Sustainable extraction of antioxidants from out-of-caliber kiwifruits

Valorisation of discarded kiwifruits is proposed by extracting bioactive compounds using sustainable solvents namely deep eutectic solvents (DES). A screening of fifteen DES and several hydrogen bonding donor solvents was carried out. Extraction efficiency was measured in terms of antioxidant activity using DPPH and FRAP tests. The influence of solvents characteristics in particular DES structure, presence of ethanol or water, and pH of DES/water mixture on the antioxidant properties of the extracts was studied. Results show that kiwi peels extracts obtained with DES based on carboxylic acids exhibit enhanced antioxidant activity compared to conventional solvents and alcohol-based DES with a maximum DPPH scavenging activity of 42.0 mg TE/g DW. Glycerol or ethylene glycol are also efficient at extracting antioxidant compounds with DPPH scavenging activity of 33.1 and 36.7 mg TE/g DW. Finally, a chemical analysis of extracts using HPTLC revealed that most active compounds extracted are polyphenolic compounds, presumably tannins.