This study developed a method for the determination of extractable and unextractable proanthocyanidins. Extractable proanthocyanidins were separated according to their degree of polymerization using normal phase HPLC. Unextractable proanthocyanidins were measured after acid-catalyzed depolymerization as flavan-3-ols (terminal units) and benzylthioethers (external units). Electrospray ionization mass spectrometry (ESI-MS) was used for the identification of proanthocyanidins in the samples. Hubaux-Vos detection limits were 0.01-0.15 ng/injection for extractable proanthocyanidins, with recovery rates from 69 to 91%. Detection limits for unextractable proanthocyanidin derivatives were 0.002-0.035 ng/injection with 80% recovery. The developed method was applied to the analysis of several fruit and berry samples. Results showed great variation in the proportion of unextractable proanthocyanidins in total proanthocyanidin content between samples, being highest in the green variety of table grape (63%) and lowest in the apple cultivar 'Valkeakuulas' (4.1%). The method reported herein is reliable and gives valuable information on the nature of proanthocyanidins in plant-derived foods.
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