What are the drawbacks of the original ORAC method?
B-phycoerythrin (B-PE), a protein isolated from porphyridium cruentum, was the chosen fluorescent probe in the original ORAC method.  However, the purity of B-PE varies from lot to lot. In addition, B-PE was found to interact with phenolic compounds that are thought to be main antioxidants derived from plants. The nonspecific protein binding can cause a falsely low ORAC value.  Therefore it is problematic to use B-PE to evaluate antioxidant activity.

Cao, G. H.; Prior, R. L. Measurement of oxygen radical absorbance capacity in biological samples. Oxidants and Antioxidants, Pt A 1999, 299, 50-62.