Sulfites in wine: Part I
While the thought of sulfur in wine might fill some with trepidation, this natural chemical element plays a vital role in winemaking: It protects from spoiling and oxidation. In Part I of our new series, Westgarth Wines expert Maurizio Broggi explains what exactly sulfur is and how it is essential for quality and style consistency.
Ancient to modern: Chemistry of preservation
Sulfur has played a vital role in the preservation of food and wine since antiquity. In ancient civilizations, burning sulfur was used to disinfect stored provisions. The Romans employed substances such as pitch, resin, and sulfur to protect wine vessels, a practice documented by authors such as Pliny the Elder.
By the 18th century, sulfur had become firmly embedded in winemaking culture, with Bordeaux cellars routinely sterilizing wooden barrels with burning sulfur wicks.
When elemental sulfur is burned in air, sulfur dioxide gas is produced, which has a pungent, irritating smell. Today, sulfur dioxide (SO₂) remains indispensable in modern winemaking, used in the vast majority of wines for its preservative and antimicrobial properties.
Beyond wine
Besides winemaking, sulfur dioxide is also widely used in food preservation and fruit processing. Indeed, dried fruit, such as raisins, dried apricots, and dried peaches, contains significant amounts of sulfur dioxide (e.g., E220), ranging from 0.5 up to 2 grams per kilogram.
Processed meat such as sausages, as well as canned and pickled vegetables, also contain sulfur dioxide, although typically at lower levels than dried fruit.
Why does winemaking need sulfur?
Sulfur dioxide fulfills important functions during winemaking, in particular as an antioxidant and as an antimicrobial.
As an antioxidant, despite common belief, sulfur dioxide does not directly react in a rapid and meaningful way with oxygen to prevent wine oxidation. Its true efficacy lies in how it interacts with the by-products of oxidative reactions. By binding to these intermediate compounds, sulfur dioxide prevents them from oxidizing other important substances in the wine.
Sulfur dioxide also inhibits polyphenol oxidase, which is the oxidative enzyme responsible for the brown color of fruit that is exposed to oxygen (e.g., a cut apple). This is why winemakers frequently add it in the form of metabisulfite to freshly harvested grapes as well as during grape crushing and pressing, to help preserve the color and aromatic integrity of the grape must.
How does sulfur prevent wine faults?
Equally important is sulfur dioxide’s antimicrobial action. It effectively inhibits or limits undesirable microbes, including spoilage yeasts like Brettanomyces, lactic acid bacteria, and acetic acid bacteria such as Acetobacter. The latter is responsible for volatile acidity — a fault that, at high levels, imparts an unpleasant vinegary note.
This microbial control is particularly valuable in preventing wine faults, though its effectiveness varies according to the sensitivity of each microorganism to sulfur dioxide. While sulfur dioxide is a powerful safeguard, it works best in concert with strong hygiene practices, sound grape quality, and temperature control throughout the winemaking process.
Sulfur and commercial vs wild yeast
Interestingly, wine yeasts (Saccharomyces cerevisiae) have higher tolerance to sulfur dioxide than most other microorganisms, including wild yeasts. Therefore, the addition of sulfur dioxide before fermentation favors a stronger population of Saccharomyces by limiting the competition with wild yeasts.
Naturally, in spontaneous fermentations, where ambient or wild yeasts carry out the fermentation, careful management of sulfur dioxide is essential, as excessive sulfur dioxide can suppress beneficial wild yeast strains, thus negatively affecting the onset and completion of spontaneous fermentations.
Chemical reaction preserves wine
Once added to grape must or wine, sulfur dioxide dissolves and divides into two forms: bound and free.
The bound form of sulfur dioxide is the portion that is no longer available, as it is chemically bound to components of grape must or wine. Since it is already chemically attached to wine compounds, it is ineffective for protection.
The free form is the portion of sulfur dioxide that is still available and provides active defense against oxidation and microbial activity.
How does pH balance influence sulfur dioxide?
A critical factor related to sulfur dioxide is a wine’s pH. This measure indicates how acidic the wine is (lower pH means the wine is more acidic).
pH affects the taste of wine by making it crisp and providing freshness, as well as contributing to its aging potential. pH also contributes to brighter and more vibrant color and wine stability.
The effectiveness of sulfur dioxide is strongly influenced by the wine's pH. Lower pH levels increase sulfur dioxide effectiveness and thus improve preservation and protection. Conversely, wines with higher pH require greater amounts of total sulfur dioxide to achieve the same protective effect.
Therefore, understanding and managing wine’s pH is critical not only for wine freshness, color, and stability, but also for maximizing the effectiveness of sulfur dioxide.
Enjoyed this introduction to sulfites and their importance to winemaking? Keep an eye out for Sulfites in Wine Part II where the cellar process, health regulations, and natural wines are in the spotlight.
Wine expert Maurizio Broggi is from Milan. With a background in engineering, he discovered his passion for wine at a sommelier training course in Italy and has since built an international career as a wine educator, ambassador, and consultant, working across his homeland, China, and the United States. He is a WSET Levels 1–4 instructor, holds the WSET Level 4 Diploma, and is pursuing the prestigious Master of Wine title. Maurizio has also completed winemaking studies at UC Davis and has been a judge at the Decanter World Wine Awards.
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