Design optimization study of the extraction of olive leaves performed with pressurized liquid extraction using response surface methodology

Pressurized liquid extraction (PLE) with environmentally friendly solvents namely water and ethanol was applied to olive leaves in order to optimize extraction yield, oleuropein content and antioxidant activity. The effect of various parameters was studied through a carefully designed optimization process. The study comprised of two distinct stages, a screening procedure using Plackett-Burman design followed by an optimization step using a central composite design (CCD). This led to the construction of a response surface indicating the optimal values for each parameter and response studied. Determination of the optimum extraction procedure was performed by comparison of the extracts' yield, oleuropein content determined by HPLC and radical scavenging activity evaluated in a DPPH assay. Extraction parameters with statistical significance were shown to be the ethanol content of the solvent mixture, the temperature of the extraction and the consecutive cycles of extractions. Concerning the extraction yield, an ethanolic extraction at 190 °C for 3 consecutive cycles was found to be optimal. Regarding the oleuropein content of the extract, a mixture of H2O/EtOH 43:57 in 190 °C for 1 extraction cycle provided the optimal results. Finally, regarding the DPPH assay, all the samples exhibited good antioxidant activity. The experimental values obtained from two different extraction procedures could be predicted within a 95% confidence interval, thus indicating the suitability of RSM in optimizing the extraction of olive leaves.