Fractional and Structural Characterization of Ball Milled and Enzyme Lignins from Oil Palm Empty Fruit Bunch Fiber

The pure milled lignin (PML), hemicellulose rich milled lignin (HRML), pure enzyme lignin (PEL), lignin rich enzyme lignin (LREL), and solubilized lignin during enzyme treatment (SLET) fractions were obtained by 90% and 50% dioxane-water extractions from 6 days' ball milled oil palm empty fruit bunch fiber and the subsequently 3 days' cellulase-treated fiber residues, respectively. The purification was performed with a two-step precipitation method instead of traditional ether precipitation. The five lignin fractions were compared using spectroscopic and degradative techniques. The pure milled lignin and pure enzyme lignin fractions showed a very low content of associated polysaccharides (2.84-2.98%), and contained a large proportion of noncondensed syringyl units with some amounts of noncondensed guaiacyl units and fewer p-hydroxyphenyl units. They are composed mainly of β-O-4 ether bonds together with small amounts of β-5 carbon-carbon linkages. The lignin fractions also appeared to be very closely associated with hydroxycinnamic acid and glucuronic acid or 4-O-methylglucuronic acid. Glucuronic acid or 4-O methylglucuronic acid and over 83% of p-coumaric acid were identified to be esterified to lignin, and over 53% of ferulic acid was found to be linked to lignin by ether bonds