Determination of Hydrolyzable Tannins (Gallotannins and Ellagitannins) after Reaction with Potassium Iodate

Paul W. Hartzfeld,† Rebecca Forkner,‡ Mark D. Hunter,‡ and Ann E. Hagerman*†
Department of Chemistry and Biochemistry, Miami University, Oxford, Ohio 45056 and Institute of Ecology, University of Georgia, Athens, Georgia 30602-2202
J. Agric. Food Chem., 2002, 50 (7), pp 1785–1790
DOI: 10.1021/jf0111155
Publication Date (Web): February 13, 2002
Copyright © 2002 American Chemical Society
†

 Miami University.

,
‡

 University of Georgia.

,
*

 To whom correspondence should be addressed. Tel.:  513-529-2827. Fax:  513-529-5715. E-mail:  hagermae@muohio.edu.

Abstract

A widely used method for analyzing hydrolyzable tannins afer reaction with KIO3 has been modified to include a methanolysis step followed by oxidation with KIO3. In the new method, hydrolyzable tannins (gallotannins and ellagitannins) are reacted at 85 °C 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 KIO3 at pH 5.5, 30 °C, forms a chromophore with λmax 525 nm, which is determined spectrophotometrically. The detection limit of the method is 1.5 μg of methyl gallate, and with plant samples, relative standard deviations of less than 3% were obtained.

Keywords: Tannin; polyphenolic compound; hydrolyzable tannin; methyl gallate; galloyl glucose; gallotannin; potassium iodate

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History

  • Published In Issue March 27, 2002
  • Received for review August 16, 2001. Revised manuscript received December 19, 2001. Accepted January 14, 2002. This project was funded by a grant from the Animal Production and Health Section of the International Atomic Energy Agency. Safety:  The methanolysis is performed at elevated temperatures with concentrated sulfuric acid in tightly capped tubes. The excess volume of the tubes is sufficiently large that excessive pressure leading to explosion is unlikely. Nonetheless, the procedure must be carried out in a chemical fume hood with adequate warning signs. Care should be taken when removing the caps from the tubes as some release of pressure is normal.

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