Antioxidant and antibacterial evaluation of Manuka leaves (Leptospermum scoparium) extracted by hydrophobic deep eutectic solvent

Manuka (Leptospermum scoparium) has been studied as a valuable shrub as its leaves are rich in bioactive components such as p-caryophyllene. This study investigated the extractability of hydrophobic deep eutectic solvent (HDES) namely menthol:lactic acid HDES for the total phenolic content (TPC) and antioxidant activity from p-caryophyllene-rich Manuka leaves. Results were also compared with ethanol and n-hexane extracts. The ethanolic extract possessed the highest amount of TPC (86.08 +/- 6.22 mg GAE/g DW) among tested extracts. The extraction of TPC using HDES was at a sufficient level (79.82 +/- 0.75 mg GAE/g DW), while n-hexane presented poor yield (2.74 +/- 0.70 mg GAE/g DW). Higher 2,2-diphenyl-1picrylhydrazyl radical scavenging activity and ferric reducing antioxidant power were found to be in ethanolic extract. The antibacterial effects of Manuka extract obtained by HDES was also assessed and the results were compared with the antibacterial effect of HDES, lactic acid, commercial Manuka essential oil, steam distilled Manuka oil, and p-caryophyllene pure oil. E. coli, Pseudomonas aeruginosa and Staphylococcus epidermidis were tested using agar disc diffusion method. Results revealed that lactic acid, HDES, and Manuka-HDES extract showed toxicity towards the tested bacteria indicating that the bactericidal effect of the Manuka extract was due to the lactic acid as a forming compound. This study verified the possibility of extracting and characterising different bioactive compounds from Manuka leaves using menthol:lactic acid HDES in a single step and thus it could be explored for broader industrial applications. (c) 2021 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Automated summary

Manuka leaves rich in p-caryophyllene were extracted using menthol:lactic acid HDES, ethanol, and n-hexane to compare total phenolic content and antioxidant activity. Ethanol extract had the highest TPC, while HDES showed promising results and n-hexane had poor yield. The antibacterial effects of Manuka extract by HDES were also evaluated, demonstrating potential for broader industrial applications.