Kraft pulping remains the main chemical pulping method in the world, the goal of which is to liberate wood fibres by degrading and solubilizing lignin. The kraft process, however, is not fully selective;  part of the carbohydrates – particularly the hemicelluloses – are affected by alkaline peeling, alkaline hydrolysis and beta-elimination, thereby reducing them to monosaccharides. These monosaccharides, in turn, are degraded by means of beta-elimination, benzillic acid rearrangement and alpha-dicarbonyl cleavage to form a mixture of low molecular-weight sugar acids such as isosaccharinic acid. A fraction of the sugar degradation products, however, is known to repolymerise, although the structure of the resulting polymer has never been fully elucidated. 
In this work, this high molecular-weight sugar degradation product was produced and isolated from the reaction of glucose in alkali. It was then analysed using nuclear magnetic resonance spectroscopy and found to be a conjugated aromatic structure rich in methyl, methylidine, alcohol and carboxylic acid groups with a molecular weight (Mn) of 31 kDa. Based on this, we propose a hypothetical structure with hydroxymethylfurfural as the repeating unit, with additional functionality provided by sugar acid substituents. 
A closer look at the NMR data of the polymer reveals striking similarities to NMR data of regular kraft lignin. Interestingly, the polymer was also found to react in the kappa number analysis, exhibiting 40% of the permanganate consumption predicted for pure lignin. These facts – along with the observation that UV-VIS data of the polymer matches the colour profile of black liquor produced from the soda cooking of cotton linters and other model systems – suggest that the structure could in fact constitute a form of pseudo-lignin. If so, the sugar polymer might form under typical kraft cooking conditions and could account for some of the signal that has traditionally been attributed to lignin.