Determination of hydrolyzable tannins (gallotannins and ellagitannins) after reaction with potassium iodate

A widely used method for analyzing hydrolyzable tannins afer reaction with KlO(3) has been modified to include a methanolysis step followed by oxidation with KlO(3). In the new method, hydrolyzable tannins (gallotannins and ellagitannins) are reacted at 85 degreesC for 20 h in methanol/sulfuric acid to quantitatively release methyl gallate. Dried plant samples can be methanolyzed under the same conditions to convert hydrolyzable tannins to methyl gallate. Oxidation of the methyl gallate by KlO(3) at pH 5.5, 30 degreesC, forms a chromophore with lambda(max) 525 nm, which is determined spectrophotometrically. The detection limit of the method is 1.5 mug of methyl gallate, and with plant samples, relative standard deviations of less than 3% were obtained.