Extraction and chemical characterization of Norway spruce inner and outer bark

Possible chemical utilization of bark requires appropriate knowledge of its composition. Extraction of valuable components before burning is an interesting option for utilization of bark. Here, Norway spruce inner and outer bark were extracted separately with a successive series of solvents of increasing polarity and the extracts, as well as the residues, were analyzed to obtain an overall picture of the bark composition. The lipophilic extractives contained the same major components as found in wood. Inner bark contained over 10% of stilbene glucosides with piceatannol (astringenin) as the main stilbene. Tannins of the proanthocyanidin type were extracted with hot water. Further extraction with pressurized hot water at 140 degrees C or 160 degrees C yielded 11-14% of non-cellulosic polysaccharides, on original bark basis, with pectic polysaccharides built up of arabinose, galacturonic acid and rhamnose dominating. Inner bark contained two times more cellulose than outer bark, but the opposite was true for lignin, determined as Klason "lignin". Among the potentially valuable components, stilbene glucosides could be extracted with water even at low temperatures, while tannins could be extracted with hot water in a second step. The pectic polysaccharides are also of potential interest and should be studied further. The amount and true chemical character of lignin is also not yet fully elucidated.