Green Extraction of Olive Leaves from Two Different Greek Cultivars: Response Surface Methodology Optimization and Valorization of Their Phenolic and Antioxidant Content

Olive oil production generates large amounts of agricultural and agro-industrial residues, with olive leaves representing one of the most abundant by-products. This study investigated the valorization of olive leaves from two Greek cultivars, Koroneiki and Megaritiki, collected from the Aeghion, Heraklion, and Megara regions, by means of an optimized green extraction process for the effective recovery of phenolic compounds. Response surface methodology (RSM) was applied to optimize solvent composition and extraction time using ethanol/water mixtures (0/100, 30/70, 50/50, and 70/30, v/v) and extraction times ranging from 15 to 90 min. Total phenolic content (TPC), antioxidant activity (DPPH and FRAP), and color parameters were determined spectrophotometrically, while individual phenolic compounds were analyzed by HPLC-DAD. Both solvent composition and extraction time significantly affected phenolic recovery and antioxidant activity. The optimal extraction conditions were 37.5/62.5 (v/v) ethanol/water and 52.5 min. Extracts from Megara showed the highest TPC (32 ± 1 g GAE kg−1 LL), antioxidant capacity (2556 ± 162 mmol Trolox kg−1 LL), and phenolic compound levels. Significant differences were observed between the two cultivars, whereas Koroneiki samples from Heraklion and Aeghion exhibited similar phenolic profiles, indicating that cultivars had a greater influence than geographical origin.