Application of deep eutectic solvents in the extraction of polyphenolic antioxidants from New Zealand Manuka leaves (Leptospermum Scoparium): Optimization and antioxidant activity

Abstract

Manuka tree (Leptospermum Scoparium) is the most widespread native shrub in New Zealand and has an economic impact as a source of honey and essential oils. In the present study, eight types of DESs based on choline chloride (ChCl) and tetrabutylammonium chloride (TBAC) as hydrogen bond acceptors (HBAs) in combination with different polyalcohols and carboxylic acids as hydrogen bond donors (HBDs) were used to assess the extraction of polyphenolic compounds in terms of total phenolic content (TPC) from dried Manuka leaves. ChCl: ethylene glycol DES showed the highest efficiency in the extraction of the polyphenolic compounds (56.87 mg GAE/g DW) followed by ChCl: lactic acid DES (52.51 mg GAE/g DW) and ChCl: 1, 3-propanediol DES (50.67 mg GAE/g DW). Furthermore, the mentioned DESs revealed a better extraction efficiency for polyphenolic compounds than ethanol as a benchmark solvent. Among the successful DESs, ChCl: lactic acid DES was preferred and studied further for optimization using response surface methodology. The highest TPC (59.82 mg GAE/g DW) was extracted at optimum conditions (extraction temperature 50.00 ℃, biomass % 5.07 and extraction time 1.07 h). Besides, the extracted polyphenolic compounds showed good stability in ChCl: lactic acid DES over eight days of storage. Moreover, after four extraction cycles using the DES, 154.65 mg GAE/g DW of TPC advises a possible scaling up of the potential DES. The antioxidant activity of extracts was evaluated using DPPH and FRAP assay. ChCl: ethylene glycol DES extract showed better free radical scavenging activity (12.78 µg TE/ml) while ChCl: lactic acid DES extracts gave higher ferric reducing power (199.78 µg TE/ml) at the same extract concentration with both being higher than the extract obtained by ethanol.