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 chromatography/electrospray ionization mass spectrometry

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.