Comparison of extraction methods for exploitation of grape skin residues from ethanol distillation

Four extraction techniques-namely, conventional maceration, ultrasound-assisted extraction, microwave-assisted extraction, and superheated liquid extraction (SHLE)-have been compared to evaluate their suitability to obtain valuable compounds from a raw material traditionally of scant interest: grape skin residues from ethanol-distillation. With this aim, red- and white-grape skins were separated from the rest of the pomace residue and subjected to extraction with 1:1 ethanol-acidic water as extractant in order to obtain the largest possible number of valuable compounds from this material, which has so far been used only as a heat source. The resulting extracts were characterized by the Folin-Ciocalteu and Ferric Reducing Antioxidant Power tests and by liquid chromatography-time-of-flight/mass spectrometry (LC-TOF/MS). The composition of the extracts under each of the optimal conditions was studied by LC-TOF/MS, and the information thus obtained compared by Venn diagrams. These diagrams, together with the extracted base peak chromatograms, were used to assess the optimal working conditions. Tentative identification of compounds was conducted using open-free databases. Grape skins from distillation industries are a source of compounds of interest for the food and nutraceutical fields.