The term sulfur is frequently misused in wine vernacular. This miscomprehension colors our understanding of winemaking and leads to confusion for both consumers and members of the trade. Indeed, it may have contributed to the current trend toward eliminating added sulfur in some wines. Therefore, it’s vital that wine professionals have a fundamental understanding of the nature and uses of various sulfur compounds in winemaking.
Sulfur was employed for household and professional purposes as early as the sixth century BCE. Mined from volcanoes, it was used by the Greeks to control rat populations and to fumigate insect- and vermin-infested ships and lodgings. It was also used as a chemical weapon in ancient wars. While debated, the first mention of sulfur in winemaking appears to be in 1487, when a German law approved the burning of sulfur-treated wood chips inside wine barrels. It is believed, however, that sulfur candles were used to prevent vinegar aromas in amphorae in Roman times, as evidenced by writings of Pliny the Elder. In other words, sulfur is a multi-century, multi-era, multi-generational industrial workhorse.
Today, two different compounds containing sulfur are commonly employed in the winery and vineyard, each serving a different purpose. In the vineyard, elemental sulfur—not sulfur dioxide—is used as an antifungal. Elemental sulfur appears on the periodic table as S, and its atomic number is 16. While sulfur is generally thought of as being “stinky,” elemental sulfur is odorless until it is exposed to heat or sunlight and volatilizes. It’s a very abundant, naturally occurring reactive chemical element (with many metals and nonmetals) that is mined from sulfur deposits and is solid at room temperature.
In the winery, sulfur dioxide appears in various types of additives and may be in gas, liquid, or solid form. Sulfur dioxide is antimicrobial, anti-enzymatic, and a preservative, serving as an antioxidant. It is colorless and, unlike elemental sulfur, has a strong noxious odor. The fumes of sulfur dioxide are toxic to inhale. In the quantities present in wine, consumption is not harmful for most people, but it may induce asthma in those with pulmonary issues. Its chemical formula is, of course, SO2: one part sulfur, two parts oxygen.
Elemental sulfur’s most important use in the vineyard is as a spray to prevent powdery and downy mildew. Powdery mildew (oidium) is a fungal disease that infects green plant tissue, meaning that grapevines are most susceptible from budbreak until veraison. It is considered the most significant grapevine disease in terms of yield and quality loss, as well as overall expense. Because it can quickly overtake a vineyard, producers typically try to prevent infections by spraying vineyards with fungicide at regular intervals throughout the growing season.
Downy mildew also affects green plant tissue, but unlike powdery mildew, it spreads through water—behavior that Dr. Akif Eskalen of the University of California, Davis, describes as “swimming” in leaf moisture and in “rain splash.” While there are studies that demonstrate sulfur alone is moderately effective in fighting downy mildew, when sulfur is mixed with copper to form copper sulfate, it is far more effective.
While mildew spores can become resistant to synthetic fungicides if sprays are too frequent in high-pressure areas, mildew has not developed a resistance to sulfur. As a result, sulfur remains the gold standard for mildew prevention even after over 100 years of use. Further, unlike synthetics, it is permitted for use in organic and biodynamic agriculture. In addition to its reliability, sulfur is much cheaper than other organic materials (such as Cinnerate and Stylet Oil). It is easy to apply, and its residue (early in the season) is not of concern because it is an element that is required for vine growth. Vineyards are generally sprayed from budbreak until veraison, typically every 10 to 14 days. In rainier climates, these increments can be even shorter, and spraying may continue through harvest. Rainy vintages in Burgundy, for instance, can instigate a six- to eight-day spraying schedule—because of both increased disease pressure and rain washing sulfur off the canopy.
To better understand the mechanics of sulfur in the vineyard, it’s necessary to examine the riveting topic of mildew spore germination. The proliferation of mildew is both temperature and moisture dependent. Peak germination temperatures for powdery mildew are between 70 and 85 degrees Fahrenheit (20 and 30 degrees Celsius). For downy mildew, this range is lower, at 55 to 75 degrees Fahrenheit (13 to 25 degrees Celsius). Powdery mildew prefers high humidity in the evening for spore production, followed by low humidity during the day, as damp conditions can inhibit spore dispersal and germination. Temperature, however, is the more important factor. Even in low humidity areas (below 40%), powdery mildew may still proliferate if the temperature rate is ideal, requiring sulfur applications in the absence of excessive moisture or humidity. Downy mildew, as its spores are waterborne, develops more readily at higher levels of humidity (around 85%). This means that downy mildew is an issue in wetter, maritime climates like Bordeaux while essentially nonexistent in dry Mediterranean climates like California, where powdery mildew is the predominant vineyard fungal disease.
