Bark characterization of a commercial Eucalyptus urophylla hybrid clone in view of its potential use as a biorefinery raw material

The bark of an Eucalyptus urophylla hybrid clone used by the charcoal industry was studied regarding anatomy and chemical composition, including the characterization of its polar extracts, in view of its potential use. The bark is characterized by a dominant homogeneous phloem with little ray expansion and alterations in the non-conducting phloem and by a rhytidome with a large portion of dead phloem tissue between the periderms. The cell proportion was axial parenchyma 37.9%, fibers 26.8%, radial parenchyma 19.4%, expanded cells 12.2%, and sieve tubes elements (3.7%). The chemical composition was ash 2.99%, total extractives 19.49% (92% polar extractives), suberin 0.57%, total lignin 25.09%, and polysaccharides 39.90%. The carbohydrates were composed mainly of glucose and xylose (83.7% and 9.7% of all monosaccharides, respectively). Polar (ethanol-water) extracts had a high phenolic content of 324.4 mg of gallic acid equivalents/g (GAE/g) of extract, flavonoids 215.0 mg of catechin equivalents/g (CE/g) extract, and tannins 325.5 mg of catechin equivalents/g (CE/g) extract. The antioxidant activity was low, corresponding to 310.1 mg Trolox equivalents/g extract and an IC50 of 10.59. In view of an integrated potential use, the bark of this E. urophylla hybrid clone is interesting as a source of tannins and a subsequent use for glucose-based applications, while the high-fiber content also allows considering it as a fiber source.