One of the most universal and widely used preservatives in winemaking is sulfur dioxide (SO2), or more commonly, sulfites. Known for its antimicrobial and antioxidant properties, it is added at specific stages of winemaking to prevent both spoilage by unwanted microorganisms and oxidation during crush. What many don’t know is that yeast actually produce SO2 during fermentation, making it a naturally occurring substance in wine. Because of this, no wine can be labeled as “Sulfite-free” or “Contains no Sulfites”.
The TTB (the governing arm of winemaking) enforces how much SO2 can legally and safely be added to wine, and requires wineries to state so on their wine labels. Many consumers have an allergy to sulfites, making it essential to include this information on wine labels. The maximum limit for SO2 in a wine is a concentration of 350 mg/L (which is rarely achieved in most wines). Any wine containing more than 10 mg/L SO2 must state so on their labels (“Contains Sulfites”). Labeling becomes a little more tricky with organic wines. To start, wines labeled “Organic” cannot contain any added SO2. Additionally, organic wines can only claim that a wine has “No Added Sulfites”, not that the wine is “Sulfite-free” (remembering that yeast produce a little sulfur!). For those who suffer from sulfites, watch out for wines labeled “Made with Organic Grapes” as they can legally contain up to 100 mg/L added sulfur!
To get the maximum benefit from sulfur without adding too much (which can alter wine quality), measuring sulfur concentrations in aging wine is routinely practiced in the cellar. We use a method called aeration-oxidation to measure sulfur levels.
Aeration-Oxidation apparatus (measuring sulfur in Syrah)
For you chemistry buffs out there, this is a distillation of SO2 followed by an acid-base titration. A wine sample with an unknown concentration of sulfur is distilled into hydrogen peroxide, where it is converted to sulfuric acid (turning the solution purple). The acid is then titrated against a base; once the titration endpoint is met, the sample turns green.
Titrating the distillate
Titration endpoint is reached
Based on how much base you used, you can calculate the sulfur in the wine. Over time, sulfur is consumed by components in the wine, so the concentration needs to be adjusted. To increase levels, the winemaker adds sulfur, usually as a salt called potassium metabisulfite. To decrease levels, winemakers can blend the wine with other low-sulfur wines, or just wait for the levels to diminish over time. Ultimately, it is the winemaker’s goal to bottle a properly sulfured wine that will age nicely for years to come.