Under ideal conditions, the time between initial plant tissue infection and the blossoming of new spores is a mere five days. Once infection launches, producers will be required to spray frequently to avoid widespread proliferation. Prevention, then, is key to the success of sulfur in the vineyard. Once established, mildew spores have a limited time (about 24 hours) to infect plant material. But if mildew spores are unable to infect plant material during this critical time, they will die.
Mildew spores are killed on contact with sulfur sprays. In the right conditions, sulfur also volatilizes. As the day heats up, sulfur will create a gas, sulfuric acid, which can destroy spores inside the canopy that are not in direct contact with the sulfur spray particulates. The optimal temperature for this volatilization is over 70 degrees Fahrenheit (20 degrees Celsius). On any given day, the temperature inside shaded parts of the canopy can be 10 to 15 degrees lower than the green parts that are fully exposed to the sun. Canopy management techniques that decrease canopy crowding, such as shoot thinning and leaf removal, are therefore crucial to improving direct contact and reducing spore germination in cool climates where volatilization does not occur.
For sulfur sprays to be most effective, the temperature needs to warm to around 75 degrees Fahrenheit (25 degrees Celsius), but this is opportune because powdery mildew spores don’t proliferate widely below 70 to 75 degrees. Sulfur is typically avoided entirely when temperatures are very hot (over 100 degrees Fahrenheit, or 38 degrees Celsius) to prevent leaf burn, or phytotoxicity. Conveniently, powdery and downy mildew spores are not tolerant of temperatures over 90 degrees Fahrenheit (32 degrees Celsius), so in the absence of considerable and unpredictable heat spikes, phytotoxicity can be avoided.
The application style of sulfur sprays can also influence their effectiveness. These styles largely fall into two categories: wettable applications and dusting applications. They are applied as their names suggest. Wettable applications include additives called surfactants, a term that refers to anything that keeps a chemical compound in suspension so it can stick to and touch more things. In this case, a surfactant mixed with sulfur makes the sulfur adhere to surfaces like leaves and clusters. Wettable sulfur is mixed with water and is more likely to be used when an infection is already present. Dusting applications are applied without a surfactant and generally used as preventative treatments.
Sulfur particle size influences its effectiveness against mildew pressure. Larger particles of sulfur are present in dusting applications, while smaller (micronized) particles are present in wettable applications. Larger particles maintain a presence on the vine for a longer period of time, particularly in relation to rain, and tend to degrade at lower rates than smaller sulfur particles. They also vaporize more slowly, which decreases their overall efficacy. Vice versa, smaller particles have greater efficacy, because they vaporize more quickly, but they also degrade faster than larger particles, especially in the presence of rain, and must be applied more frequently. Because the effectiveness of sulfur within the canopy is largely dependent on the vaporization of sulfur, the choice of whether to apply wettable or dusting sulfur will depend on the canopy management practices and temperature at spraying time.
Speaking of spraying time, the devil is in the details. Late sulfur sprays can significantly contribute to the presence of hydrogen sulfide in wine. It is generally recommended that sulfur sprays cease about five weeks prior to picking, though this can, of course, vary widely depending on climatic conditions and the grape variety. Advantageously, unless conditions are wet, the threat of infection of berries and shoots decreases as veraison nears. Winegrowers in rainy climates can apply sulfur more regularly and closer to harvest. In dry climates where sulfur is likely to “stick” to the vine for longer periods of time, it’s important to transition to other products for mildew protection, or to stop spraying, post-veraison.
