Sulfur dioxide is used in winemaking for four principal reasons. It acts as an antioxidant, an antimicrobial, and an antioxidasic, and it binds with undesirable products of oxidation, rendering them undetectable from an organoleptic perspective. Despite increasing interest in lowering additives in wine, no single alternative to sulfur dioxide has been found that will adequately perform all of these roles. However, numerous additives and processes may substitute for sulfur dioxide in one or more of them. The principal additives that are alternatives are lysozyme, Velcorin, sorbic acid, and ascorbic acid. The two processes of sterile filtration and heat treatment are alternatives as well. The advantages and limitations of each of these will be examined below. For the purposes of this essay, “sulfur dioxide” is interpreted to include all forms of sulfur used in winemaking that turn into sulfur dioxide in must or wine.
The first use of sulfur dioxide at the winery is normally when the grapes arrive to the crusher. This early addition is designed to take advantage of all four of sulfur dioxide’s properties. It will help reverse the evidence of oxidation that the grapes may be displaying due to damage in the vineyard or during harvesting, principally by binding with the products of oxidation such as acetaldehyde. In addition, most wineries prefer to kill wild yeast strains such as Kloeckera and Candida, which are known to produce more acetic acid during fermentation than commercial yeast strains. Sulfur dioxide is also effective against Lactobacillus at this stage, a desirable property for wineries wishing to delay or prevent malolactic conversion. In its third role, as an antioxidasic, sulfur dioxide used at the crusher will denature low levels of oxidase enzymes, notably laccase and polyphenol oxidase.
The amount of sulfur dioxide added at this point varies from 0 to about 100 ppm and will depend on whether the grapes arrive carrying some elemental sulfur from treatments in the vineyard, the health of the grapes, and the fermentation plan for the must. If the grapes have received a sulfur treatment in the vineyard after the last rain, less sulfur dioxide will be used at the crusher. Winemakers seeking a natural yeast ferment or to allow malolactic conversion simultaneous with the alcoholic fermentation will add smaller amounts as well. Domaine Chanoux in Bordeaux normally adds about 25 ppm at the crusher. However, in 2013, the grapes arrived in poor condition due to disease and an excessively wet vintage; thus, 70 ppm were added. As a result, the winery switched from using indigenous yeast to a cultured yeast for that year, as cultured yeasts are more resistant to sulfur dioxide’s antimicrobial properties. (Note: All examples cited are drawn from actual wineries, but due to the sensitive nature of the topic and the public nature of this essay, the names have been changed.)
Sulfur dioxide is not generally added during fermentation, as it may inhibit desirable yeast growth and produce unacceptable levels of hydrogen sulfide. Moreover, the risk of unwanted microbial growth and oxidation during this period is minimal.
Most wines receive an addition of sulfur dioxide as soon as primary fermentation is finished. At the Hans Maier Winery in Austria, the amount is very small for the Chardonnay, just 7 ppm, as the wine needs to undergo malolactic conversion. However, the dry Rieslings will receive enough to bring the molecular sulfur to a level of 0.8 ppm. At the low pH typical of Riesling, this normally occurs with about 10 to 20 ppm free sulfur dioxide.
The last addition of sulfur dioxide is at bottling and therefore beyond the scope of this essay. However, winemakers must take care not to drive up sulfur dioxide levels early in the winemaking process to make sure there is room for this final adjustment without surpassing sensory or regulatory limits.
When an alternative to sulfur dioxide is required, it is often because the must has a high pH, an environment in which sulfur dioxide is less effective. One of the more common alternatives in such a situation is lysozyme, a powerful antibacterial agent that is used to neutralize spoilage organisms such as Pediococcus. It is also effective against Lactobacillus to prevent malolactic conversion. It is legal in most jurisdictions with a few exceptions, including Canada. A significant drawback to lysozyme is that it is not effective against gram-negative bacteria, such as Acetobacter, and therefore cannot perform all antimicrobial functions of sulfur dioxide. While lysozyme is inexpensive and very effective, its popularity has been declining with interest in “vegan” wines and growing requirements for allergen labeling because of its origin in egg whites. Nonetheless, the winemaker at Allen Brown Vineyards in New Zealand uses lysozyme on several white wines to prevent malolactic fermentation; he prefers this to a high dose of sulfur dioxide, which he feels has negative organoleptic consequences.
Velcorin (the trade name for dimethyl dicarbonate, or DMDC) is an antimicrobial used as an alternative to sulfur dioxide to inhibit yeast growth. It is preferred by some winemakers because, though poisonous before it hydrolyzes, it breaks down into methanol and CO2—components that occur naturally in wine. Unlike many of the additives mentioned here, DMDC is often used on premium red wines due to its effectiveness at neutralizing Brettanomyces. However, it is a hazardous chemical that requires special handling and is expensive. It can also cause quality problems, such as increased methanol levels and undesirable organoleptic effects. Moreover, while it is a permitted additive in countries such as the United States and Australia, it is not permitted in others and is limited to 200 ppm in the EU. Whither Valley Vineyards in Paso Robles used DMDC on its Zinfandel in 2015 after a significant Brettanomyces problem caused by infected barrels.
Sorbic acid is a powerful antimicrobial that is used widely in the food and beverage industry. It is typically employed as an alternative to sulfur dioxide on off-dry, low-alcohol wines, which present a particular challenge in terms of microbial stability. It is effective against yeast and some bacteria, but not Lactobacillus. When metabolized by Lactobacillus, an off-odor characterized as geranium leaf is normally produced. Additionally, a small minority of individuals are sensitive to sorbic acid at relatively low concentrations and will perceive the wine as having a particularly unpleasant aroma. Green State Vineyards in California use sorbic acid on its bag-in-box White Zinfandel, a low-alcohol, medium-sweet wine that is difficult to stabilize.
