Fungal Biotransformation of Insoluble Kraft Lignin\ninto a Water Soluble Polymer

Abstract

Low\nsubstrate solubility and slow decomposition/biotransformation\nrate are among the main impediments for industrial scale lignin biotreatment.\nThe outcome and dynamics of kraft lignin biomodification by basidiomycetous\nfungi, <i>Coriolus versicolor</i>, were investigated in\nthe presence of dimethyl sulfoxide (DMSO). The addition of 2 vol %\nDMSO to aqueous media increased the lignin solubility up to 70%, while\nthe quasi-immobilized fungi (pregrown on agar containing kenaf biomass)\nmaintained their ability to produce lignolytic enzymes. Basidiomycetous\nfungi were able to grow on solid media containing both 5–25\ng/L lignin and up to 5 vol % DMSO, in contrast to no growth in liquid\nmedia as a free suspended culture. When a fungal culture pregrown\non agar was used for lignin treatment in an aqueous medium containing\n2–5% DMSO with up to 25 g/L lignin, significant lignin modification\nwas observed in 1–6 days. The product analysis suggests that\nlignin was biotransformed, rather than biodegraded, into an oxygenated\nand cross-linked phenolic polymer. The resulting product showed the\nremoval of phenolic monomers and/or their immediate precursors based\non gas chromatography and thermal desorption–pyrolysis–gas\nchromatography–mass spectrometry analyses. Significant intramolecular\ncross-linking among the reaction products was shown by thermal carbon\nanalysis and ¹H NMR spectroscopy. An increase in polarity,\npresumably due to oxygenation, and a decrease in polydispersity of\nthe lignin treatment product compared to untreated lignin were observed\nwhile using liquid chromatography.