Determination of an Optimum Extraction Region for the Recovery of Antioxidants From Olive Leaves (Olea Europaea L.) Using Dynamic Ple With Water/ethanol Mixtures

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Pressurized liquid extraction with water and ethanol was employed to obtain high antioxidant extracts from olive leaves (Olea europaea L.). The influence of solvent ratio (100–50% v/v water/ethanol), temperature (100–200°C), and solvent flow rate (1–5 mL min− 1) on the extraction process was investigated using a Box-Behnken experimental design. ANOVA was applied to determine the overall goodness of fit of the dependent variables (extraction yield, antioxidant capacity by DPPH, total flavonoids, total iridoid-glycoside, and total phenolic content) in second-order models. For the optimization, these models were used to find the optimal conditions using the Sequential Simplex optimization procedure combined with Derringer and Suich’s desirability function to maximize antioxidant capacity, total iridoid-glycoside recovery, and total phenolic content. The optimal conditions for extraction were determined at a solvent ratio of 53–65% v/v water/ethanol, the extraction temperature of 100–120°C, and a 3–4 mL.min− 1 solvent flow rate. Fifteen phenolic compounds were identified in the samples using HPLC-DAD-ESI/MSn, and also two phenolic acids (hydroxytyrosol and hydroxytyrosol glucoside), four flavonols (apigenin-6,8-C-dihexoside, quercetin-3-O-rutinoside, luteolin-O-hexoside, and derivatives), and nine iridoid-glycosides (verbascoside, oleuropein, and derivatives). Oleuropein derivates were identified as the major phenolic compounds in the extract obtained at optimal conditions.