Chapter 1

Chemical Components of Grapes and Wine

Volatile Sulfur Compounds in Wine

One ongoing concern during winemaking is the formation of volatile sulfur compounds (VSCs) that may negatively impact wine aroma. These compounds are either produced during primary fermentation or during wine maturation. Volatile sulfur compounds, or VSCs, originate from many sources including elemental sulfur; sulfates or sulfites; sulfur-containing amino acids by microbes; sulfur-containing pesticides; fermentation dynamics and yeast stress; and what is called reductive winemaking where wine is exposed to extremely low levels of oxygen. With very low sensory thresholds and strong unpleasant aromas, most VSCs are considered to have a negative impact on wine quality though can exhibit positive attributes when present in small quantities (e.g., varietal thiols, Section 1.5). The most common of the VSCs produced during wine production is hydrogen sulfide (H2S) followed by dimethyl disulfide, mercaptans, and diethyl disulfide..

Hydrogen Sulfide

Yeasts form hydrogen sulfide by reduction of sulfates, sulfites, and elemental sulfur during amino acid synthesis. Normally considered aversive, hydrogen sulfide can have a pleasing aromatic impact by providing a yeasty flavor to wine at low levels. Problems arise when hydrogen sulfide production exceeds its utilization and excess hydrogen sulfide leaks into the wine. In excess hydrogen sulfide can produce undesirable odor-active compound reminiscent of rotten eggs (Section 16.3). Sensory threshold levels are low, 50 to 80 ?g/L (Zoecklein et al., 1995). At lower levels hydrogen sulfide may display yeasty properties and play a role in complexity.

Dimethyl Sulfide

Dimethyl sulfide (DMS) is present in almost all wines and is probably a breakdown product of amino acids. The formation of dimethyl sulfide does not appear to be related to hydrogen sulfide production. At low levels (15 to 20 ?g/L in whites and 20 to 30 ?g/L in reds) dimethyl sulfide can contribute roundness, fruitiness, or complexity.

Mercaptans

Mercaptans are the cause of the majority of the aromas we refer to as reductive. They form when hydrogen sulfide interacts with alcohol. Wine industry and research circles have grown accustomed to some of these generic terms being used for specific compounds. Mercaptans generally refer to compounds contributing to the reductive odors reminiscent of cabbage and burnt match, while the term volatile thiols (sometimes called varietal thiols) generally refer to fruity and tropical aromas (Section 1.5). Methyl mercaptan is the most common problem sulfide compound found in post-fermentation wines.

Disulfides

Disulfides are common in wine, but are usually found below sensory thresholds. They are typically formed after fermentation from oxidation of sulfide or mercaptan precursors. They are a serious concern due to their propensity to revert back to mercaptans. This conversion is often accompanied by an increase in objectionable odors due to the lower mercaptans threshold limits. Dimethyl disulfide (DMDS) has been variously described as being onion-like or reminiscent of cooked cabbage.

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