Extraction of Phenolic-Rich Fractions from Borago officinalis By-Products with Antioxidant and Antimicrobial Activities

Borage (Borago officinalis) is an herbaceous plant recognized for its bioactive properties and, particularly, for its culinary use in Mediterranean countries. In gastronomy, the petioles are generally consumed, while a substantial proportion of leaves and other tissues are discarded. These borage by-products (BBPs) constitute a valuable source of phenolic compounds with potential biological activities, including antioxidant and antimicrobial effects. Accordingly, this study evaluated both conventional solid–liquid extractions and an advanced technology, ultrasound-assisted extraction (UAE), to obtain bioactive BBP fractions. Different hydroethanolic mixtures were initially assessed. Although extractions using 25% ethanol did not yield the highest total phenolic content (TPC), they produced extracts with the strongest antioxidant capacity, as reflected by the highest Trolox equivalent antioxidant capacity (TEAC) values against DPPH● and ABTS●+ (24 and 117 µmol/g). Response surface methodology (RSM) was employed to establish the most suitable extraction time and temperature (78 min, 70 °C) to maximize extraction yield, TPC, and radical-scavenging activity. In contrast, UAE enabled the use of milder conditions (45 min, 25 °C) while still achieving comparable TPC and TEAC values (15%, 29 and 246 µmol/g). Phenolic characterization of selected extracts revealed the presence of nine compounds, with epigallocatechin and rosmarinic acid identified as the major constituents. These extracts exhibited antibacterial activity against Staphylococcus aureus and Escherichia coli, whereas no inhibitory effect was observed against Listeria innocua. Overall, these results highlight the bioactive potential of BBP extracts and encourage further investigations into their functional properties, as well as sensory and consumer acceptance studies.