Tree bark characterization envisioning an integrated use in a biorefinery

Tree barks are considerable waste streams in forest industries, and a potentially interesting resource for biorefineries. Barks of six relevant tree species, namely poplar ( Populus × canadensis ), black locust ( Robinia pseudoacacia ), red oak ( Quercus rubra ), willow ( Salix sp.), Corsican pine ( Pinus nigra subsp. laricio ), and larch ( Larix decidua ), were anatomically and chemically characterized. Anatomical analysis illustrated the structural heterogeneity of the different barks with occurrence of various cell types and their arrangement in phloem, periderm, and rhytidome. Summative chemical analysis showed a high extractive content (14–30 wt%) for all barks. Black locust bark presented a substantial suberin content (10 wt%). The lignin content was similar for most barks (22–29 wt%), except for Corsican pine bark (45 wt%). Analytical pyrolysis demonstrated that softwood bark lignin was mostly G type with a small amount of H units, whereas for hardwood species S, G, and H units were present, with low S/G ratios (0.3–0.7). The cell wall structural polysaccharides were rather low. Total monosaccharides ranged from 35 to 44 wt%, with glucose being predominant for almost all barks, followed by xylose in hardwood and mannose in softwood barks. Identification of the lipophilic extractives highlighted the predominance of resin acids for softwood barks, and fatty acids and triterpenoids for all barks. Analysis of the polar bark extracts revealed large variations in the content of phenolics (265–579 mg gallic acid equivalent/g extract), flavonoids (177–391 mg catechin equivalent/g extract), and condensed tannins (88–670 mg catechin equivalent/g extract). Furthermore, the polar extracts presented a high antioxidant potential (500–1209 mg trolox equivalent/g extract), as determined by the FRAP assay. Additionally, a very strong antioxidant activity (AAI > 2), as evaluated by the DPPH assay, was observed for all barks. In summary, the results highlight the marked anatomical and chemical interspecies variability in barks, thus suggesting the need for tailored biorefining approaches. Graphical abstract