The state and spatial distribution of extractives during birch kraft pulping, as evaluated by staining techniques

A set of staining techniques selective for extractives such as sterols and fats were applied to native birch wood and mill sampled unbleached and hydrogen peroxide bleached birch kraft pulps. Observations showed that extractives remaining in parenchyma cells of unbleached pulps were present as spheres and/or smaller agglomerates. These structures were larger than the simple pit openings of the parenchyma cells indicating a physical hindrance to the deresination of the pulp. When comparing unbleached pulps washed at low and high dilution factor respectively, it was observed that the spheres were much more effectively removed in the latter case. This was interpreted as an effect of increased soap solubility in the digester washing, zone, allowing micelles to diffuse through the parenchyma cell walls and pits and solubilizing the spheres/aggloinerates. In bleached pulps most of the sterols had been removed from the parenchyma cells. The material remaining inside the cells was mainly saturated fatty acids. Numerous particles of unsaturated as well as saturated fatty acids, most likely present as metal soaps, were observed outside the parenchyma cells in both the bleached pulp and hand sheets of mill refined paper. The relative abundance of sterols and unsaturated as well as saturated fatty acids correlated with the amounts detected by gas chromatography analysis. We conclude that the combined information from gas chromatography analysis and the specific staining methods made it possible to understand not only how much of the extractives are removed during the kraft process but also details about the mechanisms of removal