Galactoglucomannan stabilization during the initial kraft cooking of Scots pine

Abstract

The behaviour of wood components was studied during kraft cooking of Wiley milled Scots pine (Pinus sylvestris L.) at low temperatures characteristic for the initial phase, but comprising alkali concentrations ranging from 0.31 mol l^-1 to 1.55 mol l^-1. The results confirmed the extensive dissolution/degradation rate of galactoglucomannan (GGM) already in the beginning of a kraft cook applying standard conditions. More than 70% of GGM was eliminated along with lignin removal of approximately 30%. A relatively high GGM yield increase of almost 2.5% on o.d. (oven dry) wood has been observed when both the temperature and the alkali concentration was raised to 130°C and to approximately 1.5 mol l^-1, respectively, suggesting that stopping reactions were favoured over peeling reactions. Parallel to the pronounced stabilization of GGM, arabinoxylan experienced an additional loss of approximately 0.5–0.7% on o.d. wood, presumably owing to dissolution rather than alkaline degradation. A comprehensive kinetic model describing the alkaline degradation of GGM along with the lignin removal has been developed based on Wiley milled Scots pine raw material. The proposed kinetic model combines the alkali-dependent equilibria between ionized species of carbohydrate components with the time-dependent rate expressions of peeling, stopping, and alkaline hydrolysis. The model successfully describes the removal of GGM and lignin during initial kraft cooking. The kinetic expressions for the alkaline hydrolysis of carbohydrates were, however, less reliable owing to the limitation of the experiments to low temperatures only.