COMPARISON OF EXTRACTION METHODS AND ASSAY PROCEDURES FOR THE DETERMINATION OF THE APPARENT TANNIN CONTENT OF COMMON BEANS

The biological utilisation of legume seeds is limited by the presence of polyphenols, among other antinutritional factors. Tannins, compounds of the complex polyphenol family, interact with dietary proteins and digestive enzymes, decreasing the nutritional value of ingested foods. The quantification of polyphenols in grains and seeds is problematic due in part to the diversity of methods used and the presence of interfering factors. In this study the most commonly used assay procedures were compared and validated. Black bean polyphenols, fractionated by adsorption chromatography into tannins and non-tannins, were assayed with Prussian blue, Folin-Denis, and corrected and uncorrected vanillin tests. The results obtained suggest that the Prussian blue assay is the most convenient for tannin determination. In spite of its low specificity it reacted preferentially with this polyphenol group and correlated well with the other procedures. Neither the corrected nor the uncorrected vanillin assay is recommended for use in beans due to interference by non-tannin compounds present in raw extracts.