Extractability of Oleuropein, Hydroxytyrosol, Tyrosol, Verbascoside and Flavonoid-Derivatives from Olive Leaves Using Ohmic Heating (a Green Process for Value Addition)

The aim of this study was to determine the influence of ohmic heating (OH) as a green processing approach on the extraction of principal polar phenols from olive leaves, with special attention on oleuropein content. The concentrations were quantified using ultra-high-performance liquid chromatography (UHPLC). Other target analytes including total flavonoid content (TFC) and total condensed tannins (TCT) were also assessed as secondary response variables. The key predictor variables were (i) extraction method: ohmic heating (OH) and conventional heating (conven), (ii) solvent ratio (40–80% aqueous ethanol), and (iii) extraction temperature: 45–75 °C. In addition, control samples were prepared through solvent extraction at room temperature using the same solvent ratios applied for OH and conven. The length of processing time (15 min) was the same for all extraction methods. Overall, the data showed that intensified extractability of major polyphenols from olive leaves is readily achievable by using ohmic heating rather than the other aforementioned methods (p < 0.05). Importantly, the recovery of oleuropein through the ohmic system, that reached up to 26.18 mg per g extract at 75 °C with 80% ethanol was significantly higher than other examined approaches (7.98–14.55 mg per g in conven groups, and 8.64–10.81 mg per g in control groups). Other major polyphenols that reached the maximum levels via the ohmic approach, were as follows: (i) luteolin 7-O-glucoside (4.12 mg per g extract), apigenin 7-O-glucoside (3.47 mg per g extract), rutin (3.78 mg per g extract), and tyrosol (0.34 mg per g extract) using 60% ethanol at 55 °C, (ii) verbascoside (1.04 mg per g extract) using 80% ethanol at 75 °C, and (iii) hydroxytyrosol (1.38 mg per g extract) using 80% ethanol at 55 °C. The findings of this study demonstratethat ohmic heating is potentially a method of preference for efficient recovery of representative phenols of olive leaves.