Antioxidant activities of six natural phenolics against lipid oxidation induced by Fe2+ or ultraviolet light

The mechanisms and antioxidant activities of six ics against lipid oxidation induced by ultraviolet (UV) radiation or Fe2+ were studied. An oil emulsion was prepared with flax oil and the thiobarbituric acid-reactive substances (TBARS) method was used to determine lipid oxidation. The antioxidant activities of the six phenolics against UV-induced lipid oxidation were as follows: quercetin > rutin = caffeic acid = ferulic acid = sesamol > catechin. The inhibitory concentrations (IC50) showed that the effectiveness of these antioxidants against Fe2+-induced lipid oxidation was in the order quercetin (1.7 mu M)> rutin (10.3 mu M)> catechin (14.9 mu M)> sesamol (18.5 mu M) > caffeic acid (19 mu M)> ferulic acid (>250 mu M), and quercetin was more efficient than butylated hydroxy toluene (BHT) (2.9 mu M). Quercetin and rutin had absorption maxima at the UV-A (320-380 nm) region, while the other phenolics tested had absorption maxima near (catechin, 278 nm) or at the UV-B (280-320 nm) region. The stoichiometric ratios of quercetin, rutin, catechin, and caffeic acid to Fe2+ were 3:1, 2.1, 1:1, and 1:1, respectively. Although free-radical scavenging capability of antioxidants was the most critical, UV absorption and/or Fe2+-chelation properties of natural phenolics also contributed significantly to the control of lipid oxidation induced by UV or Fe2+ in oil systems.