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

Ball-milled and enzyme lignins were produced from abaca fibre via ball milling 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 (LREL) fractions using a two-step precipitation method instead of a traditional ether precipitation procedure. The yield and chemical composition of the resulting lignin samples are reported. The ML and EL fractions contained low 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 polysaccharides (35.6-38.3%), and showed high molecular weights (8800-25000). The four lignin fractions are composed of a large proportion of syringyl units with fewer guaiacyl and p-hydroxyphenyl units. The ML is mainly composed of β-O-4 ether bonds between the lignin structural units. The less common β-β, β-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 are esterified to lignin in the three lignin-rich fractions of ML, EL and LREL. This level increased to over 90% in the hemicellulose-rich fraction of HRML. 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 fraction.