Ascorbic acid and its isomer, erythorbic acid, deserve a mention. Ascorbic acid was once widely used, particularly in Australia, on white wines because of its potent antioxidant capacity (greater than that of sulfur dioxide) but has fallen out of favor due to its limitations. If molecular sulfur dioxide levels fall too low after bottling, hydrogen peroxide, a powerful oxidizing agent, may be produced. It is also less effective on wines below 11.5% ABV, which constitute most wines in need of exceptional measures to ensure stability. McPaul Brothers in Australia previously used it extensively but has reduced use to just a few off-dry whites due to these limitations. Since ascorbic acid must be used in conjunction with sulfur dioxide, it is not technically an alternative.
In terms of processes that present an alternative to sulfur dioxide, one of the most common is sterile filtration followed by aseptic bottling. This process requires exceptional measures to avoid any contamination of the wine or bottle after filtration with a membrane of 0.45 microns. Sterile filtration is less common on structured red wines. While there is much research to the contrary, many winemakers believe it has a negative effect on texture and aroma. While sterile filtration will prevent microbial growth in the bottle, sulfur dioxide may still be used to prevent oxidation. The equipment required for sterile filtration came into common use after WWII and allowed the production of light, off-dry wines such as Lancer’s Rosé and Riunite Lambrusco. These wines are also bottled with some residual CO2, which reduces the amount of sulfur dioxide needed to keep them fresh.
A somewhat harsher alternative to using sulfur dioxide to eliminate microorganisms is to heat wine or must through processes such as flash pasteurization, tunnel pasteurization, or thermotic (hot) bottling. Heat treatment kills microbes (bacteria as well as yeast) and denatures oxidase enzymes, but these processes reduce fruit character and can create unwanted aromas and flavors. From an organoleptic standpoint, it is far preferable to heat treat juice than wine, but this is not as secure a method of reducing microbial activity as heat treatment following bottling. The négociant Louis Couture in Burgundy was known to flash pasteurize many of its musts in previous decades. Improvements in the vineyard in recent years have resulted in healthier grapes, rendering the practice unnecessary.
Winemakers may also choose not to use sulfur dioxide or any of its alternatives. “No added sulfur” wines will often display more volatile acidity and show developed aromas and flavors earlier than wines treated with sulfur dioxide. An increasing number of winemakers and consumers interested in “natural” wines believe the trade-off is justified and may even prefer the oxidative aromas. Yet these wines are more susceptible to damage due to poor shipping conditions. Villa Stranoggia in Italy makes an organic Cabernet Sauvignon without added sulfur. The wine has shown a reduced fruit profile in some export markets.
Despite the many downsides to sulfur dioxide and the current focus on reducing additives in the food and beverage sector, it does not appear that “no sulfur dioxide added” wines will become the norm anytime soon. Nonetheless, better work in the vineyards, improved cellar hygiene, and more protective handling of musts and wines have dramatically reduced the use of sulfur dioxide in the winery over the past 50 years. Multiple alternatives to sulfur dioxide exist, but all of them have substantial drawbacks, which prevent their universal adoption. No one alternative to sulfur dioxide can fulfill all four of its important roles of antioxidant, antimicrobial, antioxidasic, and neutralizer of undesirable products of oxidation.
Jean, thank you for this article. As a new minted MW student seeing examples of how to formulate a brief, example rich essay is incredible valuable, especially with topics that are as broad as sulfur dioxide use.
Mevushal Kosher wines made via flash pasteurization will often state this in their marketing materials because this is less damaging to quality than other methods of making a wine Kosher. Although most Rabbis believe that flash pasteurizing makes a wine Mevushal, there is not universal agreement. It is difficult to get concrete data on non-Kosher use of flash pasteurizing as it is not something wine companies want to share. My anecdotal impression is that it has been decreasing over the past few decades.
Thank you for taking the time to comment, James. The essays in this section of GuildSomm have a word limit - which this essay already exceeds. The idea here is to give students, particularly MW students, an idea of how to address the most important aspects of a given question while working under a significant time constraint. Passing a question like this is as much about what is left out as what is included. Sulfur is a huge topic; I'm sorry I couldn't cover it to your satisfaction within the constraints of this section.
I value the perspective that MWs bring to this series, and the sober, fact-based quality of the writing. And this is full of interesting information, yet frustratingly incomplete.
Fundamentally, I don't understand why it spends a short paragraph addressing winemakers who don't add sulfur at any step (and conflating them with "natural" wine) when the essay limits itself to sulfur addition until the end of fermentation--which was also a bummer. (The article, after all, is "SO2 and Its Alternatives"). What about the use of sulfur at racking? What about bottlings with excess CO2? Why is sulfur at bottling beyond the scope of the essay? Sterile filtration, which happens right before bottling, merited inclusion--and rightly so, although it is strange to see it presented as an alternative to sulfur when it's usually accompanied hand in hand.
Would have loved to see additional information on, for example the form in which sulfur is added, and how it's chemically bonded, which vigerons I've talked to have opinions about. The article also loses a chance to talk about the concept of "free" sulfur, and I'd love to learn more about how much is a typical byproduct of fermentation. And--well, look, I don't want to criticize a piece on not talking about things I wanted it to talk about. I just wish there'd been a little more context for a subject which sees a lot of polemic, consumer attention, and misunderstanding at all levels.
Brilliant article, thank you!