Biotransformation of (R)-(+)- and (S)-(−)-citronellol by Aspergillus sp. and Penicillium sp., and the use of solid-phase microextraction for screening

The biotransformation of (R)-(+)- and (S)-(−)-citronellol by fungi was studied. For screening experiments, solid-phase microextraction (SPME) was used as analytical sampling technique. It was found that sporulated surface cultures of Aspergillus niger were able to convert the substrate into cis- and trans-rose oxides and nerol oxide. The relative contents in the headspace SPME extract of the three bioconversion products cis- and trans-rose oxide and nerol oxide were up to 54, 21 and 12%, respectively. Rose oxide is found in minor amounts in some essential oils, such as Bulgarian rose oil and geranium oil and contributes to its unique odor. It is one of the most important fragrance materials in perfumery in creating rosy notes. Other bioconversion products were 6-methyl-5-hepten-2-one, 6-methyl-5-hepten-2-ol, limonene, terpinolene, linalool and α-terpineol. These bioconversion reactions were confirmed by sporulated surface cultures on larger scale and sampling by dynamic headspace sweep and steam distillation solvent extraction. The same conversions were noticed with A. tubingensis and Penicillium roqueforti. This bioconversion was enantioselective since more of the chiral cis- than trans-rose oxide was obtained (cis/trans ratio up to 95/5). Submerged liquid cultures of P. roqueforti yielded two unidentified metabolites after conversion of citronellol (yield up to 5%). The stability and acid-catalyzed conversion of citronellol was also investigated. No chemical oxidation or auto-oxidation products were detected in acidified liquid control broths up to pH 3.5. However, when control tests were run with solid media, acid-catalyzed conversion of the substrate to small amounts of cis- and trans-rose oxides, nerol oxide, linalool and α-terpineol was observed at pH 3.5 and when heat treatment (steam distillation solvent extraction) was applied.