Spruce bark as an industrial source of condensed tannins and non-cellulosic sugars

• Spruce bark is a promising industrial source of condensed tannins.
• Bark was extracted in bench- and pilot scale with unpressurized hot water.
• Extracts with 50% tannin content and 22–35% sugar content were produced.
• Selective extraction of only tannins or sugars was not possible.
• Glycome profiling identified certain non-cellulosic carbohydrates in bark.

Norway spruce (Picea abies) is an important raw material for the forest industry in Nordic countries. The chemical composition and hot water extraction of spruce bark was studied to find out its potential as an industrial source of condensed tannins. Industrial bark was found to contain a high amount of wood (up to 21%), a sufficient amount of tannin for industrial extraction (10.7% of wood-free bark), and a high amount of non-cellulosic glucose, varying according to the felling season (7.7–11.5% of wood-free bark). Temperature had a major effect on the overall extraction yield. Selective extraction of only tannins or water-extractable carbohydrates was not possible. The extraction was scaled up to pilot-scale and an extract was produced having a promising 50% tannin content. Glycome profiling performed on bark and hot water extracts showed the presence of xyloglucan, pectic polysaccharides and arabinogalactan in bark. In addition the extracts were characterized using size exclusion chromatography and 31P nuclear magnetic resonance spectroscopy. Spruce bark appears to be a promising new source of tannins, however the high content of free, glycosidic, and polymeric sugars in the raw extract may need to be tackled prior to use in applications.