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The Folin–Ciocalteu reagent (FCR) or Folin's phenol reagent or Folin–Denis reagent, also called the gallic acid equivalence method (GAE), is a mixture of phosphomolybdate and phosphotungstate used for the colorimetric in vitro assay of phenolic and polyphenolic antioxidants. It is named after Otto Folin, Vintilă Ciocâlteu, and Willey Glover Denis.
The reagent does not only measure phenols, and will react with any reducing substance. It therefore measures the total reducing capacity of a sample, not just phenolic compounds. This reagent is part of the Lowry protein assay, and will also react with some nitrogen-containing compounds such as hydroxylamine and guanidine. The reagent has also been shown to be reactive towards thiols, many vitamins, the nucleotide base guanine, the trioses glyceraldehyde and dihydroxyacetone, and some inorganic ions. Copper complexation increases the reactivity of phenols towards this reagent.
This reagent is distinct from Folin's reagent, which is used to detect amines and sulfur-containing compounds.
A 1951 paper entitled "Protein measurement with the Folin phenol reagent" was the most cited paper in the 1945–1988 Science Citation Index, with 187,652 citations.
Physiologic significance
Because it measures anti-oxidant capacity in vitro, the reagent has been used to assay foods and supplements in food science. The oxygen radical absorbance capacity (ORAC) used to be the industry standard for antioxidant strength of whole foods, juices and food additives. However, the United States Department of Agriculture (USDA) withdrew these ratings in 2012 as biologically invalid, stating that no physiological proof in vivo existed to support the free-radical theory. Consequently, the ORAC method, derived only in in vitro experiments, is no longer considered relevant to human diets or biology.
The Trolox equivalent antioxidant capacity assay is an alternative in vitro measurements of antioxidant capacity.