Dynamic superheated liquid extraction of anthocyanins and other phenolics from red grape skins of winemaking residues

Grape skins from a grape pomace were subject to extraction with superheated ethanol–water mixtures for quantitative extraction of anthocyans and other phenolic compounds. The variables affecting dynamic extraction of these compounds were studied and identification and quantification of the extracted compounds were performed by both direct spectrophotometry or after HPLC separation using UV or MS detectors. The optimal working conditions for total extraction of anthocyans were: 1:1 (v/v) ethanol–water acidified with 0.8% (v/v) HCl, 120 °C, 30 min, 1.2 ml/min and 80 bar. The yields of anthocyanins, total phenolics and flavanols thus obtained were much higher (3 times for anthocyanins, 7 times for total phenolics and 11 times for flavanols) than those provided by dynamic conventional solid–liquid extraction. Several sample preparation procedures for skins as alternatives to free-drying were also investigated and drying at 40 °C for 24 h provided the best results. Extraction with acidified water provides similar composition and poorer efficiency than 1:1 ethanol–water; also similar to two commercial grape skin extracts used as natural colorants.