All the phenolic compounds contained in the wine are oxidised by the Folin-Ciocalteu reagent. This reagent is formed from a mixture of phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40) which, after oxidation of the phenols, is reduced to a mixture of the blue oxides of tungsten (W8O23) and molybdenum (Mo8O23).
