A Comparative Evaluation of Bio-solvents for the Efficient Extraction of Polyphenolic Phytochemicals: Apple Waste Peels as a Case Study

Apple peels were used as a model solid matrix for the comparative evaluation of aqueous mixtures of three bio-solvents, including ethanol, 2,3-butanediol (butanediol) and glycerol, with regard to polyphenol recovery. The evaluation process was based on a step-by-step optimisation of the extraction, taking into consideration basic factors, including solvent composition, liquid-to-solid ratio and temperature. A further detailed description of the extraction capacity of each solvent was carried out using kinetics. Optimal solvent concentrations were 50 % (v/v), 50 % (v/v) and 70 % (w/v) for ethanol, butanediol and glycerol, respectively, while optimal liquid-to-solid ratio and temperature were 100 mL g(-1) and 80 degrees C for all solvents tested. The kinetic assay showed that extraction yields in total polyphenols obtained with these solvent systems displayed no statistical difference, although the overall extraction rate constant and diffusion coefficient were significantly higher for the extraction performed with 50 % (v/v) butanediol. The liquid chromatography-mass spectrometry examination of the extracts obtained under optimised conditions revealed that their polyphenolic profile was composed, in total, of 12 principal substances, but some qualitative differences among the extracts were also observed. The reducing power of the extract obtained with 50 % (v/v) ethanol was significantly higher, a finding attributed to possible synergistic effects among polyphenols.