Volatile Sulfur Compounds: Incidence and factors affecting their formation in Californian Wines
The formation of volatile sulfur compounds during fermentation was studied using juice from grapes with a history of production of sulfide odors. Juice was supplemented with different levels of glutathione (GSH) and glutamic acid to investigate the role of these compounds in production of sulfur vblatiles. Juices supplemented with high levels of GSH produced the highest levels of H2S during fermentation. For Cabernet, the control produced no ethanethiol (EtSH), whereas other supplemented juices produced about 20 ppb EtSH. In contrast for Merlot musts, all produced ethanethiol. The low level GSH must fermented faster than the control (17 days vs. 22 days), whereas supplementation with a nitrogen equivalent amount of glutamic acid had no effect on fermentation rate. This Finding may be important since the control juice had not only a sulfide odor problem but also slow fermentation. The 300 pprn GSH supplemented juice produced eight different type of thiol compounds with high concentrations whereas lower addition of GSH and GLU produced only three thiol compounds at much lower concentrations. Model media with different nitrogen levels were investigated for their pattern of H2S production and the preliminary result showed the production of H2S was directly related to the levels of nitrogen content with the highest levels when yeast actively growing. In no case was H2S present at the end of fermentation.Further experimental fermentation will be conducted based on this preliminary result. To compare the effect of the size of fermentation on the formation of sulfur volatiles, wines from lab scale fermentations were compared with commercial wine produced in large scale fermentation of the same grapes. The commercial wine had a strong sulfide odor and an extremely high level (about 1 ppm) of H2S and lesser amounts of methyl and ethyl mercaptan, whereas wines from our small fermentor did not have any off-odor and lower concentrations of sulfur volatiles, suggesting the importance of redox potential and dissolved oxygen differences between the two systems.