Development of volatile sulfur compounds (VSCs) during winemaking is an ongoing challenge faced by winemakers. Many factors leading to the formation of these compounds are unknown or poorly understood. To address this issue, Dr. James Osborne and Dr. Michael Qian are studying how VSCs develop in the early stages of barrel aging.

Their initial results indicated that the amount of lees present after fermentation can affect the concentration of compounds that serve as pre-cursors for certain VSCs. However, increased settling times at the end of fermentation reduced the amounts of compounds present in the wines. Further analysis of samples tested during barrel aging also revealed that the early formation of reductive smells that occur soon after going to barrel are likely a result of H2S (rotten egg smell) and/or methyl thioacetate (MTA – the pre-cursor for the cooked cabbage and rotten egg-smelling compound methanethiol). This finding led them to research factors that impact H2S and MTA production in late fermentation and early wine aging.

In addition, the concentration and type of yeast assimilable nitrogen (YAN) present during fermentation and the presence of elemental sulfur significantly impacted the production of H2S and the concentration of MTA. The amount of MTA in the wines often correlated with high production of H2S during fermentation. In particular, the addition of diammonium phosphate (DAP) to a pinot noir fermentation resulted in increased H2S production while the addition of amino acids (to the same YAN concentration as the DAP addition) reduced the amount of H2S. However, in some cases high YAN (whether caused by DAP or amino acid addition) resulted in high amounts of methyl acetate in the wines even when lower H2S was produced in the ferments.

These results demonstrate techniques winemakers can use to reduce VSC formation. Minimizing residual elemental sulfur on grapes, understanding the type and concentration of YAN present during fermentation, using low H2S producing yeast strains, and increasing settling times after fermentation to reduce wine lees levels during aging will all lead to a reduction of VSCs. While the formation of VSCs in the winery is challenging, all is not lost – winemakers can mitigate the problem by implementing the practices above.