From Extraction to Valorization: Unlocking the Potential of Bark-Derived Extraction Residues for Sustainable Material...

Tree bark, a renewable byproduct of the forest industry, has long been recognized as a rich source of bioactive and structural compounds, including polyphenols, tannins, triterpenes, and suberinic acids. Over recent decades, numerous studies have explored bark extraction processes aimed at recovering these valuable substances. However, the substantial quantities of solid and liquid residues remaining after extraction are still largely overlooked despite their significant potential for further valorization. This review summarizes the current state of research on bark extraction, highlighting the diversity of applied techniques from conventional solvent extraction to advanced green methods such as organosolv, subcritical water, and supercritical CO₂ extraction. Particular emphasis is placed on post-extraction residues, which remain rich in lignocellulosic, suberinic and phenolic compounds suitable for the development of bio-based materials, composites and functional chemicals. Importantly, this review introduces a novel perspective by evaluating post-extraction residues with the same significance as primary bark extracts, emphasizing their un-tapped potential within emerging bark biorefinery concepts. The review identifies existing knowledge gaps related to the chemical characterization, recovery strategies and industrial integration of these byproducts. Finally, it outlines future research directions focused on transforming bark extraction residues into high value sustainable materials fully aligned with the principles of the circular bioeconomy and zero waste processing.