Reexamination of the ORAC assay: effect of metal ions

• Published in: Analytical and bioanalytical chemistry Vol. 400; no. 5; pp. 1451 - 1458
• Main Authors: Nkhili, E., Brat, P.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer-Verlag 01.05.2011; Springer
• Subjects: Analytical Chemistry; Antioxidants - metabolism; Ascorbic Acid - metabolism; Beverages - analysis; Biochemistry; Characterization and Evaluation of Materials; Chelating Agents - metabolism; Chemistry; Chemistry and Materials Science; Dehydroascorbic Acid; Edetic Acid - metabolism; Exact sciences and technology; Food Science; Laboratory Medicine; Monitoring/Environmental Analysis; Original Paper; Quercetin - metabolism; Reactive Oxygen Species - metabolism; Spectrometric and optical methods; ORAC; Phenolic compounds; Antioxidant activity; Metal chelator; Fruit juices; Ascorbic acid; Chemical analysis; Metal ion; Metal; Probe; Free radical; Chemical enrichment; Autoxidation; Oxidation; Hydrates; Trace analysis; Complexation; Oxygen; Stability; Sample; Use; Matrix effect; Foodstuff; Method; Antioxidant; EDTA; Fruit juice; Biological compound; Absorption spectrometry; Apple; Fluorescein; Quercetin; Chelating agent; Phenols; Xanthene dye
• ISSN: 1618-2642; 1618-2650
• DOI: 10.1007/s00216-011-4884-8

The oxygen radical absorbance capacity (ORAC) assay method has been employed extensively in the field of antioxidant and oxidative stress. It uses fluorescein as probe for oxidation by peroxyl radical. Hundreds of reports have been published on the use of this method to determine antioxidant capacity in food and biological samples. The question is whether the results of all these reports are influenced by antioxidant autoxidation, which occurs during the ORAC test. Indeed, the presence of metal ions in the studied matrix will influence antioxidant stability, thereby leading to the underestimation of their antioxidant properties. Ethylenediaminetetraacetic acid hydrate (EDTA) can be used as a metal complexation agent. This paper examines the effect of the addition of EDTA on the ORAC values of pure compounds (quercetin, ascorbic, and dehydroascorbic acid) and five food juices (kiwi, orange, tomato, red grape, and apple). Metal complexation by EDTA (80 μM) clearly increased the ORAC values, given that the antioxidant was protected against rapid autoxidation incited by trace metal ions within samples and then by free radicals. Our finding also undoubtedly demonstrated that the number of literature values is potentially underestimated.