Aromatic aldehydes are important aromatic compounds for the flavour and fragrance industry. In this study, a parallel cascade combining aryl alcohol oxidase from Pleurotus eryngii (PeAAOx) and unspecific peroxygenase from the basidiomycete Agrocybe aegerita (AaeUPO) to convert aromatic primary alcohols into high-value aromatic aldehydes is proposed. Key influencing factors in the process of enzyme cascade catalysis, such as enzyme dosage, pH and temperature, were investigated. The universality of PeAAOx coupled with AaeUPO cascade catalysis for the synthesis of aromatic aldehyde flavour compounds from aromatic primary alcohols was evaluated. In a partially optimised system (comprising 30 μM PeAAOx, 2 μM AaeUPO at pH 7 and 40 °C) up to 84% conversion of 50 mM veratryl alcohol into veratryl aldehyde was achieved in a self-sufficient aerobic reaction. Promising turnover numbers of 2800 and 21,000 for PeAAOx and AaeUPO, respectively, point towards practical applicability.

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.