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

Abstract

Abstract The mechanisms and antioxidant activities of six natural phenolics against lipid oxidation induced by ultraviolet (UV) radiation or Fe 2+ 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 (IC 50 ) showed that the effectiveness of these antioxidants against Fe 2+ ‐induced lipid oxidation was in the order quercetin (1.7 µM) > rutin (10.3 µM) > catechin (14.9 µM) > sesamol (18.5 µM) > caffeic acid (19 µM) > ferulic acid (>250 µM), and quercetin was more efficient than butylated hydroxytoluene (BHT) (2.9 µM). 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 Fe 2+ 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 Fe 2+ ‐chelation properties of natural phenolics also contributed significantly to the control of lipid oxidation induced by UV or Fe 2+ in oil systems.