Valorization of Raspberry ( Rubus Idaeus L.) Leaves via Ultrasound‐Assisted Extraction and Preparative HPLC Fractionation: Antioxidant Kinetics, Antimicrobial Activity, and Performance in Bulk Oils and Pickering Emulsions

ABSTRACT Raspberry ( Rubus idaeus L.) leaves are an abundant agri‐food by‐product with high potential as a source of natural bioactive compounds. In this study, ultrasound‐assisted extraction (UAE) was applied as a green approach to recover phenolic‐rich extracts, which were subsequently characterized by HPLC–DAD and high‐resolution mass spectrometry (HPLC–HRMS). To elucidate structure–activity relationships, the extract was fractionated by preparative HPLC into phenolic acid/catechin‐, ellagitannin‐, and flavonoid‐rich fractions. Antioxidant activity was evaluated using conventional spectrophotometric assays, kinetic stopped‐flow DPPH measurements, and isothermal calorimetry in bulk sunflower oil and Pickering emulsions stabilized by zein nanoparticles. The ellagitannin‐rich fraction, dominated by sanguiin H‐6, exhibited the highest radical scavenging activity and largely accounted for the antioxidant behavior of the whole extract. While moderate inhibition of lipid oxidation was observed in bulk oil, a marked enhancement of antioxidant efficiency and induction time was achieved in Pickering emulsions, highlighting the importance of interfacial localization effects. Antimicrobial activity tests showed significant inhibition of Gram‐positive foodborne pathogens, particularly Bacillus cereus and Staphylococcus aureus . Overall, this work demonstrates the potential of raspberry leaves as a sustainable source of multifunctional bioactives and underscores the role of green extraction, fractionation, and appropriate delivery systems in maximizing their effectiveness for food applications.