Engineering volatile thiol formation in yeast

Aims: Volatile thiols are very potent aroma molecules which contribute to the aroma of many beverages. The characteristic thiols of certain wine varieties like Sauvignon Blanc are partly released during the yeast-based fermentation from plant-synthesized glutathione- or cysteine-conjugated and dipeptic precursors present in the must. In this work, we aimed at the construction and characterization of yeast strains with the ability to synthesize volatile thiols from respective precursors.

Methods and results: Besides genome integration of the Escherichia coli gene tnaA, which encodes an enzyme with high β-lyase activity, a glutathione synthetase and glutathione-S-transferases were overexpressed. Up to 8.9 µg L-1 3-mercaptohexan-1-ol could be formed with the strain from externally added trans-2-hexen-1-ol. Well characterized thiols such as 2-methyl-2-butanethiol, 3-mercapto-3-methylbutan-1-ol and 8-mercapto-p-menthan-3-one, as well as several so far undescribed thiol compounds could be synthesized.

Conclusion: Volatile thiols could be produced by feedingalcohol, alkenol, aldehyde or ketone precursors like trans-2-hexenal, trans-2-hexen-1-ol, cis-2-hexen-1-ol, 3-methyl-2-buten-1-ol, 3-buten-2-one and pulegone to the optimized yeast cells.