Study on green extraction of limonene from orange peel and cascade catalysis to produce carvol and carvone in deep eutectic solvents

Carvol and carvone are oxidation products from the natural product limonene. They are important raw materials for the flavours and fragrances industry and also act as pharmaceutical active ingredients. Orange waste peels possibly represent an attractive source for limonene, but studies on valorizing orange peel wastes are rare. In this study, we report a new enzymatic cascade system for the in-situ conversion of limonene from orange peel into valued-added carvol and carvone. The use of deep eutectic solvents (DES) allows for efficient in-situ extraction of limonene from waste orange peels. We propose a dual function use of DES as solvent for the extraction and the biocatalytic oxidation of limonene as well as cosubstrate to promote the oxidation reaction. Using ChCl-Pro-H₂O DES for the extraction of limonene from waste orange peels, approximately 17 milligrams of limonene per gram of orange peel was achieved at 40°C for 24 h. Then, with ChCl-Pro-H₂O DES as the extractant and reaction medium, a cascade reaction system of choline oxidase (ChOx) and unspecific peroxygenase (UPO) was established to catalyse the conversion of limonene into carvol and carvone. The concentration of the final products was up to about 1.6 mmol L⁻¹. This study showed a biocatalytic transformation pathway and provides technical support for the high-value utilization of waste in orange peel.