Differentiation of proanthocyanidin tannins from seeds, skins and stems of grapes (Vitis vinifera) and heartwood of Quebracho (Schinopsis balansae) by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and thioacidolysis/liquid c

To control and identify with confidence the principal enological tannins (ETs), we have devised a specific method based on the characterization of proanthocyanidin composition. We started by controlling the red colouring produced by hydrochloric acid butanolysis (e.g. Bate–Smith reaction), due to the formation of anthyocyanidins, typical of proanthocyanidin tannins. Using thioacidolysis/liquid chromatography/electrospray ionization mass spectrometry we were able to identify: (i) the nature of the flavan-3-ols (catechin, epicatechin for procyanidins tannins; gallocatechin, epigallocatechin for prodelphinidins tannins), (ii) the degree of galloylation, (iii) the average degree of polymerization (mDP). We also performed a complete structural study by matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry (MALDI-TOF-MS). By comparing the chromatographic profiles of standard proanthocyanidins prepared in our laboratory with those of a variety of commercial enological tannins, we were able to identify their origin: seed proanthocyanidins (PA): only procyanidins, high level of galloylation, with a large amount of epicatechin, and a low mDP corresponding to a majority of oligomeric tannins; skins PA: a mixture of procyanidins and prodelphinidins, with a predominance of procyanidins, a low level of galloylation, with a large amount of epicatechin, and a very variable mDP; stems PA, mixture of procyanidins and prodelphinidins, little galloylation, with a very low level of epicatechin in the terminal unit, and a medium value for mDP (>5); Quebracho PA: results show no known flavan-3-ols, and according to MALDI-TOF-MS, the main structure is attributed to a large amount of profisetinidin, corresponding to a resorcinol proanthocyanidin.