Deep Eutectic Solvents as a Potential Alternative Extraction Technique for the Isolation of Phenolic Compounds from Economically Important European Tree Species

The by-products of the wood-processing industry are still predominantly used for energy generation, despite being a rich source of high-value phenolic compounds. This review focuses on the valorization of bark from economically crucial European tree species. Based on an extensive literature survey, three deciduous species (Fagus sylvatica, Quercus robur/petraea, Carpinus betulus) and three coniferous species (Pinus sylvestris, Picea abies, Abies alba) were selected on the basis of their distribution in the European Union, their industrial relevance, and the composition and bioactivity of their extractive phenolic fractions. Conventional and nonconventional extraction techniques are briefly compared, with particular emphasis on deep eutectic solvents (DESs) as emerging green media for the selective isolation of phenolics from bark and other lignocellulosic residues. DESs are typically renewable, nontoxic, biodegradable, and nonflammable, and their tunable composition allows them to be tailored to specific target compounds. The literature data demonstrate that DES-based extractions can provide phenolic-rich extracts with high antioxidant and antimicrobial activities and, in some cases, can outperform conventional solvents. Finally, the potential applications of bark-derived phenolic extracts in the pharmaceutical, agricultural, food, polymer processing, and cultural heritage sectors are outlined. The review also identifies knowledge gaps in DES selection, extract purification, and solvent recovery, highlighting future research needs for integrating DESs into sustainable wood-biomass biorefineries.