Changes in grape polyphenols (V. vinifera L.) as a consequence of post-harvest withering by high-resolution mass spectrometry: Raboso Piave versus Corvina.

Grape dehydration is an oenological process used for the production of high-quality reinforced and sweet wines. Corvina and Raboso Piave are two red grape varieties used for production of high-quality Italian wines, such as Recioto, Amarone di Valpolicella and Raboso Passito. Changes of polyphenolic composition of the grapes as a consequence of the withering were studied by ultra-high performance liquid chromatography-quadrupole time of flight mass spectrometry (UHPLC/QTOF); for identification of compounds a homemade HR-MS database of grape and wine metabolites, was used. Concomitant with trans-resveratrol and viniferins, relevant increases of other stilbenes (piceatannol, resveratrol trimers and tetramers) and antioxidant compounds (quercetin, syringetin and tamarixetin) were observed. These compounds are part of the induced metabolism occurring during the withering process and in general improve the nutraceutical properties of grapes and wines. On the other hand, longer processes showed to decrease flavan-3-ols and glycoside flavonols. Constant increase of E/Z ε-viniferin ratio was observed in all samples, and this parameter can be used to monitor the process.