Polyphenols from apple skins: A study on microwave-assisted extraction optimization and exhausted solid characterization

Apple processing industry can be improved by the valorization of its residues. Apple skins could be used for the recovery of high added value compounds for the formulation of new antioxidant products (dietary supplements, cosmetics, drugs). In this study, apple skins from Jonagold cultivar were selected as raw material for an optimization study on microwave-assisted process parameters, using green solvents and an inert atmosphere combined to hard operating conditions (long time and high temperature), with the aim to enhance the mass transfer in the extraction process without significant loss in biomolecule activity. Response surface modeling was employed for the experimental planning and multi-variable process optimization. The effect of time, temperature and solvent composition on extraction performances and exhausted solid elemental composition were evaluated. The extract obtained after optimization showed very high extraction yields (50.4 mggallic acid equivalents/gdry biomass; 13.9 mgcatechin equivalents/gdry biomass) and remarkable antiradical properties. Moreover, after the extraction the solid residue exhibited higher carbon content (52.7%) and calorific value (24.6 MJ/kg) than the untreated apple skins.