Unlocking the potential of acorn oak cupules as a bio-based feedstock

Abstract Acorn cupules are the structures that originally protect and connect the acorn to the tree. Currently, they are considered a residue of the acorn sector. In the present work, cupules from six Portuguese autochthonous species of oak ( Quercus suber , Q. rotundifolia , Q. pyrenaica , Q. faginea , Q. coccifera , Q. robur ) were chemically (summative composition, lignin composition, apolar extracts composition, phenolic contents, and antioxidant activity of polar extracts) and thermally (proximate analysis and higher heating value) analysed to enable the evaluation for best deconstruction pathways and end purposes toward a full use of these available resources. The results revealed variations among the six species for every parameter examined. Q. robur stood out for the amount of total extractives (37.2%) and content in condensed tannins (235 mgCE/g Ext ). Q. coccifera demonstrated a higher percentage of lignin (34.7%), and Q. rotundifolia was the species with a higher percentage of polymeric sugars (50.2%) and flavonoids (450mgCE/g Ext ). Q. suber showed the highest antioxidant capacity (AAI = 7.7; 1.9gTE/g Ext ) and the highest content in total phenols (622mgGAE/g Ext ). Cupule’s lignin polymer proved to have a low S/G ratio (from 0.4 to 0.7). Higher heating value was similar for all six species (19.4 to 20.6 MJ/Kg), but Q. suber had a higher fixed carbon content, 28.2%, compared to 21.5% ( Q. coccifera and Q. pyrenaica) to 25.4% ( Q. rotundifolia ). These findings highlight the potential of acorn cupules as a source of lignin or hemicellulose oligosaccharides. Q. suber demonstrated a very high antioxidant capacity with potential to be utilized in the cosmetic or pharmaceutical industry.