Characterisation of Mediterranean Grape Pomace Seed and Skin Extracts: Polyphenolic Content and Antioxidant Activity

Grape pomace seeds and skins from different Mediterranean varieties (Grenache [GRE], Syrah [SYR], Carignan [CAR], Mourvedre [MOU] and Alicante [ALI]) were extracted using water and water/ethanol 70% in order to develop edible extracts (an aqueous extract [EAQ] and a 70% hydro-alcoholic extract [EA70]) for potential use in nutraceutical or cosmetic formulations. In this study, global content (total polyphenols, total anthocyanins and total tannins), flavan-3-ols and anthocyanins were assessed using HPLC-UV-Fluo-MSn. In addition, extract potential was evaluated by four different assays: Oxygen Radical Absorbance Capacity (ORAC), Ferric Reducing Antioxidant Potential assay (FRAP), Trolox equivalent antioxidant capacity (TEAC) or ABTS assay and 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging assay. As expected, seed pomace extracts contained higher amounts of polyphenols then skin pomace extracts. Indeed, seeds from Syrah contained a particularly important amount of total polyphenols and tannins in both type of extract (up to 215.84 +/- 1.47 mg of gallic acid equivalent [GAE]/g dry weight (DW) and 455.42 +/- 1.84 mg/g DW, respectively). These extracts also expressed the highest antioxidant potential with every test. For skins, the maximum total phenolic was found in Alicante EAQ (196.71 +/- 0.37 mg GAE/g DW) and in Syrah EA70 (224.92 +/- 0.18 mg GAE/g DW). Results obtained in this article constitute a useful tool for the pre-selection of grape pomace seed and skin extracts for nutraceutical purposes.