Distillery Residues from Cistus ladanifer (Rockrose) as Feedstock for the Production of Added-Value Phenolic Compounds and Hemicellulosic Oligosaccharides

Cistus ladanifer residues obtained after essential oil distillation were extracted with ethanol and water (CLRext) and subsequently hydrothermally treated (autohydrolysis) in order to selectively hydrolyze hemicelluloses. The extraction removed a significant amount of potentially valuable compounds (40% w/w, dry basis), foremost, phenolic compounds (0.363 and 0.250 g gallic acid equivalent/g extract, respectively, for water and ethanol). Autohydrolysis was studied under diverse severity factors (log Ro), in the temperature range of 150 to 230 °C. The hydrolyzates mainly contain oligosaccharides, reaching the highest concentration (23.5 g/L) for log Ro of 3.07 (190 °C), corresponding to a yield of 15 g oligosaccharides/100 g dry feedstock. The processed solids are enriched in glucan and lignin. The maximum glucan content (35%) was attained at log Ro of 3.51 (205 °C). Py-GC/MS confirmed the reduction of pentose-derived carbohydrates in the solid after hydrothermal treatment and an increase of syringil units in the lignin compared to the untreated biomass. These results show the potential use of this C. ladanifer residue for the production of phenolic extracts, and hemicellulosic oligosaccharides, together with the production of a cellulose- and lignin-rich solid stream.