Bark-based biorefineries: anatomical and chemical characterization of the bark of endemic Quercus vulcanica of Turkey

Abstract

The detailed anatomical and chemical features of the bark from endemic Quercus vulcanica in Turkey are reported here for the first time and discussed in the perspective of integration into a bark-based biorefinery system. The bark of Q. vulcanica trees was collected and studied through observations using light and scanning electron microscopy, wet-chemical analysis, inorganic elemental and FTIR analyses, GC–MS determinations of lipophilic extractives and suberin monomers, as well as TBARS antioxidant activity of hydroethanolic extracts. The bark of Q. vulcanica comprises phloem and a rhytidome with thin periderms and a few cork layers. The ash content is high (16.4%), primarily consisting of calcium oxalate crystals. Extractives were present in a high amount (23.1%) of which 88% corresponded to hydrophilic extractives (10.3% ethanol, and 10.1% water solubles). The suberin content is low (3.7%), which aligns with the small proportion of cork in the bark rhytidome. The composition of suberin is characterized by similar proportions of α, ω-alkanoic diacids and ω-hydroxyalkanoic acids, with 18-hydroxy-9-octadecenoic acid (26% of monomers) and octadec-9-enedioic acid (20.6%) as the main monomers. The lignin content is 21.9%, and the monomeric composition of polysaccharides includes glucose, xylose, arabinose, galactose, rhamnose, and acetyl groups. The lipophilic extractives are mainly composed of terpenoids (72.2% of all compounds), with friedelin and friedelanol as the main compounds. Hydroethanolic extracts, obtained under mild conditions with a yield of 10.2%, exhibited antioxidant activity (TBARS assay, EC₅₀ value of 55 μg/mL). The overall chemical and structural properties of Q. vulcanica bark indicate promising potential for biorefineries.