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The ORAC test allows to determine the antioxidant power of an ingredient, a compound, a formula, based on their capacity to trap free radicals.
The ORAC method aims to evaluate the ability of a sample to limit the oxidation of a fluorescent reagent such as fluorescein which is a probe sensitive to oxidation. The oxidation of fluorescein is accompanied by a decrease in fluorescence measured over time at an excitation wavelength of 485nm and an emission wavelength of 520nm. This change in fluorescence is directly related to the number of free radicals. Indeed, the presence of antioxidants leads to an inhibition of free radicals that damage the fluorescent compound. The standard antioxidant used is TROLOX, a water-soluble vitamin E analog. Fluorescence decay kinetics are assessed every minute for one hour.
The higher the ORAC index, the more antioxidant properties the test sample possesses in a quantitative way. This index or relative value of ORAC is defined as a ratio between the difference between the areas under the curves of the test sample and the blank, and the difference between the areas under the curves of the Trolox and the blank. The results are therefore expressed as molarity of Trolox per molarity or mass of sample tested.