Antioxidant Activity and Chemical Characteristics of Supercritical CO₂ and Water Extracts from Willow and Poplar

Renewable feedstock from perennial industrial crops, including those cultivated on marginal land in a short-rotation coppice system, could be an important contribution to the bioeconomy. The majority of data available on the topic are limited to the production of bioenergy from this type of biomass. According to the concept of bioeconomy, biomass-based bioproducts have priority over energy production. This paper characterizes the chemical composition and antioxidant activity of extracts from bark (b), wood (w) or a mixture of bark and wood (b + w) from Salix purpurea, Salix viminalis and Populus nigra obtained using supercritical carbon dioxide (scCO₂), scCO₂ and water (1%, w/w) or only water. Generally, a high concentration of polyphenols was obtained after extraction with scCO₂ and water, while the lowest concentration was found in extracts obtained with scCO₂. The highest concentration of polyphenols (p < 0.05) was obtained in an extract from P. nigra (b) (502.62 ± 9.86 mg GAE/g dry matter (d.m.)) after extraction with scCO₂ and water, whereas the lowest polyphenol concentration was observed in an scCO₂ extract from S. purpurea (b) (6.02 ± 0.13 mg GAE/g d.m.). The flavonoids were effectively separated by extraction with scCO₂ (0.88-18.37 mg QE/g d.m.). A positive linear relationship between the antioxidant activity determined by DPPH and ABTS assays and the concentration of polyphenols was demonstrated, R² = 0.8377 and R² = 0.9568, respectively. It is most probable that the concentration of flavonoids, rather than the concentration of polyphenols, determines the chelating activity of Fe²⁺. The Fe²⁺-chelating activity of scCO₂ extracts ranged from 75.11% (EC₅₀ = 5.41 mg/cm³, S. purpurea, b + w) to 99.43% (EC₅₀ = 0.85 mg/cm³, P. nigra, b + w). The lowest chelating activity was demonstrated by the extracts obtained with scCO₂ and water (maximum 26.36%, S. purpurea, b + w). In extracts obtained with scCO₂ and water, p-hydroxybenzoic acid (210-428 µg/g), p-coumaric acid (56-281 µg/g), saligenin (142-300 µg/g) and salicortin (16-164 µg/g) were the dominant polyphenols. All of these chemical compounds occurred mainly in the free form. The S. purpurea, S. viminalis and P. nigra biomass proved to be an attractive source of biologically active compounds for various possible applications in food, drugs or cosmetics. These compounds could be extracted using an environmentally friendly method with scCO₂ and water as a co-solvent.