Wine, all on its own, is a fairly good antiseptic. The tartaric acid in wines made from grapes is a relatively strong organic acid that helps keep the pH of the wines low, which in itself is a good way to inhibit microbes. Add to that the antimicrobial properties of alcohol, and you have a beverage that could help you survive through a plague. However, that’s not to say that nothing will survive in wine. Acetic Acid bacteria, Brettanomyces, and other spoilage organisms can literally turn a wine sour and make it generally unpleasant to drink. This is where the use of sulfur dioxide (SO2) in the winery is imperative. In addition to antimicrobial properties, SO2 is also an antioxidant and antioxidasic. It is arguably the most important additive in wines and, except for alcohol, is the only component in wine that requires a warning statement on the label. Thus, it is important that wineries not only ensure that they are correctly dosing and monitoring their wines with SO2, they also need to ensure that they are properly measuring it as well.

Using SO2 in the winery. Sulfur dioxide is a pretty noxious gas, and is not typically used in its pure form in small wineries. Most often, wineries employ it by adding a potassium salt of sulfurous acid, known as potassium metabisulfite (KMBS). It is important to understand that by weight, KMBS is about 57% sulfur dioxide. Thus, for every 100 grams of KMBS added to a wine, 57 grams of SO2 is added. To complicate matters, the majority of that 57 grams of SO2 becomes chemically bound to certain compounds in wine when it is first added, rendering it useless to protect the wine!

SO2 Binding in wine and why we want it free! Any compound in wine with a carbonyl function will bind sulfur dioxide. While many of you have no idea what that means, just know that there are many compounds in wine that have a carbonyl group. Glucose, acetoin, diacetyl, galacturonic, α-ketoglutaric and pyruvic acids, and acetaldehyde are all compounds that will bind SO2. This effectively does exactly what it sounds like: it “ties the hands” of sulfur so that it is unable to do its job! The good thing to know is that once all the binding sites are filled with sulfur, the remaining sulfur is floating around in the wine, free to do its job!

The term “free sulfur” is used to describe the unbound or “working” portion of sulfur in wine. In reality, only a small percentage of your free sulfur is actually “working” against microbes, and this working portion depends on a wine’s pH.  At a lower pH, more of the free sulfur is in the SO2 form, while at a higher pH, more of it is in the form of bisulfite (H2SO3–), which is essentially ineffective in wine (see table 1).

Adding and monitoring Sulfur in wine. To follow good winemaking practices, there are three critical times when a winemaker should think about sulfur addition: at crush, following the completion of alcoholic fermentation or malolactic fermentation, and any time a wine is moved (exposed to oxygen). In unfermented juice or must, a small amount of added sulfur will help kill spoilage bacteria and provide some protection from oxidation. Generally, 30 mg/L of total sulfur is sufficient to halt bacterial problems without hindering fermentation in low pH wines from quality fruit (absence of rot). Following fermentation, the quantity of sulfur to add is not quite as formulaic.