Flavor-active Wine Yeasts: Sulfur Compounds

I recently blogged about Where Does Hydrogen Sulfide Come From?, placing the blame on the production of this rotten egg smell squarely on the yeast. Yeast metabolism during the fermentation of grapes to wine gives rise not only to hydrogen sulfide but other sulfur containing compounds not necessarily desirable in wine, as shown below:

But not all sulfur compounds are repugnant and we have the yeasts to thank when we drink a glass of Sauvignon Blanc and detect passionfruit, grapefruit, gooseberry, guava and box hedge in the wine. The compounds that give rise to these aromas/flavors are called polyfunctional thiols, (also called mercaptans) and they are shown circled in red in the structures below:

These polyfunctional thiols, 3MH, 3MHA and 4MMP are extremely potent having perception thresholds in the parts per trillion range. In order for a wine to have these aromatic attributes, the winemaker must select the right yeast and the proper nutrient addition to the yeast rehydration media, control fermentation temperature, and ensure that the pre-fermentation operations such as skin contact, oxygen, phenol, and sulfur dioxide content are appropriately managed.¹ Not an easy task!
References:
1. Antonio G. Cordente, Christopher D. Curtin, Cristian Varela, Isak S. Pretorius, Flavor-active Wine Yeast, Applied Microbiology and Biotechnology, November 2012, Volume 96, Issue 3, pp 601-618.
All structures were drawn by the freely available drawing program from ACD Labs called ACD/ChemSketch Freeware.