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

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.