Characterization of Brauns’ lignin from fresh and vacuum-dried birch (Betula pendula) wood

Brauns’ lignins present in the methanol extracts of fresh birch (Betula pendula) xylem and of sawn birch board subjected to vacuum drying were characterized by ¹H and ¹³C NMR spectroscopy (1D and 2D), IR spectroscopy, gel permeation chromatography (GPC) and colour measurements (CIELab) in order to find out whether Brauns’ lignin could contribute to the colour change of sawn timber that occurred during vacuum drying. The two Brauns’ lignin samples contained about equal amounts of syringylpropane and guaiacylpropane units linked with β-O-4 and β–β side-chain structures. Molecular weight of the Brauns’ lignin of vacuum-dried birch board (acetylated: 5,200 g mol⁻¹) was higher than that of the Brauns’ lignin of fresh birch wood (acetylated: 4,400 g mol⁻¹). The Brauns’ lignin of vacuum-dried wood was also clearly darker and more prominently yellow and red; \( \Delta E^{*}_{{ab}} \) between the Brauns’ lignin samples was 23.59. The differences in the molecular weights and colours suggest that the Brauns’ lignin underwent a chemical change during vacuum drying of the wood and that this change may have affected the colour of the wood.