Chemical and anatomical characterization, and antioxidant properties of barks from 11 Eucalyptus species

Abstract

Bark is an interesting and available byproduct from the forest industry that can be used as biomass for biorefineries. The barks from 11 Eucalyptus species (E. botryoides, E. camaldulensis, E. globulus, E. grandis, E. maculata, E. ovata, E. propinqua, E. resinifera, E. rudis, E. saligna and E. viminalis) were studied for 4-year-old trees regarding their structural and chemical features, including the composition of ethanol–water extracts and their antioxidant properties. The bark of these young trees had a thin or inexistent rhytidome, with a reduced periderm with low suberized and lignified cells. The barks were structurally similar and characterized by a homogeneous phloem with little ray expansion and alterations in the collapsed phloem. Photomicrographs documented the main structural differences. There was a between-species variation in chemical composition: extractives ranged from 5.5 to 18.6% (the hydrophilic extractives were dominant) and lignin from 14.4 to 23.5%. The carbohydrates composition also showed between species variation, for example the glucose-to-xylose ratio ranged from 3.8 to 12.1, allowing to consider carbohydrate targeted utilizations. Suberin content was in all cases very low (< 1%). The between-species variation of the ethanol–water bark extracts was also high: total phenolics ranged from 283 to 917 mg GAE/g of extract, flavonoids from 121 to 387 mg CE/g of extract and tannins from 94 to 545 mg CE/g extract. The antioxidant activity ranged from 368 to 1042 mg Trolox/g of extract and IC50 values from 3.4 to 8.6 µg extract/ml (the value for trolox was 2.6). The bark extract of E. rudis was particularly interesting as regards its antioxidant capacity, as well as that of E. maculata and E. propinqua.