An alternative colorimetric method relies on the reaction with vanillin
under acidic conditions. A 2-mL aliquot of a freshly prepared solution of
vanillin (1 g/100 mL) in 70% sulfuric acid is added to 1 mL of aqueous plant
extract. The mixture is incubated in a 20°C-waterbath and after exactly 15
min. the absorbance at 500 nm read. The concentration of proanthocyanidins
is expressed as (+)-catechin equivalents (used for the standard curve). This
assay is specific for flavonols. As a consequence, when using this assay to
determine the concentration of condensed tannins, widely distributed
monomeric flavonols, such as catechin (1.39) and epicatechin (1.90), can
interfere (Hagerman and Butler, 1989).