Halting sulfur sprays closer to harvest is particularly crucial for red grapes. Red juice is more susceptible to issues with hydrogen sulfide than white grapes. Because red grapes ferment on their skins, and sulfur is present on the skin of the grape, any sulfur residue that comes in on the grapes will remain in the ferment. For white grapes, which are pressed upon arrival to the winery (orange wines aside), settling overnight then racking the following day will remove the juice from many of the sulfur particulates, which will fall to the bottom of the tank with the other solids. Those seeking match-stick reduction characteristics in white wines may choose to introduce sulfur sprays later in the growing season, but this should be handled with a good deal of caution. It’s difficult to control and cherry pick reduction characteristics.
We now know how elemental sulfur works in the vineyard, but how does sulfur dioxide work in the winemaking process? It’s worth repeating that sulfur dioxide, composed of one sulfur molecule and two oxygen molecules, is a completely different chemical from elemental sulfur.
SO2 may be added to fruit during processing, to aging wine, and to finished wine prior to bottling. These additions can occur in the form of powder as potassium metabisulphite (K2S2O5) or liquid (liquid sulfur dioxide). Potassium metabisulphite is far safer than liquid SO2; using the liquid form safely requires taking hazmat-like precautions.
Sulfur dioxide is very reactive, which is both a blessing and a curse. After it’s added to wine, a portion of the sulfur dioxide (roughly one-third) binds with many chemicals in wine. SO2 can be divided into two categories: free SO2 and bound SO2, which together are known as total SO2. Free SO2 is simply non-bound SO2, and its levels generally fall between 20 and 50 milligrams per liter in finished wine.
Free SO2 is the portion that offers protection to the wine and is represented by the three chemical forms of SO2, which are formed when it is dissolved in water:
All three species are in a continual state of cleaving and combining with H2O, but at a given pH, reach an equilibrium such that the proportion of molecular versus bisulfite ions is relatively constant. Very importantly, because the wine’s pH determines the amount of each of these three species of SO2 that is present, and because each plays a different role in protecting the wine, SO2’s effectiveness is pH dependent. As pH increases, the bisulfite ion concentration increases and the molecular form of SO2 decreases. Because the molecular form is more abundant at lower pH, SO2 is more effective at achieving microbial stability at a lower pH.
Bound SO2 is of no consequence to the protective nature of SO2 relative to oxygen and microbes. Free SO2, which can be further divided into molecular and bisulfite forms, is the portion that is relevant to the winemaker.
Though SO2 is characterized as an antioxidant, it has very little direct interaction with oxygen. However, the bisulfite ion does react with many other compounds in a wine, including acetaldehyde, quinones, monomeric anthocyanins, uronic acids, keto acids, xylosone, and glucose. Compounds like acetaldehyde and quinones are responsible for oxidative properties in wine, and when the bisulfite ion intercepts either of these compounds, oxidative processes are hindered.
The role of molecular SO2 is best analyzed in the context of wine pH. At higher pH levels, molecular SO2 is barely present in wine and therefore has little protective capacity. For full microbial effectiveness, the levels of H2SO3 must be in concentration of 0.825 milligrams per liter (equivalent to parts per million). While this is not difficult to achieve for a crisp, high-acid white wine, for reds where pH generally ranges from 3.5 to 4 after malolactic fermentation, it’s essentially impossible.
For example, at a pH of 3, 6% of the SO2 in wine will be in molecular form. A winemaker would need to achieve 14.8 milligrams per liter of free SO2 to reach 0.825 parts per million of molecular SO2, fully protecting the wine. However, at a pH of 4, only 0.6% of SO2, is in molecular form. To raise the molecular form to 0.825 parts per million, the winemaker would need to attain 141 milligrams per liter of free SO2. Yet at this level, which is well above what is acceptable in good winemaking practice, sulfur dioxide would be perceptible. (The maximum legal limit in the United States for SO2 additions to wine is 350 milligrams per liter.) For red wines, a target of 0.5 parts per million molecular SO2 is more realistic, but at this level, the wine is not fully protected and there is the possibility for microbial activity.
Additionally, sulfur dioxide is much more effective against bacteria than against yeast. For example, free SO2 of 50 parts per million causes only a slight inhibition of Saccharomyces cerevisiae, while 30 parts per million of SO2 will likely inhibit the performance of lactic acid bacteria (Oenococcus oeni). Additionally, yeast sensitivity to sulfur dioxide is strain dependent. Saccharomyces cerevisiae is more tolerant than spoilage yeast. Yet even within the rogue yeast category, two common spoilage yeasts, Dekkera anomala and Brettanomyces bruxellensis, have shown various tolerances to SO2, with Dekkera being more tolerant than Brettanomyces.
