Highly Efficient Extraction of Antioxidant Polyphenols from Olea europaea Leaves Using an Eco-friendly Glycerol/Glycine Deep Eutectic Solvent

A number of biological activities have been attributed to polyphenolic substances occurring in the leaves of the olive tree (Olea europaea), an olive oil industry waste material, and for this reason several extraction processes have been developed for the efficient recovery of these phytochemicals. The study presented herein describes for the first time the use of an eco-friendly deep eutectic solvent (DES) as a highly effective solvent for the extraction of polyphenols from olive tree leaves. Initially, a central composite design was implemented to optimise solvent composition and liquid-to-solid ratio and then a kinetic assay was performed to investigate diffusivity (De) and temperature effects. The maximum total polyphenol yield was 106.25 mg gallic acid equivalents per g dry weight, achieved at 70 °C. This value was by 18% higher than that achieved with 60% aqueous methanol. Furthermore, the DES extract exhibited significantly higher antiradical activity and reducing power. Temperature increase up to 70 °C was shown to boost De and the activation energy determined for the process was 29.55 kJ mol−1. Characterisation of the extract by means of liquid chromatography–diode array–mass spectrometry showed oleuropein and certain flavone glycosides to be the major polyphenolic constituents.