Characterization of phenolics in red grape skins recovered from winemaking by-products employable as a food ingredient or as a nutraceutical preparation

The grape transformation by wine industry generates a large volume of by-products, such as grape pomace, which still contains polyphenols and other beneficial compounds for the human health. Thus, the recovery of the active compounds for the production of ingredients for food and nutraceuticals might be of interest. The purpose of our research was to define the qualitative and quantitative composition of the polyphenols in products to be used later for the formulation of foods high in added value. Red grape skins samples of Barbera, Croatina, Freisa, Dolcetto, Grignolino, Neretto, Nebbiolo and Pinot Nero were recovered from winemaking by-products, within the Valorvitis project (AGER 2010-2222). Samples were analyzed for total phenolics (Folin Ciocalteu assay), proanthocyanidins (Porter assay), and individual polyphenols by UPLC coupled to diode array and mass spectrometry detector. A wide variability was observed in the contents of total phenolics (12-54 mg gallic acid Eq/kg), proanthocyanidins (0.7- 5%), total anthocyanins (0.2-1.4%) and flavonols (0.3-2.5%). Glucoside of malvidin, delphinidin, peonidin, petunidin and cyanidin were the main anthocyanins, respectively. Additionally, lower amount of these compounds was found esterified with acetic, p-coumaric and caffeic acid. The main flavonols were the aglycones quercetin and kaempferol, respectively, but also lower amount of their derivatives such as glucuronides and glucosides were found. Regarding proanthocyanidins, the found amount were comparable with those reported in literature and the chromatographic analysis showed that catechin, epicatechin and epigallocatechin-gallate monomers were the main constituents of the polymers. Among the samples considered, the cultivar Barbera, which is largely cultivated, was found to be one of the most promising sources of phenolics. Thus, this well characterized winemaking by-product could be used as food ingredient or in nutraceutical preparations.