Limitation of the butanol-hydrochloric acid-iron assay for bound condensedtannins

The butanol-HCl-iron method is widely used for measurement of extractable c ondensed tannins (syn. proanthocyanidins) in foods and feeds. As the method is based on acid catalysed oxidative depolymerization of condensed tannins into anthocyanidins, this method has also been used for determination of b ound condensed tannins. The recovery of bound condensed tannins by the buta nol-HCl-iron assay was monitored by subjecting the residues left after the assay to solid-state C-13 NMR spectroscopy. The signal at delta 155.0, indi cative of condensed tannins, remained relatively high in the residues follo wing the butanol-HCl-iron assay, suggesting an incomplete recovery of bound condensed tannins by the assay. The results obtained using the butanol-HCl -iron assay should therefore be interpreted with caution. (C) 1999 Elsevier Science Ltd. All rights reserved.