Abstract
Polyphenol-containing extracts from olive (Olea europaea) leaves (OLL) were obtained using a glycerol-based deep eutectic solvent (DES) and a combination of DES with methyl β-cyclodextrin (m-β-CD). The extracts were stored at various temperatures for a period of 20 days and the reducing power (PR) was monitored to trace changes in the antioxidant potency of the extracts. Over the examination period and at every temperature tested, PR displayed a constant decline, which followed pseudo zero-order kinetics. The determination of the decay constants indicated that the presence of m-β-CD acted protectively, slowing down the progression of the PR decline. Examination of the polyphenolic profiles using liquid chromatography–diode array–mass spectrometry showed that after storage for 20 days at 50 °C, some major polyphenols occurring in OLL suffered extended degradation. The formation of a yellow pigment in the extracts stored in DES but not in aqueous ethanol suggested that polyphenol oxidation did occur during storage. It was concluded that the oxidation of some OLL components was rather responsible for the PR decline observed.
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