Vol. 67, Nr. 3, 2002—JOURNAL OF FOOD SCIENCE 943 © 2002 Institute of Food Technologists Food Chemistry and Toxicology JFS: Food Chemistry and Toxicology Ferric Ions Reduce the Antioxidant Activity of the Phenolic Fraction of Virgin Olive Oil T. KECELI AND M.H. GORDON ABSTRACT: The antioxidant activity of relatively polar extracts from virgin olive oil was investigated in sunflower oil stripped of tocopherols and in tocopherol-stripped sunflower oil-in-water emulsions. The extracts were found to be effective as antioxidants in both media in the absence of added metal ions. However, the antioxidant activity was markedly reduced by the presence of added ferric chloride. In sunflower oil-in-water emulsions (pH 5.4) containing ferric chloride, all concentrations of olive oil polyphenols exhibited pro-oxidant effects. It appears that the reducing action of olive oil polyphenols accelerates oxidation of oil and especially of emulsions containing Fe (III) by reducing ferric ions to ferrous ions, which are effective pro-oxidants during storage. Keywords: ferric ions, olive oil, phenolic, iron, antioxidant Introduction P HENOLIC COMPOUNDS IN VIRGIN OLIVE OIL ARE IMPORTANT components because they contribute significantly to flavor (taste and aroma), oxidation stability, as well as the nutritional and biological value of the oil (Montedoro and others 1993; Vi- sioli and Galli 1998a, b). The phenolic compounds found in virgin olive oils are simple phenols (hydroxytyrosol, tyrosol), phenolic acids (hydroxycinnamic and hydroxybenzoic acids), and oleuropein, tyrosol, and hydroxytyrosol derivatives, and the latter are thought to be responsible for the antioxidant stability of virgin olive oil (Montedoro and others 1993; Tsimi- dou and others 1996). Polyphenols from olive oil are believed to be effective through their free radical scavenging and metal chelating properties (Chimi and others 1991). Metal ions can react with hydroperoxides, promoting their decomposition due to their oxidation-reduction poten- tial, so they serve as excellent catalysts for lipid oxidation re- actions (Benjelloun and others 1991; Mei and others 1998). Transition metals are effective in catalyzing free radical reac- tions in the presence of hydroperoxides (Yamamoto and Niki 1988; Yamauchi and others 1988). Since transition metals, es- pecially iron, are common contaminants of water, it would be expected that water-soluble metal ions could catalyse the oxidation of dispersed lipids at the oil-water interface. The autoxidation of lipids could be attributed to the decomposi- tion of lipid hydroperoxides by ferric and ferrous ions to give lipid peroxyl and alkoxyl radicals which initiate the free radi- cal chain oxidation (Yamamoto and Niki 1988). LOOH + Fe 3+ ® LOO • + H + + Fe 2+ LOOH + Fe 2+ ® LO • + HO – + Fe 3+ It is mainly the ferrous ion that induces lipid peroxidation, since the reaction of ferric ion with lipid hydroperoxides is much slower. The reaction, however, may proceed faster with ferric ions in the presence of reducing antioxidants such as -tocopherol and ascorbate which reduce Fe 3+ to the Fe 2+ state to catalyze hydroperoxide decomposition (Yamamoto and Niki 1988). Phenolic compounds in some circumstances act as antioxidants by chelating metal ions, but this effect may be less significant if they reduce metal ions back to their most active pro-oxidant state (Brune and others 1991; Diea- na and others 1995; Solinas and others 1996; Moran and oth- ers 1997). Most studies of lipid oxidation in foods have been con- cerned with oxidation of bulk oils but many foods are com- plex heterogeneous systems such as emulsions. Lipid oxida- tion and the activity of antioxidants are more complex when studied in emulsions since factors including emulsifier, pH, droplet size and concentration, temperature, total oxygen pressure, metal ion concentrations, and the effects of com- plex interfaces may have an effect (Coupland and Mc- Clements 1996). Therefore, the aim of this study was to investigate the ef- fect of iron on the antioxidant activity of olive oil extracts in oil and in oil-in-water emulsions and to determine the inter- actions between iron and olive oil extracts. Materials and Methods Materials Folin-Ciocalteu reagent, caffeic acid, -tocopherol, ferric chloride, ferrous gluconate, tyrosol, p-coumaric, homovanil- lic, vanillic acid, p-hydroxyphenylacetic, ferulic, caffeic, gal- lic, trans-cinnamic, o-coumaric and syringic acids were pur- chased from Sigma Chemical Co. (Poole, Dorset, United Kingdom) and oleuropein was supplied by Extrasynthese (Lyon-Nord, France). Hydroxytyrosol was donated by Mrs Paiva-Martins, Univ. of Porto, Portugal. All chemicals and solvents used throughout the study were HPLC or analar grade obtained from Sigma, Merck, or Rathburn, United Kingdom. Extra-virgin olive oil and sunflower oil of a brand known to lack added antioxidants were purchased from a lo- cal retail outlet in the United Kingdom. Stripping of tocopherols from sunflower oil Tocopherols were removed from the sunflower oil by the procedure of (Yoshida 1993). Complete removal of toco- pherols was confirmed by HPLC. Extraction and analysis of phenolics from extra-virgin olive oil Extraction with aqueous methanol and purification of the polar extract from extra-virgin olive oil was carried out ac-