Ultrasound-assisted solid liquid extraction (USLE) of olive fruit (Olea europaea) phenolic compounds

A new method of ultrasound-assisted solid liquid extraction (USLE) of olive fruit phenols is described. Phenolics were extracted using high intensity probe ultrasonication and analysed by HPLC-DAD-FLD-MS/MS. Four USLE parameters – sonication time (4, 15, 20, 30 min), temperature (25, 45 °C), solvent composition (80%, 100% methanol) and extraction steps (1–5) were studied and optimised on the basis of nine major olive fruit phenols. A three-step extraction of 20 min with pure methanol (25 mL) at 45 °C was needed for sufficient phenol recoveries (94.1–98.7%) from 1.5 g of freeze-dried olive fruits. The proposed USLE method was more efficient in comparison to US bath and agitation, with up to 33% and 80% enhancement in the case of oleuropein, respectively. In addition, the overall method provided high selectivity, precision and sensitivity with LODs/LOQs ranging from 0.66–4.92 μg g⁻¹ and 2.00–14.77 μg g⁻¹ of olives DW, respectively.