What is the principle of the ORAC assay?
The ORAC assay depends on the free radical damage to a fluorescent probe through the change in its fluorescence intensity.  The change of fluorescence intensity is an index of the degree of free radical damage. In the presence of antioxidant, the inhibition of free radical damage by an antioxidant, which is reflected in the protection against the change of probe fluorescence in the ORAC assay, is a measure of its antioxidant capacity against the free radical (Figure 1).

Figure 1.

The uniqueness of the ORAC assay is that the reaction is driven to completion and the quantitation is achieved using "area under the curve" (AUC) (Figures 2-3).

Figure 2. Antioxidant activity of tested sample expressed as the net area under the curve (AUC)

In particular, the AUC technique allows ORAC to combine both inhibition time and inhibition percentage of the free radical damage by the antioxidant into a single quantity (Figure 3).

Figure 3. Calculation of ORAC values. AUC=0.5 + F1/F0 +F2/F0+F3/F0+.......+Fn/F0; Relative ORAC value = [(AUCSample- AUCBlank)/(AUCstandard- AUCBlank) x (molarity of standard/molarity of sample)