Bark from Pinus Pinaster is an interesting source of polyphenolic natural
compounds, that can be used successful1y as total or partial replacement of
conventional phenolic resins. These compounds, among other applications, are
used as adhesives in the wood agglomerate industry. In this kind of application
some problems remain to be solved in order to obtain a Pine extract of
commercial value. It is necessary to optimise tije extraction procedure and
select a suitable method for the quantification of the tannin content of the bark.
In order to study these problems, the tannin extraction from the Pine bark was
tested with an alkaline solution (NaOH), and with a fractionation procedure
based on a sequence of an organic (ethanol) and aqueous extraction. The
phenolic content of each extract or fraction was evaluated by the FolinCiocalteu
colorimetric assay for total phenols and two procedures using the
Stiasny reaction: the gravimetric Stiasny method and the indirect colorimetric
procedure that uses the Folin-Ciocalteu reagent to evaluate the total phenols
present in the extract solution before and after it condenses with formaldehyde.
The yield value when the alkaline extraction is used is substantially higher
than the values obtained with organic or aqueous solutions. However, the
selectivity of the process is low. In fact, it was found that the alkaline extract
Formaldehyde Condensable Phenolic Material (FCPM) content represents 9596
% of the total phenols content of the extract but this fraction is only ~ 40 %
of the total mass of extract. So, the alkaline extract is relatively poor in
phenolic material, exhibiting a large variety of non-phenolic extractives. On
the other end, ethanol provides a very rich phenolic extract, in which 96 % of
total phenols are condensable with formaldehyde, but exhibits a relatively low
extraction yield. The aqueous extract presents the lowest extraction yield with
low content either in phenolic material as in FCPM, but, as most of the phenolics had already been extracted by the previous organic extraction,
especially the low molecular weight fractions, this result was predictable.
