The Role of Sulfur Dioxide in Wine
A small amount of SO2 is produced naturally as a by product of fermentation, but most of the SO2 has been added by the winemaker. During white wine production, it is added at almost every stage of the process, and is more or less required after malolactic fermentation is complete. It is used to a lesser extent during red wine production, but is still a necessary component in many production techniques.
The Up Side
Sulfur Dioxide is a simple molecule, two Oxygen atoms bonded with a Sulfur (S-O-S). Its Chemical properties are actually quite similar to that of Carbon Dioxide, one of the most abundant molecules on Earth. But in terms of winemaking, using it effectively is critical in producing a wine that will stand the test of time.
The most important mechanism of action for Sulfur Dioxide is as an anti-microbial agent. It regulates the growth of harmful yeast and bacterial growth in the wine. However, the “good” yeasts used in the winemaking process have developed a resistance to SO2 over the years, allowing them both to live in harmony with each other. This gives the “good” yeasts as competitive advantage over the harmful yeasts in the fermentation process.
Another important role of Sulfur Dioxide lies in its anti-oxidant properties. This guards against browning and protects the fruit-like qualities of the wine. SO2 can bind with a molecule called acetaldehyde. Many of you have experienced the smell of this molecule from a brown, bruised apple. It is also produced when a wine undergoes oxidation. When SO2 reacts with acetaldehyde, they bond together, producing a harmless, odorless molecule.
The Down Side
If a winemaker uses too much SO2, it can kill the “good” yeast, haulting fermentation before the desired end point. It can also stop malolactic fermentation from completing, yield wines that taste unfinished.
You can tell a wine that has too much Sulfur Dioxide by its characteristically pungent odor. It smells similar to that of a recently struck match. Although many people have trouble putting their fingers on this smell, it is largely sensed as an irritation the nasal passage membranes.
Using Sulfur Dioxide in Red Wine presents a unique set of problems for winemakers. SO2 binds to a group of molecules called anthocyanins; which give red wines their color. When SO2 reacts with anthocyanins, it renders them inert, and they lose their color and properties. Luckily for winemakers, a large percentage of anthocyanins are bound to tannin molecules, safe guarding them from the effects of SO2.
SO2, Sulfites and Humans
When a wine label says “contains sulfites” it is referring to the fact that sulfur dioxide has been added. Since 1987, wines sold in the United States must have a warning label that they contain sulfites. The EU has required a similar warning label since 2005. Because sulfites can be harmful to some people in trace amounts, the American Government requires that all wine that contain them to say “contains sulfites” on the wine label.
Because Sulfites are naturally occurring in the fermentation process, a wine free of sulfites is a natural anomaly. Sulfites produced from fermentation range from 6 to 40 parts per million (ppm). So almost every wine has sulfites in it, but it is within the discretion of the winemaker to add more.
United States policy on sulfites in wine allows up to 350ppm. When below this level, people without allergies to sulfites should not have adverse reactions to the sulfites from wine.
Some people have adverse reactions to sulfur dioxide, but their numbers are small. Most winemakers take great care to add as little sulfur dioxide as possible. It is usually added both before and after the fermentation process. It is possible, but quite difficult to make a stable wine without adding sulfur dioxide.
Sulfur Dioxide Mechanisms
Because of its chemical and physical properties, Sulfur Dioxide is naturally a gas. But like many other gasses, it can also be dissolved in liquids; in our case, wine. In its gaseous form, it is orders of magnitude more effective at as an anti-microbial. In its dissolved form, however, it is much more effective as an anti-oxidant. This works perfectly for winemakers trying to find a delicate balance between these two properties.
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