Winemakers come from many different disciplines. I’ve known English teachers, architects, plumbers and even philosophy majors who were all excellent winemakers. Another thing they all had in common was a weak knowledge of chemistry. While a chemistry background is helpful in winemaking, it isn’t a prerequisite. Even though nearly everything we do with wine deals with chemistry and biochemistry, we do not need to understand these reactions on a molecular level to make use of the results.
HIGHLIGHTS
- How much sulfur dioxide to add and when are probably the most-asked questions from beginning winemakers.
- The author explains the difference between free SO2 and bound SO2 as well as the importance of pH in the efficacy of SO2.
- Additions pre-fermentation and immediately post-fermentation are discussed. SO2 during aging and bottling will be covered in part two.
Like so many wine lovers, I enjoy cooking, but I’ve never taken a cooking class in my life. Despite this, I can still crank out a mean pot of gumbo (albeit a beer dish). I can do it from memory, adjusting the ingredients by taste. But for the first pot of gumbo I ever made, I followed a recipe to the letter, carefully measuring every ingredient. This recipe provided a proven starting point from which to build my knowledge and, eventually, my own style.
Wine has far fewer ingredients than a pot of gumbo. An ingredients label for wine might simply read: grape juice, yeast and sulfur dioxide. In the wine cellar, the winemaking process could be pretty straightforward in regards to the amount of yeast and when and how to add it, but when it comes to sulfur dioxide, there are an endless number of caveats. When and how much sulfur dioxide to add are probably the most-often-asked questions from beginning winemakers. Why does this one ingredient have to be so complicated? One thing is for sure: If gumbo had an ingredient as complex as sulfur dioxide, I would never have attempted it in the first place.
Like sulfur dioxide, electricity can be pretty complicated, but we all use it successfully, and so I believe we can do the same with sulfur dioxide. This article provides a strategy for the successful use of sulfur dioxide in winemaking, allowing for its caveats while providing a recipe of sorts as a framework into which you may weave your experience, individual requirements and style.
The most important additive
Sulfur dioxide (SO2 for short) is by far the most important additive used in wine. Many times it is the only additive. Its value derives from its ability to perform several critical functions. It preserves a wine’s freshness and fruit character by virtue of antioxidative, antimicrobial and anti-enzymatic properties. There is no other additive available to winemakers that can singlehandedly play all of these roles.
The judicious use of SO2 is required to make high-quality, shelf-stable wine. That said, it is important to note that wine can be ruined by excessive use of SO2. As winemakers, it is our task to produce good wines, and good winemaking does not use SO2 as a substitute for sloppy habits.
To use SO2 judiciously, we need to understand two basic characteristics of this “complex” molecule. The first is the difference between free and bound SO2. The second is the role of pH in the efficacy of SO2.
It is because SO2 is a highly reactive molecule that it performs so many functions in the process of making wine. It bonds with numerous things, most notably acetaldehyde and anthocyanins. Anthocyanins are responsible for the color in red wine, while acetaldehyde gives wine a nutty, oxidized, sherry-like aroma. Yeast, bacteria and the chemical oxidation of ethanol produce acetaldehyde. When SO2 is added to wine, it reacts with acetaldehyde, greatly reducing its aroma impact, but SO2 reacted in this way is rendered unavailable to perform its other functions and is considered “bound.”
On the other hand, SO2 that is un-reacted is considered “free.” Free SO2 is what is still available to protect the wine. The term “total SO2” refers to the sum of both “free” and “bound” SO2.
SO2 is typically added at various points in wine production: pre-fermentation, post-fermentation, aging and bottling. Each of these stages has a different decision-making process for determining how much SO2 to add.
Pre-fermentation additions
SO2 has been traditionally added to crushed grapes or freshly pressed juice to inhibit undesirable yeast and bacteria that have been shown to produce excessive levels of odiferous esters and outright spoilage. SO2 also decreases enzymatic browning in must and potential loss of fruit character. When I worked at the Ohio Agricultural Research and Development Center many years ago, we did some experiments examining timing and amount of initial SO2 additions. In the study, hand-harvested Seyval and Vignoles in sound, ripe condition were divided into four different lots, each of which received an initial dose of 50 ppm SO2 at different points of production.
Lot 1: SO2 added directly after crushing and before pressing.
Lot 2: SO2 added directly after pressing and before juice clarification.
Lot 3: SO2 added after clarification and before fermentation.
Lot 4: SO2 added after fermentation at first racking.
The SO2 was adjusted to the same level in all lots for aging and bottling.
