The ORAC assay depends on the free radical damage to a fluorescent probe, such as fluorescein. As a result, there is a change in fluorescent intensity. Hence, the degree of change is indicative of the amount of radical damage. The presence of antioxidants results in an inhibition in the free radical damage to the fluorescent compound. This inhibition is observed as a preservation of the fluorescent signal.
It is possible to quantify the protection by calculating the area under the curve (AUC) from the experimental sample. After subtracting the AUC for the blank, the resultant difference would be the protection that the antioxidant compound confers. Trolox®, a water-soluble vitamin E analog, is used as the calibration standard and ORAC results are expressed as Trolox® equivalents.
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