Accessibility of Cell Wall Lignin in Solvent Extraction of Ultrathin Spruce Wood Sections

Abstract

Wood is a naturally occurring composite, comprising cellulose, hemicellulose, and lignin. The tightly arranged cell wall components make the fibers resistant against chemical and microbial degradation. This natural resisting power of fibers is a technical obstacle during the degradation of cellulose into sugars. Therefore, removal of cell wall lignin is necessary in order to make the cellulose accessible. In this study, ultrathin sections of Norway spruce (Picea abies) branch wood were examined using Raman and transmission electron microscopy (TEM) before and after extracting the sections with 1,4-dioxane without resin embedding in order to study the accessibility of native lignin. The progress of extraction of lignin was followed by measuring its Raman scattering intensity at the ∼1600 cm–1 band. It was found that lignin was extracted not only from the compound middle lamellae but also from other layers of the cell wall. Changes in the contrast of TEM images confirmed a decrease in lignin concentration after solvent extraction. Observed ruptures in the S1 layer indicated that extraction weakened this layer in particular.