40 isoeugenol-tolerant yeasts were isolated from the rhizosphere soil samples which in turn were collected from aromatic plants in different regions of Iran, and further tested for their ability to grow on a minimal medium containing isoeugenol as the sole carbon and energy source. Nine isolates which were able to grow on isoeugenol were examined for their ability to convert isoeugenol into vanillin under growing cell experiments. Of the tested yeasts, the highest conversion efficiency was observed in isolate MP24. The isolate was identified as Trichosporon asahii based on morphological, biochemical and molecular (ITS region) characters and tested to effectively convert isoeugenol into vanillin under resting cell system. A comparative analysis of thin layer chromatography (TLC), UV–Vis spectrometry, and high-performance liquid chromatography (HPLC) verified that vanillin and vanillic acid are accumulated as two major metabolites using T. asahii strain MP24 resting cells. In the presence of 7.5 g/l of wet weight cells of the strain MP24 pre-grown on isoeugenol and harvested at the end of the exponential growth phase, the optimal concentration of vanillin reached 2.4 g/l with a molar conversion of 52.5% in the potassium phosphate buffer (100 mM, pH 5.8) supplemented with 5 g/l of isoeugenol and 2% (v/v) N,N-dimethylformamide (DMF). The total concentration of vanillin and vanillic acid obtained from the bioconversion process was 4.2 g/l (total molar yield of 88.3%). Until now, no data has been published on the conversion of isoeugenol into vanillin by the strains of the genus Trichosporon.
