Analysis of Sulfide-Off Odors Produced During Sur Lie Aging
The first goal of this project was to validate the solid phase microextraction (SPME) method developed in the Ebeler laboratory for the analysis of S-volatiles produced both during fermentation and sur lie aging, and to determine if the method displayed the dynamic range of sensitivity needed to undertake an analysis of the product-precursor relationships for the volatile S-containing compounds that appear during aging on the yeast lees (sur lie). The SPME method has been validated over two years and displays the level of sensitivity needed to undertake the analysis of the chemistry of formation of the S-volatiles under natural or un-spiked conditions.
The second goal of this experiment was to examine the role of several factors on the formation of S-volatiles during sur lie aging. The role of nitrogen, sulfate and vitamin levels and effect of supplementation with methionine, cysteine and glutathione were evaluated and found to not impact the appearance of S-volatiles in synthetic juice media. Likewise the presence of hydrogen sulfide formation during primary fermentation was not correlated with the appearance of S-volatiles sur lie in either synthetic or natural juices. The presence of elemental sulfur impacted H2S formation but to date the presence of elemental sulfur during the primary fermentation has not resulted in the appearance of S-volatiles sur lie. Similarly juice pH has impacted the appearance of negative notes in the wines but these characters do not appear to be associated with S-volatiles. The grape solids level during the primary fermentation did impact the level of S-volatiles formed, but appreciable S-volatiles were found only at the higher solids levels, levels that would not be normally used in production. These wines likewise developed off-characters that could not be correlated with S-volatiles. The off-notes found are reminiscent of S-compounds so it is not surprising that their appearance is blamed on the presence of S-volatiles. However this research suggests that S-volatiles play a more minor role in the appearance of off-characters sur lie than previously reported.