The selectivity of dissolution of carbohydrates and lignin during a kraft cook proceeds in three
distinct phases with the first one merely being an extraction of both types of components (initial
phase). When around 20 % of both carbohydrates and lignin have gone into solution, the kinetics
changes dramatically and a rather selective lignin dissolution takes place until approximately
90 % of all lignin has been dissolved (bulk phase). The final portion of the lignin can,
however, only be removed with great difficulty and at the expense of a large carbohydrate loss
(final phase). In practice, the cook is interrupted at the transition point to the final phase in order
not to loose in pulp quality or yield (Figure 5.3).