Fractionation and characterization of ball-milled and enzyme lignins from abaca fibre

Ball-milled and enzyme lignins were produced from abaca fibre via ball mill ing for 6 days followed by cellulase treatment for 3 days. The crude lignin preparations were fractionated into milled lignin (ML), enzyme lignin (EL) , hemicellulose-rich milled lignin (HRML), and Lignin-rich enzyme lignin (L REL) fractions using a two-step precipitation method instead of a tradition al ether precipitation procedure. The yield and chemical composition of the resulting lignin samples are reported. The ME and EL fractions contained l ow amounts of associated neutral sugars (2.0-3.3%) and uronic acids (1.4-1. 5%), and showed relatively low average molecular weights (2500-2660), while the LREL and HRML fractions contained large amounts of bound polysaccharid es (35.6-38.3%), and showed high molecular weights (8800-25000). The four l ignin fractions are composed of a large proportion of syringyl units with f ewer guaiacyl and p-hydroxyphenyl units. The ML is mainly composed of beta- O-4 ether bonds between the lignin structural units. The less common beta-b eta, beta-5 and 5-5' carbon-carbon linkages are also present in the lignin molecules. It was found that uronic acids and 41-63% of p-coumaric acids ar e esterified to lignin in the three lignin-rich fractions of ML, EL and LRE L. This level increased to over 90% in the hemicellulose-rich fraction of H RML. For ferulic acids, 92-97% were found to be etherified to Lignin in the three lignin-rich fractions of ML, EL and LREL, while in the hemicellulose -rich fraction of HRML this reduced to only 13%, suggesting that a majority of the ferulic acids are esterified to hemicelluloses or lignin in this fr action. (C) 1999 Society of Chemical Industry.