Isolation and evaluation of antioxidants from leaves of a Tunisian cultivar olive tree

Quantitative and qualitative determinations of phenolic compounds were carried out on Chemlali olive leaves, a by-product of olive tree pruning. The evolution of the amount of the major phenolic compound was monitored from July 2003 to March 2004. Quantitative analysis was performed by high-performance liquid chromatography (HPLC), which revealed that oleuropein was always the major compound. Its concentration reached 6.8 g per 100 g of fresh olive leaves, while monomeric phenols were the minor compounds. Beside oleuropein, six flavonoids (luteolin 7-O-glucoside, luteolin 7-O-rutinoside, apigenin 7-O-glucoside, rutin, luteolin and apigenin) were identified. The identification was based on separation by HPLC equipped with a diode array detector, followed by liquid chromatography-mass spectrometry analysis. A relative high amount of purified hydroxytyrosol (2.3g per 100g of fresh olive leaves) was obtained in short time by a simple hydrolysis reaction of Olea europaea leaf extract and by purification using a C-18 silica gel column. The antioxidant activities of the extracts and the purified compounds were evaluated by measuring the radical-scavenging effect on 1,1-diphenyl-2-picrylhydrazyl and by using the P-carotene linoleate model assay. Hydroxytyrosol and hydrolysate products have the highest antioxiclant activities.