Wines & Vines

March 2015 practical winery & vineyard 53 levels of sugar and ethanol mask them. 2,15 Winemakers expect an increase in VA of about 60 to 120 mg/L in barrel-aged wine after one year. This is not necessar- ily due to microbial spoilage, but rather the degradation of the hemicellulose of the oak barrel itself. Also, phenolic com- pounds can oxidize over time to form peroxide, which oxidizes to acetaldehyde and, after, to acetic acid. 15 Sulfur compounds Sulfur compounds can be pleasant or disagreeable and generally have a low threshold of detection. Volatile sulfur compounds, such as thiols, are respon- sible for the ripe/fruity aromas of Sauvignon Blanc and are formed during fermentation. 12 They can also contribute a box-tree-like aroma in the same variety. 11 Sulfurous compounds at low concen- trations may cause a perceived "min- erality" in some wines. 5 Additionally, Lactobacillus can metabolize methionine, which forms volatile sulfur compounds such as methanethiol, dimethyl disulfide and propionic acid. Oenococcus oeni also metabolizes methionine. Current research suggests that the most significant byproduct is propionic acid, which contributes a choc- olate aroma and may be partially respon- sible for the pleasing and complex aroma profile of malolactic fermentation. 9 The H 2 S molecule is the most stud- ied sulfur compound. Normally consid- ered aversive, H 2 S can have a pleasing aromatic impact by providing a yeasty flavor to wine at low levels. Higher con- centrations of H 2 S have a rotten egg aroma and a very low sensory threshold of 10 to 100 µg/L. 8 In order to synthesize sulfur-containing amino acids, yeast can reduce sulfite to sulfide, which is then enzymatically combined with a nitrog- enous compound to form cysteine or methionine. However, if the nitrogenous com- pounds are not present, the result is release of hydrogen sulfide, which freely bypasses the cell wall. 6 Deficiencies in vitamin B5 have been found to be lim- iting in juices that produce H 2 S. This vitamin is important for the formation of Coenzyme A (CoA), which is neces- sary for the formation of methionine and cysteine. Without this enzyme, these amino acids cannot form, and the sulfur produces hydrogen sulfide. 13 However, a vitamin deficiency is extremely rare and difficult to test for in a lab. 1 H 2 S can also result from: • Reduction of elemental sulfur from spray residues (relatively uncommon). • Presence of other sulfur-containing compounds (glutathione). • High or very low juice turbidity (about 0.5% turbidity is recommended). • Low redox potential of must (reduc- tively held or in tall/skinny tanks). • Release of bound sulfurous com- pounds in yeast lees during lees aging. The impact of lees on H 2 S production is not always negative. The mannopro- teins of yeast can form disulfide bridges with sulfur compounds and lessen their aromatic impact. Excessive SO 2 use leads to the for- mation of H 2 S by inhibiting acetalde- hyde reduction to ethanol. If a deficiency is also present in O-acetylserine and O-acetylhomoserine (precursory com- pounds necessary for the formation of sulfur-containing amino acids), H 2 S is produced from the enzymatic reduction of sulfite (from SO 2 ). This gives the yeast a sulfur source to produce the amino acids. 7 A high metal ion concentration (such as residual copper from Bordeaux mix- ture) within the must suppresses cellu- lar respiration, which lowers the redox potential and, ultimately, elevates H 2 S W I N E m A k I N G

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