Formation of Volatile Sulfur Compounds in Pinot Noir Post-Fermentation

Development of volatile sulfur compounds (VSCs) post-fermentation can be a significant issue during both red and white winemaking. Unfortunately our understanding of contributing factors or conditions that impact VSCs is limited due in part to the complexity of their formation. This study focuses on the development of VSCs in Pinot noir during post-fermentation aging. During the first year of the study the impact of lees levels and composition on formation of VSCs was determined. Results showed that although lees levels and yeast strain impacted the amount of sulfur containing amino acids (pre-cursors for the formation of volatile sulfur compounds) in the wine, this did not result in an increase in the formation of volatile sulfur compounds. Wine samples were also provided by collaborating wineries in 2013 and assessed for VSCs so as to determine the cause of early reduction issues in barrel. Wineries were instructed to take juice and wine samples from lots that traditionally had issues with VSCs. Wine samples were taken after pressing and after one, three, and nine months barrel aging. Analysis of these samples indicated that the early formation of reductive smells soon after going to barrel were most likely due to H2S rather than the formation of more complex volatile sulfur compounds such as mercaptens and disulfides. Where this H2S is derived from and what factors impact its formation became the focus of future experiments. Firstly, experiments investigating the role of YAN concentration and content were undertaken. A synthetic grape juice was prepared where the amount and type of YAN (primary amino acids vs. ammonia from diammonium phosphate (DAP)) could be varied. H2S production was measured throughout fermentation and finished wines were assessed for a range of other VSCs. Variation in YAN concentration as well as whether YAN was derived from amino acids or DAP impacted H2S production during fermentation as well as formation of volatile sulfur compounds post-fermentation. In particular, DAP supplementation increased the amount of H2S formed late in fermentation and resulted in the highest amount of methyl thioacetate in the wines post-fermentation. Experiments investigating the role of elemental sulfur in the formation of H2S and other volatile sulfur compounds post-fermentation were also undertaken.

Pinot noir grape fermentations  were undertaken where an addition of 0, 5 or 15 ug/g elemental sulfur was made to the grapes. Fermentations were conducted by a high H2S producing yeast strain (UCD522 ) or a no-H2S producing yeast strain (P1Y2). Addition of elemental sulfur to the grapes resulted in H2S formation during the alcoholic fermentation independent of which yeast strain was used. H2S production was higher in fermentations performed by UCD522 with increasing amounts of elemental sulfur resulting in increased production of H2S. In addition, higher elemental sulfur additions also resulted in higher H2S production late in fermentation. This is particularlyimportant as H2S formation late in fermentation is more likely to be retained in the wine due to the reduced production of CO2 by yeast. Higher elemental sulfur also resulted in wines containing higher concentrations of methyl thioacetate post-fermentation. Both of these findings suggest an important role for elemental sulfur in the formation of volatile sulfur compounds during and after fermentation. Overall, this study to date has demonstrated that lees levels impact the concentration of sulfur containing amino acids in the wine but may not directly impact formation of volatile sulfur compounds. Instead, the formation of H2S late in fermentation or early post-fermentation may be the main cause of post-fermentation reduction soon after wine goes to barrel. Current experiments are investigating the impact of YAN, yeast strain, and elemental sulfur on the formation of H2S and other volatile sulfur compounds post-fermentation. This work includes an ongoing effort to measure the amount of elemental sulfur present on grapes at harvest.