Extraction and recovery of phenolic compounds from olive leaves

Abstract

There is currently a great interest on the use of the residual biomass originated from the agricultural and food sectors as a bioresource instead as a waste, due to its high potential for the recovery of high added-value compounds. Olive leaves, an olive oil processing residue, contain high amounts of phenolic compounds with health-promoting properties, such as oleuropein, tyrosol and hydroxytyrosol. These compounds can be extracted from olive leaves to be added later in processed foods and thus increase their healthy properties. In this work, the recovery of phenolic compounds present in previously ground olive leaves (size less than 1 mm) was carried out by solid-liquid extraction using several solvents (hexane, ethanol, methanol and ethyl acetate). The olive leaves used were kindly provided by the cooperative "Pagos de Benaval" from olive trees of the variety "Serrana de Espadán", endemic of the Sierra de Espadán natural park (Castellón, Spain). The total phenolic content (TPC) in extracts was determined according to the Folin- Ciocalteau method. Olive leaf extract profile and quantitative determination of oleuropein and hydroxytyrosol were performed in an HPLC-DAD system, and also in a LC-MS system. The antioxidant activity of the extracts was determined by the DPPH and ABTS assays. The best results, with a high antioxidant activity and also a high oleuropein and hydroxytyrosol content in the extracts, were obtained when ethanol and methanol were used as solvents. Finally, a central composite design was performed to determine the effect of three factors (extraction temperature (25-40 ºC), solvent volumeleaf mass ratio (5-15 mL/g) and alcohol-to-water ratio (50-90 vol. %)) on TPC, antioxidant activity, and oleuropein and hydroxytyrosol content in the extracts, in order to optimize the extraction process.