Antioxidant activity of the phenolic compounds released by hydrothermal treatments of olive tree pruning

The liquors from liquid hot water treatment (LHW) and from steam explosion (SE) of Olea europea pruning were characterised for phenolic content and in vitro antioxidant activity. Rising the temperature treatment enhanced the release of phenolic compounds to the aqueous phase and the fraction of them which are soluble in ethyl acetate was increased. Up to 2.29 and 1.88 g gallic acid equivalents (GAE)/100 g pruning were solubilised in LHW and SE, respectively, 80% of these compounds were recovered in the ethyl acetate extract. Syringol and syringaldehyde were the most abundant lignin derived volatile products identified. Hydroxytyrosol, oleuropein, syringaldehyde and tyrosol were the major phenolic compounds identified by HPLC. The antioxidant activity of the extracts, comparable to that of synthetic antioxidants, depended on the extract concentration but was scarcely affected by treatment temperature.