Polysaccharide Degradation and Lignin Modification during Brown Rot of Spruce Wood: A Polarised Fourier Transform near Infrared Study

Abstract

Radial thin sections of spruce wood that had been degraded by brown-rot fungi (Gloeophyllum trabeum or Poria placenta) exhibiting mass losses up to 16% were investigated by polarised Fourier transform near infrared (FT-NIR) transmission spectroscopy. This method allowed for the analysis of the changes within wood caused by the decay fungi. The main features of brown rot visible in the FT-NIR spectra were the reduction of amorphous wood polysaccharides in preference to crystalline, an overall loss of molecular orientation of the structural polymers, and the formation of phenolic groups in lignin through cleavage of aryl–ether bonds or demethoxylation. Furthermore, polarised FT-NIR spectroscopy allowed us to examine the assignments of the first overtones of O–H stretching vibrations in more detail and to suggest a number of new band assignments in this spectral region. Among them were the bands of the 0(3)–H(3)···O(5) intramolecular hydrogen bond of glucomannan and the strong parallel intramolecular hydrogen bond 0(2)–H(2)···O(6), which is only accessible with polarised NIR spectroscopy because it overlaps with a number of perpendicular bands, among them the perpendicular O(6)–H(6)···O(3)′ intermolecular hydrogen bond of crystalline cellulose. First overtones of strongly H-bonded O–H groups of cellulose I_β (6340 cm⁻¹) and cellulose I_α (6270 cm⁻¹) were tentatively assigned.