Each lot was done in triplicate, and the finished wines were presented to a tasting panel for evaluation. The results showed that the earlier SO2 was added in the winemaking process, the better the finished wine was rated. Volatile acidity was also lowest when SO2 was added directly after crushing. Based on this information, I now make my initial SO2 addition as early as possible.
In a follow-up experiment we looked at the impact of various amounts of SO2 added at crush. Additions of zero, 20, 40, 80 and 120 ppm SO2 were added. In this case, the tasting panel preferred wines with additions of 20-80 ppm, with zero and 120 ppm treatments being significantly less desirable. These results give a reasonable range of SO2 to consider as pre-fermentation additions.
But how does a winemaker decide on the most appropriate dose? Two important factors to consider are the condition of the fruit and whether or not the wine will go through malolactic fermentation.
Since the most important function of a pre-fermentation SO2 addition is to inhibit undesirable microorganisms, grapes in less-than-stellar condition or with a higher pH should receive SO2 at the higher end of this range. Warm grapes are susceptible to faster microorganism growth and also should receive a higher dose. Cold, clean fruit, fresh from the vineyard, can get by with a dose closer to the lower end of this range. When adding SO2 to crushed grapes, thorough mixing is required. Good results can be achieved by adding the SO2 upstream of the destemmer-crusher and letting the crusher and must pump do the mixing for you.
Post-fermentation additions
As mentioned earlier, yeasts produce a sulfur-binding compound called acetaldehyde. As this compound is produced through the course of alcoholic fermentation, it is bound up by the available SO2. If you measured the SO2 immediately after fermentation, you would find little to no free SO2 and a total SO2 less than half of your pre-fermentation addition. The resulting loss would be due to binding, precipitation and vaporization. What this means is that for all practical purposes, the winemaker is starting from square one again.
Unless there is a compelling reason, wine destined for malolactic fermentation (ML) should probably not receive any SO2 at this point. Lactic acid bacteria are highly susceptible to SO2. Instead, initiation of ML should take place as soon as possible.
All other wines—whites especially—should receive sulfur at their first racking. The following philosophy makes this addition fairly routine for most wines:
1. I want enough SO2 present to protect the wine.
2. With alcoholic fermentation complete, the wine can be kept cold to retard microbial activity.
3. In most cases the wine won’t be consumed for several months at the earliest, and a slightly elevated SO2 at this stage is not an issue.
4. Better protection is afforded by one larger dose of SO2 than by more frequent smaller doses.
The above philosophy, combined with experience, has led me to a standard addition at first racking of 80 ppm. The dosage of SO2 is calculated and added to the receiving tank as it is filling. I understand a one-size-fits-all answer is simplistic, but an addition at this level usually provides adequate protection through spring without going too high.
Extenuating factors might include plastic tanks, warm cellars, a pH above 3.5, or late-harvest wines, all of which might call for 10-20 ppm more SO2.
This same addition is acceptable for wines following completion of ML. When using paper chromatography to determine a wine’s malate status, be aware that a wine can read malate negative and still contain traces of malate. For this reason, I like to wait a week or two after a negative malate reading to ensure completion of the secondary fermentation. If diacetyl (buttery aromas) levels are excessive at the end of ML, delay SO2 additions to give the yeast a little more time to metabolize the compound.
As noted earlier, SO2 will bind with anthocyanin, resulting in a phenomenon known as sulfite bleaching. The end product of this reversible reaction is a colorless compound called anthocyanin-4-bisulfite. This can be an issue in lighter reds and rosés, and care should be taken not to overdo SO2. This may be a case where a lower SO2 addition is in order. Lab trials can be used to predict the amount of color loss you can expect. With time, most anthocyanins will revert to their colored form.
When stopping fermentation with refrigeration, it is best to chill the wine to cessation before adding SO2 to avoid stimulating acetaldehyde production by the yeast.
This significant SO2 addition early in a wine’s life will afford some degree of comfort if you get busy with other projects. With blending, clarifying and aging, the wine enters the next stage of production that requires a more individualized approach to SO2 usage. Ways to monitor and adjust SO2 during aging and bottling will be discussed in part two of this article.
Chris Stamp is president and winemaker at Lakewood Vineyards in Watkins Glen, N.Y. He started his career in wine as winemaker at Plane’s Cayuga Vineyards in the Finger Lakes, and in 1986 took a position as enology research and extension associate at the Ohio Agricultural Research Development Center in Wooster, Ohio. When his family opened Lakewood Vineyards winery in 1988, he returned to New York to become the winemaker there.
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