Valorisation of Quercus ilex acorn shells through green extraction: impact on phenolic composition, antioxidant and antimicrobial activities

Acorn shells from Quercus ilex represent an underexploited by-product with potential as a source of bioactive phenolic compounds. This study evaluated ultrasound-assisted extraction (UAE) and microwave-assisted extraction (MAE) using different ethanol-water mixtures to optimize phenolic recovery, antioxidant capacity, and antimicrobial activity. Hydroalcoholic solvents outperformed water. Hydroalcoholic solvents improved phenolic recovery compared with water. MAE with 25% ethanol provided the highest total phenolic content and antioxidant capacity, whereas UAE with 80% ethanol yielded a phenolic profile enriched in bioactive fractions. Advanced analysis (HPLC-ESI-QTOF-MS) identified a diverse phenolic composition, including phenolic acids, hydrolysable tannins, proanthocyanidins, flavonoids, and stilbene-related compounds. Furthermore, C18 solid-phase extraction (SPE) successfully purified and concentrated these extracts. Purified extracts showed concentration-dependent antimicrobial effects against food-borne pathogenic bacteria and postharvest filamentous fungi pathogens, mainly expressed as delayed growth onset and reduced overall growth. Antimicrobial performance was strongly microorganism-dependent, with UAE-SPE generally showing greater activity than MAE-SPE, particularly against Escherichia coli, Listeria spp., Monilinia fructicola , and Penicillium expansum . Overall, these findings support the valorisation of Q. ilex acorn shells as a sustainable source of phenolic-rich extracts with antioxidant and antimicrobial potential, although further studies in real food systems are required to confirm their technological applicability.