Integrated Hot-Compressed Water and Laccase-Mediator Treatments of Eucalyptus grandis Fibers: Structural Changes of Fiber and Lignin

Eucalyptus grandis fibers were treated with hot-compressed water (HCW) and laccase mediator to enhance the fiber characteristics and to produce an active lignin substrate for binderless fiberboard production. The composition, morphology, and crystallinity index (CrI) analysis of fibers showed that the HCW treatment increased the CrI and lignin content of the treated fibers through partial removal of hemicelluloses. Simultaneously, the HCW treatment produced some granules and holes on the surface of the fibers, which possibly facilitated the accessibility of the laccase mediator. Milled wood lignins and enzymatic hydrolysis lignins isolated from the control and treated fibers were comparatively characterized. A reduction of molecular weight was observed, which indicated that a preferential degradation of lignin occurred after exposure to the laccase mediator. Quantitative C-13, 2D-HSQC and P-31 NMR characterization revealed that the integrated treatment resulted in the cleavage of beta-O-4' linkages, removal of G' (oxidized alpha-ketone) substructures, and an increase in the S/G ratio and free phenolic hydroxyls.