An enzymic treatment of olive oil by-product using culture broths of Aspergillus niger enriched in cinnamoyl esterases was tested with its potential to release free simple phenolic compounds, and especially hydroxytyrosol. After characterizing enzymes able to hydrolyse ester bonds between phenolic compounds and polysaccharides, the culture broth of A. niger BRFM442 was selected in view of the specificity of its enzymes for orthodiphenolic compounds. It was tested on a phenolic extract of olive oil by-product, obtained by ethanolic extraction. Large amounts of free hydroxytyrosol were released and to a less extent p-tyrosol, caffeic acid, p-coumaric acid and ferulic acid. A two-step chromatographic treatment on HP-20 resin and Sephadex LH-20 allowed the recovery of an aqueous fraction of hydroxytyrosol (1.4 g/kg dry olive oil by-product) at 85% purity. The antioxidant activity of hydroxytyrosol was higher than that of antioxidants such as ascorbic acid and BHT. Its good solubility in oily and aqueous media makes natural hydroxytyrosol, recovered with chemical-free methods, useful for application in multi-component foods. (c) 2004 Elsevier Ltd. All rights reserved.