Ultrasound-Assisted Green Extraction of Eggplant Peel (Solanum melongena) Polyphenols Using Aqueous Mixtures of Glycerol and Ethanol: Optimisation and Kinetics

Eggplant peels were used to evaluate glycerol and ethanol for their ability to recover polyphenolic antioxidants. The evaluation was based on optimisation by a Box-Behnken experimental design and kinetics. The results showed that extraction with water/ethanol and water/glycerol mixtures, under optimised conditions, afforded virtually equal yield in total polyphenols, which was 13.40 and 13.51 mg caffeic acid equivalents per g dry weight, respectively. The extraction kinetics revealed that diffusion of phenolics in water/glycerol mixtures was slower (0.85 x 10(-12) m(2) s(-1)) compared with the one attained with water/ethanol (2.23 x 10(-12) m(2) s(-1)), yet the ability of both systems to recover essentially the same levels of total polyphenols was confirmed. The determination of total chlorogenates, total flavonoids and total pigments indicated that water/glycerol might be a more effective solvent system, but controversies were observed with regard to the antiradical activity and reducing power. The analytical polyphenolic profile of both extracts was dominated by chlorogenic acid and no major differences were recorded, a finding indicating that none of the solvent systems displayed selectivity. The results suggested that glycerol may be an ideal candidate for use in eco-friendly extraction processes.