Factors influencing removal of pentosan from birch wood: Evidence on the lignin‐carbohydrate bond

Abstract

AbstractThe effect of particle size, alkali concentration, and temperature on the rate of extraction of pentosans and on the amount of pentosans extractable from birch wood meal has been studied. In general, a very rapid extraction of accessible pentosans is followed by a slow, almost negligible, diffusion of inaccessible pentosans. There is, therefore, almost a limiting amount of pentosans not extractable under each given set of conditions. The effect of decreasing particle size is to increase the amount of pentosan extractable under any given set of conditions. The optimum sodium hydroxide concentrations at 0, 20, 40, 60 and 80 degrees are about 2, 4, 7, 10 and 12%, and under the last of these conditions at least 80% of the pentosans can be extracted rapidly from the wood meal. Pentosan is made more accessible by increasing the alkali concentration up to these levels, but thereafter the amount of accessible pentosan is decreased by an increase in alkali concentration. The decrease in extractability has the same concentration and temperature dependence as the increase in cellulose swelling under the same conditions. Therefore, pentosan removal is inhibited by excessive swelling restricted within the rigid lignin and fiber structure. Similar but less marked inhibition by swelling occurs even after lignin removal.Even under conditions of low alkali concentration, when no inhibition of solution by restricted swelling occurs, neither the temperature nor alkali dependence of rate is typical of a chemical process. The factors which inhibit the removal of pentosan are therefore physical and the inhibition of solution of pentosan from birch wood gives no support to the concept of a lignin‐carbohydrate bond.