This investigation was carried out to throw light on the mechanism of difficulty to pulp larch heart wood (I). Ezomatsu (Picea jezoensis Carr.) wood (II), I and larch sap wood were cooked with four different sulphite pulping liquors (Ca-, Mg-, Na- and NH₃-base). The same cooking condition was used for every autoclave cooking : 7% T-S0₂, C-SO₂, 1:6 wood to liquor ratio, 140°C (6+α hrs.). As shown in Tab. 1, larch sap wood was pulped very well as II, but I scarcely pulped by Ca-base while relatively well by NH₃- or Na-base. Mg-base is situated between Ca and monovalent bases for all wood species.
Lignosulphonate (measured by colloid titration with glycol-chitosan (20)), S₂O₃″ and SO₄ ″ in liquor were measured at appropriate intervals during cooking (see Tab. 2 to 4 and Fig. 1). In the case of I, rise of acidity (SO₄ ″ -formation due to decomposition of cooking liquor by taxifolin, ε-galactan and others) comes at relatively late stage of cooking (7.5 hrs.). It is only 1.5 hrs.earlier than that in the case of II. Delignification in cooking of I is, however, very lower from the early stage to the end than that of II. If, at the stage of rising acidity of I (7.5 hrs.), its delignification is comparable to that of II at 7.5 hrs., I might be pulped better, regardless of existence of taxifolin and ε-galactan. It is, therefore, believed that main reason of difficulty to pulp I is due to its slow delignification, which may be caused by hard penetrability of I. Cooking result of I has been improved with soluble bases having good penetrability or by combination of Ca-base and various processes promoting penetration such as Va-purge, vacuum-impregnation and etc. (15). The authors believe that it is unreasonable to explain the difficulty to pulp I only by decomposition of cooking liquor with taxifolin and e-galactan contained in I as believed generally up to date.