Effect of antioxidant oxidation potential in the oxygen radical absorption capacity (ORAC) assay

Roger H Bisby; Rachel Brooke; Suppiah Navaratnam

doi:10.1016/j.foodchem.2007.12.012

Effect of antioxidant oxidation potential in the oxygen radical absorption capacity (ORAC) assay

Food Chem. 2008 Jun 1;108(3):1002-7. doi: 10.1016/j.foodchem.2007.12.012. Epub 2007 Dec 14.

Authors

Roger H Bisby¹, Rachel Brooke², Suppiah Navaratnam³

Affiliations

¹ Biomedical Sciences Research Institute, University of Salford, Salford M5 4WT, UK. Electronic address: r.h.bisby@salford.ac.uk.
² Biomedical Sciences Research Institute, University of Salford, Salford M5 4WT, UK.
³ Biomedical Sciences Research Institute, University of Salford, Salford M5 4WT, UK; FRRF, STFC Daresbury Laboratory, Warrington WA4 4AD, UK.

PMID: 26065764
DOI: 10.1016/j.foodchem.2007.12.012

Abstract

The "oxygen radical absorption capacity" (ORAC) assay (Ou, B., Hampsch-Woodill, M., Prior, R.L. (2001). Development and validation of an improved oxygen radical absorbance capacity assay using fluorescein as the fluorescent probe. Journal of Agricultural and Food Chemistry 49, 4619-4626) is widely employed to determine antioxidant content of foods and uses fluorescein as a probe for oxidation by peroxyl radicals. Kinetic modeling of the ORAC assay suggests that the lag phase for loss of fluorescence results from equilibrium between antioxidant and fluorescein radicals and the value of the equilibrium constant determines the shape of the lag phase. For an efficient antioxidant this constitutes a "repair" reaction for fluoresceinyl radicals and produces a well defined lag phase. The lag phase becomes less marked with increasing oxidation potential of the antioxidant. Pulse radiolysis confirms that fluoresceinyl radicals are rapidly (k∼10(9)dm(3)mol(-1)s(-1)) reduced by Trolox C, a water soluble vitamin E analogue. ORAC assays of phenols with varying oxidation potentials suggest that it might be employed to obtain an estimate of the redox potential of antioxidants within food materials.

Keywords: Antioxidant; Fluorescein; ORAC; Oxygen; Pulse radiolysis; Radical; Reduction potential.