Formation of Volatile Sulfur Compounds in Pinot Noir Post-Fermentation – Part 2: Lees Level and Contact Time on Volatile Sulfur Compounds in Wine
The effect of lees contact time during wine barrel aging on volatile sulfur compounds was investigated in this study. Pinot noir wines were made using grapes from the Oregon State University vineyard and fermented with two different commercial yeast strains. One set of wines was produced using the low/no H2S producing yeast strain Saccharomyces cerevisiae P1Y2 , and the other set was produced using Saccharomyces cerevisiae RC212. Fermentations were conducted in triplicate. At the completion of alcoholic fermentation, wines were pressed and split into three different lees treatments based on settling time (0, 24 and 72 hrs) which were named as heavy, medium and light lees treatment respectively in this progress report. Samples were collected at 0, 2 weeks, and 1, 2, 3, 6, 9 months. Volatile sulfur compounds were analyzed by solid phase microextraction-gas chromatography-pulsed flame photometric detection (SPME-GC-PFPD) method. The results showed that hydrogen sulfide (H2S), methylmercaptan (methanethiol) and methyl thioacetate (sulfur containing ester) were the major sulfur compounds of the concern in the wines. Moreover, the concentration of H2S was directly influenced by the type of yeasts in the first month of storage. Wines made from Saccharomyces cerevisiae P1Y2 generally had lower concentration of H2S than those from Saccharomyces cerevisiae RC212. In addition, lower H2S concentration was observed in light lees load than in medium or heavy lees load treatment from Saccharomyces cerevisiae P1Y2 during the first two weeks of aging, whereas more H2S were generated in higher lees loading samples from Saccharomyces cerevisiae RC212. H2S accumulated with time during early aging, and reached to the maximum at one month, then decreased afterwards regardless of types of lees or the amount of lees loaded. Methanethiol also accumulated during aging, and reached its maximum at 2-3 months, then decreased slowly afterwards.
High level of methyl thioacetate was detected in the experimental wines, wines from Saccharomyces cerevisiae RC212 had substantially higher level of methyl thioacetate than those from Saccharomyces cerevisiae P1Y2 regardless of lees level, and the concentration of methyl thioacetate stayed consistent during barrel aging. Heavy lees generally lead to more dimethyl sulfide (DMS) accumulation. During the aging storage, DMS reached to peak level at 2-3 months, decreased at 6 months, and then continued accumulating at 9 months. The levels of other sulfur compounds (carbon disulfide, dimethyl disulfide and dimethyl trisulfide) were very low for flavor contribution. The samples from collaborating wineries were also analyzed, but the results were complicated due to various treatments and remedies performed at winery. H2S was the major volatile sulfur compound in those winery samples, especially at the beginning of the barrel aging. Some winery samples also had high levels of methyl thioacetate and methanethiol. Although various remedies were performed in wineries to remove H2S , the levels of methyl thioacetate were very high in many of the wines after 6 to 9 months of aging. High concentration of methanethiol was also observed in many wines. More winery samples were studied for 2014 harvest and high levels of hydrogen sulfide, DMS and methyl thioacetate were detected in most of those wines. We collaborated with Dr. James Osborne further studied the effect of YAN on volatile sulfur formation. Detailed results were submitted separately as a single report. The study showed that the levels of YAN did not affect the generation of MeSH, CS2, DMDS and DMDS. However, the amount of YAN and the type of YAN did affect the formation of thioactates by Saccharomyces cerevisiae UCD522. DAP addition generated higher thioacetates. We also studied the effect of elemental sulfur on volatile sulfur composition. Although the residual sulfur affected hydrogen sulfide production during fermentation (see the separate report), sulfur addition did not affect the volatile sulfur compounds in the final wine except for methyl thioacetate. Sulfur addition generated more methyl thioacetate by both Saccharomyces cerevisiae P1Y2 and UCD522. High levels of methyl thioacetate could be an important issue for winery. Methyl thioacetate impart sulfur off-flavor in wine, it can also be converted to methanethiol that has a very low sensory threshold. The results suggested the methyl thioacetate and methanethiol could be the major culprits for sulfur off-flavor development during barrel aging. This new finding is significant and needs to have further investigation in future.