquid fraction, it is necessary to separate lignin and hemicellulose-derived polysaccharide. Therefore, the following processes were applied: membrane separation with nanofiltration (NF) and enzymatic hydrolysis by hemicellulase. To clarify lignin-concentrated fraction obtained during these processes, the fates of lignin and polysaccharide components were pursued by a solution NMR method and confirmed by compositional analysis of each fraction.
  
  
  RESULTS
  After hydrothermal pretreatment of rice straw, the NF concentrate of the supernatant of liquid fraction was hydrolyzed by hemicellulase and the resulting black precipitate was recovered. In this black precipitate, the intensity of NMR spectra related to lignin aromatic regions increased and those related to polysaccharides decreased, compared to rice straw, the solid fraction after hydrothermal pretreatment, and the NF concentrate. The lignin content of the black precipitate was 65.8 %. Lignin in the black precipitate included 52.9 % of the acid-insoluble lignin and 19.4 % of the soluble lignin in the NF concentrate of supernatant of liquid fraction.
  
  
  CONCLUSIONS
  A precipitate with high lignin content was obtained from supernatants of the liquid fraction. These results suggested that precipitation of lignin was enhanced from concentrated mixtures of lignin and hemicellulosic polysaccharides by hydrolyzing the polysaccharides. Precipitation of lignin can contribute to lignin recovery from lignocellulosic biomass and, at the same time, allow more efficient ethanol production in the subsequent fermentation process.
authors.affiliation: Organization of Advanced Science and Technology, Kobe University, 1-1 Rokkodaicho, Nada-ku, Kobe, Hyogo 657-8501 Japan.
authors.forename: Kengo
authors.initials: K
authors.lastname: Sasaki
authors.affiliation: RIKEN Biomass Engineering Program, 1-7-22 Suehiro-cho, Tsurumi-ku, Yokohama, Kanagawa 230-0045 Japan.
authors.forename: Mami
authors.initials: M
authors.lastname: Okamoto
authors.affiliation: RIKEN Biomass Engineering Program, 1-7-22 Suehiro-cho, Tsurumi-ku, Yokohama, Kanagawa 230-0045 Japan.
authors.forename: Tomokazu
authors.initials: T
authors.lastname: Shirai
authors.affiliation: Organization of Advanced Science and Technology, Kobe University, 1-1 Rokkodaicho, Nada-ku, Kobe, Hyogo 657-8501 Japan.
authors.forename: Yota
authors.initials: Y
authors.lastname: Tsuge
authors.affiliation: Department of Chemical Science and Engineering, Graduate School of Engineering, Kobe University, 1-1 Rokkodaicho, Nada-ku, Kobe, Hyogo 657-8501 Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Teramura
authors.affiliation: Organization of Advanced Science and Technology, Kobe University, 1-1 Rokkodaicho, Nada-ku, Kobe, Hyogo 657-8501 Japan.
authors.forename: Daisuke
authors.initials: D
authors.lastname: Sasaki
authors.affiliation: Department of Chemical Science and Engineering, Graduate School of Engineering, Kobe University, 1-1 Rokkodaicho, Nada-ku, Kobe, Hyogo 657-8501 Japan.
authors.forename: Hideo
authors.initials: H
authors.lastname: Kawaguchi
authors.affiliation: Organization of Advanced Science and Technology, Kobe University, 1-1 Rokkodaicho, Nada-ku, Kobe, Hyogo 657-8501 Japan.
authors.forename: Tomohisa
authors.initials: T
authors.lastname: Hasunuma
authors.affiliation: Department of Chemical Science and Engineering, Graduate School of Engineering, Kobe University, 1-1 Rokkodaicho, Nada-ku, Kobe, Hyogo 657-8501 Japan.
authors.forename: Chiaki
authors.initials: C
authors.lastname: Ogino
authors.affiliation: RIKEN Biomass Engineering Program, 1-7-22 Suehiro-cho, Tsurumi-ku, Yokohama, Kanagawa 230-0045 Japan ; Department of Bioinformatic Engineering, Graduate School of Information Science and Technology, Osaka University, 1-5 Yamadaoka, Suita, Osaka 565-0871 Japan.
authors.forename: Fumio
authors.initials: F
authors.lastname: Matsuda
authors.affiliation: RIKEN Center for Sustainable Resource Science, 1-7-22 Suehiro-cho, Tsurumi-ku, Yokohama, Kanagawa 230-0045 Japan.
authors.forename: Jun
authors.initials: J
authors.lastname: Kikuchi
authors.affiliation: RIKEN Biomass Engineering Program, 1-7-22 Suehiro-cho, Tsurumi-ku, Yokohama, Kanagawa 230-0045 Japan ; Department of Chemical Science and Engineering, Graduate School of Engineering, Kobe University, 1-1 Rokkodaicho, Nada-ku, Kobe, Hyogo 657-8501 Japan.
authors.forename: Akihiko
authors.initials: A
authors.lastname: Kondo
keywords: Black precipitate; Enzymatic hydrolysis; Hydrothermal pretreatment; Lignin; Nanofiltration; Rice straw