Improved procedure to determine non-extractable polymeric proanthocyanidins in plant foods

Proanthocyanidins (PA) or condensed tannins, a major group of oligomeric and polymeric dietary polyphenols, have an essential role on the organoleptic and health-related properties of plant foods. Their content is usually determined by HPLC analysis of aqueous-organic extracts. However, appreciable amounts of polymeric PA that remain in the residues of extraction usually are not considered for the analysis. A complete quantification of PA requires an additional determination of these non-extractable PA (NEPA). The objective of this work was to develop a new procedure to determine the content of NEPA, based in depolymerization by butanolysis, which yields anthocyanidin monomers and xanthylium compounds. Samples and standard are treated with butanol/HCl with FeCl₃ (100 °C, 60 min), and absorbances at 555 nm (anthocyanidins) and 450 nm (xanthylium compounds) are measured in the hydrolysates. NEPA content determined in some plant foods suggests that procedures based just on anthocyanidin concentration overestimate the actual content.