Reduced pressure extraction of oleuropein from olive leaves (Olea europaea L.) with ultrasound assistance

In this study, oleuropein was extracted from Frantoio olive leaves with ultrasound-assisted and reduced-pressure extraction (URPE). Then, it was characterized using HPLC-UV-vis detection. The effects of several important factors, i.e., the type and concentration of extraction solvent, extraction time, extraction temperature, ultrasonic power, liquid to solid ratio and the extraction pressure on the extraction yield of oleuropein from olive leaves, were investigated. The optimal conditions were obtained as follows: ethanol concentration 75% (v/v), extraction temperature 50 °C, ultrasonic power 600 W, extraction time 3 min, liquid to solid ratio 30:1 (ml/g), and extraction pressure 25 kPa. Under these optimal conditions, the oleuropein extraction was 7.08 ± 0.05%. A total yield of oleuropein of 7.67 ± 0.02% was obtained after three extractions using URPE. The extraction efficiency reached 92.3% after one extraction. Additionally, compared with the other extraction techniques, i.e., conventional solvent extraction (CSE) under atmospheric pressure, reduced-pressure boiling extraction (RBE), and ultrasound-assisted extraction (UAE), the oleuropein yield and extraction time by URPE was the highest and shortest, respectively. In addition, scanning electron microscope (SEM) imaging of olive leaves after the extraction was performed. URPE could break the olive leaf cell effectively. The results indicated that URPE was an extremely useful and important extraction method for natural products.