Chemical characterisation of bark and of alkaline bark extracts from maritime pine grown in Portugal

The chemical composition of maritime pine (Pinus pinaster) bark and of alkaline bark extracts has been investigated with bark from trees grown in Portugal. Bark is composed of lignin and polyphenolics (ca. 44%), polysaccharides (ca. 39%), dichoromethane, ethanol and water extractives (ca. 17%) and ashes (ca. 1%). The lignin content determined by the Klason method, after the extraction of polyphenolics (tannin-rich fraction) from the bark by alkali solution, is 33.2%. Cellulose content is about 24%. Hemicelluloses (about 15% of bark dry weight) are suggested to be predominantly composed of arabinoglucuronoxylans and minor amounts of galactoglucomanans. The bark lignin is composed by p-hydrophenylpropane and guaiacylpropane units in proportions of 20:80. The condensed tannins are constituted mainly by catechin-type structural units. The direct extraction of bark with aqueous alkaline solutions yields extracts which contain sugars, lignin and condensed tannins. The number average molecular weights (Mn) of alkaline extracts are in the range 1000–1500. The composition and Mn of extracted substances depends on the alkaline extraction conditions.