For these reasons, and because the amount of SO2 in bottle decreases over time due to oxidation, sulfur dioxide cannot be relied upon to prevent microbial activity once the wine is in bottle. In the presence of active microbial populations, other measures, like sterile filtration, must be employed to prevent microbial issues. Moreover, SO2 cannot be relied upon as an antioxidant without other steps taken in the winery to avoid oxygen ingress: topping up, sparging headspace with inert gasses, and storing wine at cool temperatures. As wine oxidizes, more and more SO2 becomes bound, and its effectiveness wanes.
It goes without saying that SO2 does not exist in a vacuum. Its interactions with other chemical compounds in wine can change color, flavor, and texture.
The primary aldehyde in wine is acetaldehyde. Acetaldehyde may be generated from yeasts during fermentation, by acetic acid bacteria, or through the oxidation of phenols and/or ethanol. SO2 is particularly reactive with aldehydes. This is important both for reds and whites, but white wines are more affected by aldehydic characteristics than reds because of their lack of protective polyphenols. In a young white wine, aldehydes are present at levels under 100 milligrams per liter (for comparison, Fino Sherry typically has a concentration of aldehydes at levels of 250 to 500, or even higher).
The form of sulfur dioxide that is reactive with acetaldehyde is the bisulfite ion. O2 binds with acetaldehyde during both fermentation and élevage, reducing the drinker’s perception of its nutty “oxidative” and “bruised apple” character. In other words, on its own, acetaldehyde produces an off-aroma of oxidation, but when it binds with SO2, it loses its odor.
Phenols describes a group of various chemical compounds that influence the texture and color of wine. Phenols are found in all parts of the grape: skin, seed, pulp, and stem. In extremely simplistic terms, the presence of acetaldehyde, phenols, and SO2 in red wine influences the color and texture of a wine. Left to their own devices, tannin and anthocyanins will bond together via bridges created by acetaldehyde molecules. These are called polymeric compounds, and the process of these molecules combining is called polymerization. When SO2 is added to red wine, the bisulfite ion attaches to the anthocyanin molecule in the same place that tannin would bind, resulting in the reduction of these polymeric compounds.
Anthocyanins are pigment compounds in juice and wine responsible for color in young wines. When SO2 is added to the fermentation tank, it will quickly bind with anthocyanin, “bleaching” the color of the juice as at wine pH, monomeric anthocyanins are colorless. This process is reversible under the right conditions, and the bond may break, returning the juice or wine to its original color.
In the presence of the bisulfite ion, less polymerization will occur, which may lead to wine with more unbound tannin molecules, potentially producing more bitter and astringent wines. However, it’s worth noting that this is an extremely complicated subject and the presence of many other compounds that are reactive with tannin molecules, namely proteins and anthocyanins (resulting in the formation of polymeric pigments), may mitigate this effect.
Thiols are chemical compounds that produce a wide range of aromas, from fruity and flinty qualities when they are present in small amounts, to garlic or cooked cabbage when present in larger amounts. Esters are organic acids that also contribute to fruity aromas and are born during alcoholic fermentation. Both are influenced by sulfur dioxide additions. Thiols in particular are prone to oxidation. Esters are ephemeral and may be quite pungent in very young wine but will naturally wane over time. Both are present in higher quantities in aromatic varieties, though it’s important to note that this is yeast-strain and fermentation-temperature dependent. Given all these parameters, ester and thiol aromas will be preserved with greater sulfur dioxide additions. This is particularly important in aromatic varieties that are aged in stainless steel with little oxygen contact, such as New Zealand Sauvignon Blanc. Barrel aging has the opposite effect. It will reduce thiol concentration.
The presence of sulfur in wine is incredibly complex, influencing various chemical reactions that affect the style and flavor profile of a finished wine. Sulfur, though often disputed and misunderstood, is an essential winemaking tool: a single compound that has transformed our ability to produce wine that is stable, free of many faults, and ageworthy.
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Amy... excellent article. Thank you!
This was such a great article!! Super helpful in my understanding of SO2, I will never titrate the same again!
Thank you for the excellent article Amy, very informative!