Ultrasound-Assisted Extraction of Polyphenolic Antioxidants from Olive (Olea europaea) Leaves Using a Novel Glycerol/Sodium-Potassium Tartrate Low-Transition Temperature Mixture (LTTM)

Olive leaves (OLL) represent a major waste generated during the production of olive oil, but there is a great potential for their valorization, because they provide important content in polyphenolic phytochemicals, which possess several bioactivities. In spite of the high number of studies dealing with polyphenol recovery from olive leaves, green processes involving environmentally benign solvents are scarce. In this study, a novel renewable natural low-transition temperature mixture (LTTM), composed of glycerol and sodium-potassium tartrate, was tested for its efficient ability to extract polyphenolic substances from OLL. The extraction process was optimised by using response surface methodology and the maximum yield in total polyphenols was 26.75 +/- 3.22 mg caffeic acid equivalents per g dry weight, achieved with 50% (v/v) aqueous LTTM, liquid-to-solid ratio of 45 mL g(-1) and at 73 degrees C. The LTTM was proven to be equally effective with 60% aqueous methanol, but it displayed inferior antioxidant properties. Liquid chromatography-diode array-mass spectrometry analyses revealed no significant qualitative differences between the LTTM and the aqueous methanolic extract.