Aroma Compounds
/Volatile Sulfur Compounds
Have you ever smelled a glass of wine and thought whoa! That smells stinky or sour? Odds are you are smelling the volatile sulfur compounds that are present in wine. What are volatile sulfur compounds? There are generally two types of sulfur compounds in wine. The first are sulfites, which indicate the presence of excess sulfur dioxide and smells like the familiar rotten eggs that we all associate with sulfur. The second are sulfides, which consists of volatile sulfur compounds that can be created through chemical reactions by the yeast during the fermentation process.
Hydrogen Sulfide
The Science
Must is freshly pressed grape juice that contains the skins, seeds, and stems of the fruit. Usually, by itself must contains so few sulfur compounds that they are unrecognizable to the human palate. The sulfur compounds begin to come out during the fermentation process.
While the yeast are breaking down the sugars and converting them to ethanol, a process named sulfide reduction sequence also occurs. During the sulfide reduction sequence, amino acids are created which allow the yeast to break these sulfur compounds into generic sulfide. This process generally goes off without a hitch, and the result is wine free of any harsh sulfur smells. Occasionally, the must does not have the requisite nitrogen required by the yeast to complete the sulfide reduction sequence. When this occurs, the resulting chemical reaction produces hydrogen sulfide and gives the wine sulfur aromas.
The Result
Sulfur compounds are yet another struggle that the winemaker must face while producing their wine. They are one of the many reasons why chemistry plays such a big role in winemaking. If a winemaker is unaware of the lack of nitrogen in the must, then a whole batch of wine can be spoiled and smell of rotten eggs, garlic, or onion.
Sulfur Compound Aromas
In a wine, sulfur can take on the aromas of rotten eggs, leeks, onions, chives, garlic, scallions, or a burnt match.