Limitation of the butanol–hydrochloric acid–iron assay for bound condensed tannins

The butanol–HCl–iron method is widely used for measurement of extractable condensed 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 bound condensed tannins. The recovery of bound condensed tannins by the butanol–HCl–iron assay was monitored by subjecting the residues left after the assay to solid-state 13C NMR spectroscopy. The signal at δ 155.0, indicative of condensed tannins, remained relatively high in the residues following 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.