The hypothesis of higher lignin reactivity (higher S/G
ratio) would cause a higher alkali concentration at the end
of the cooking (lower alkali consumption) and, consequently,
a slightly higher polysaccharides degradation and
lower pulp yield. As this was not the case, the hypothesis of
a more favourable S/G ratio alone cannot justify the
observed behaviour. Extractives can play a significant role
and a low extractives content will be related to higher pulp
yields, as the work of Lourenc¸o et al. [18] clearly demonstrated.
However, the representation (not shown) of pulp
yield as a function of ethanol/toluene extractives content
for the seven wood samples (taken from different sites and
height levels in the tree) characterized in Table 4 did not
support a significant negative correlation (r2 = 0.097). The
reason for this apparent contradiction may be due to the
fact that heartwood and sapwood proportions in each
sample were not considered. In fact, heartwood varies
within the tree and samples taken at different height levels
may have different heartwood proportion [19]. Lourenc¸o
et al. [18] worked with wood samples only from the 5 %
height level and separating sapwood and heartwood in each
wood disc. In our case, if the sample with 538 kg m-3
wood density is excluded, the negative correlation was
revealed (r2 = 0.65).