On chromophores and leucochromophores formed during the refining of wood

The discoloration reactions taking place when wood is converted to thermomechanical pulp (TMP) were studied using milled wood lignin (MWL), from spruce wood chips and from first-stage refined pulp respectively. As analytical tools, advanced NMR techniques and UV-VIS spectroscopy were used, to detect differences in the chemical composition. It was found that the most probable reason for the discoloration reactions is an oxidation of lignin, giving rise to small amounts of aryl-alpha-carbonyl structures, together with the formation of coloured metal ion complexes. Despite the results from earlier works, on beta-1 and beta-5 lignin model compounds, no stilbenes seemed to be formed during refining.