Taken from MoreWine's Free Guide to Red WineMaking Written by Shea Comfort
Sulfur Dioxide, or SO2, is a chemical compound used by winemakers to help keep their wine protected from the negative effects of oxygen exposure as well as spoilage microorganisms. Sulfur Dioxide is known by a variety of different names to winemakers, the most common being “SO2”, “Metabisulfite”, and just plain “Sulfite.” In winemaking, the SO2 concentration in a wine is measured in Parts per Million, or ppm, which refers to the number of Parts of Sulfite per Million parts of wine. This unit of measure is equivalent to mg/L, or milligram of SO2 per Liter of wine.
Sulfite Management is one of the toughest aspects of home winemaking to master, but also one of the most critical aspects of creating a high quality wine – commercial or home-made. Proper sulfite levels in a wine create a protective buffer that helps the wine withstand any accidental oxygen or microbial exposure that may occur during the ageing/storage process. The sulfite acts as an intermediary force that quickly intercepts and reacts with the offending element or organism before it can damage the wine. However, this is a one-way ticket. Once the sulfite becomes used-up, it is no longer available to react with future threats. In its most basic form, SO2 management simply comes down to understanding how to create and maintain a small, stable reserve of free SO2. However, just having a generic amount free SO2 in a wine does not guarantee you are safe. SO2 levels required to protect a wine are pH dependent. As the pH goes up, a higher free SO2 level is needed to protect the wine. This means that you can have 25 ppm free SO2 in your wine, but if your pH is above 3.3, the wine is still not adequately protected… Let’s take a moment to examine the nature of sulfites and how we as home winemakers can best manage them in order to help us make the best wine possible.
The basic timeline of sulfite additions needed to keep the appropriate levels is actually not as simple as it might seem. Many of the solids and chemical compounds in the wine interact with sulfite, and their concentration/presence has a direct effect on how any given sulfite addition behaves. To begin with, no two wines are ever the same; each one possesses a unique ratio of chemical compounds and solids that are present at varying concentrations. Depending on winemaking techniques, handling, or even sanitation issues, the differences can be quite pronounced. Because of the differences between two seemingly identical wines, they will often end up with different free SO2 levels after equal sulfite additions. This is important because it means that if we want to be accurate in our sulfite management, each wine will have to be evaluated individually once the SO2 has been added. Then, based on test results, additional sulfur can be added to achieve the target level. The simple formula used to make sulfite additions is a great starting point, but we cannot rely on this theoretical calculation alone in order to achieve our desired free SO2 level – it must be tested and corrected if we want accuracy. Remember: It does not matter whether based on your calculations you have added enough SO2 to achieve 35 ppm free if your test lets you know you only have 10 ppm free in the wine!
The discrepancy mentioned above between what we’ve calculated on paper and the amount of free SO2 we actually wind up with in the wine is coming from a phenomenon known as “binding” and is based on the fact that when sulfur is added to a wine, portions of the addition react with and become chemically bound to the aldehydes, acids, furfural, sugars (glucose), solids, yeast/bacteria, etc. Binding continues until all of the various reaction-able elements in the wine have either become bound up or there is no more free sulfite to interact with. This binding action actually serves to protect the wine; as long as there is free sulfur present, it is available to react with and effectively neutralize both oxidation and microbial spoilage threats. In effect, free SO2 can be viewed as an insurance policy that the winemaker takes out in case the wine has any problems during its lifetime: As long as you have the recommended amount of free SO2 your wine is protected.
Note: Barrels and tanks with headspace tend to lose their free SO2 more quickly than fully topped inert vessels do, due to the wine’s interaction with oxygen in the environment/headspace. In addition, even if there is no oxygen exposure, free SO2 levels can still decrease gradually during ageing due to normal chemical reactions taking place in the wine as it continues to evolve. Therefore, it’s a good idea to check the free SO2 levels of all of your vessels even if they are topped-up and have not been opened since you last checked them.
Maintaining this reserve of free sulfur means that once a wine starts to lose its free SO2 content, we are obligated to add more to raise it back up again. Yet, there can be too much of a good thing. If we don’t monitor our amounts and keep adding sulfur to the wine in an effort to maintain the required pool of free SO2, it is possible to add so much that the sulfur becomes detectable in taste, negatively impacting the wine. This is one of the fine lines that we as winemakers walk, one more example of the junction between artistry and science that is winemaking. We need to have a sufficient quantity of sulfur present in order to maintain the free SO2 levels needed to protect the wine, but we don’t want the levels to be so high as to be noticeable when we drink it. Therefore the goal of proper sulfite management in winemaking is learning to create the required amount of free SO2 in the wine while using the lowest total amount of sulfite possible. In order to help us do this it is important to take a further look into the implications of the binding process.
Please bear with us. We know that this is complex, but we have found again and again that increasing your understanding makes it more likely that you will be able to respond to a problem situation quickly and correctly. If you feel like your head is spinning a little bit from all this, we recommend that you go get a glass of wine – or in extreme cases perhaps even coffee…
Ok, back to it: Post fermentation, when we make our first SO2 addition (by calculating, testing the results and correcting to our desired level if necessary) we establish our starting point for free SO2 in the wine. If the wine remains completely sealed, apart from the normal and slight drop in the free SO2 levels, the level of free SO2 will remain fairly stable over time. However, as soon as we start to open the vessels up for tasting, testing, blending, fining or topping up barrels, we will begin to see a drop in the free SO2. This drop can be slight or quite drastic depending on how the wine is being handled. There are three main causes of the binding phenomenon responsible for a drop in the wine’s free SO2 levels: aldehyde formation, spoilage organisms, and the introduction of solids into the wine.
1. Aldehydes: When a wine is exposed to oxygen the alcohol in it oxidizes into chemical compounds called aldehydes. Aldehydes are a class of chemical compound that bind with SO2, resulting in lower free SO2 levels. In fact, there is a snowball effect often associated with aldehyde formation: As the aldehydes develop and react with the free SO2 in the wine, less SO2 is available to intercept oxygen. As a result, the incoming oxygen then reacts with more alcohol to create more aldehyde – and so on and so forth. This is the most common cause of a drop in a wine’s free SO2 and the most common cause of oxygen-related spoilage that we see in homemade wines.
2. Spoilage Organisms: If conditions are favorable and spoilage organisms contaminate the wine, this can create a cell mass that binds with the SO2. The end result is a lower free sulfur level in a wine. Most commonly these organisms will be Acetobacter (vinegar bacteria), Lactobacillus, or Pediococcus. Spoilage problems usually gain a foothold when depleted free SO2 levels – usually due to excessive oxygen exposure - make the wine vulnerable.
3. Introduction of solids: Any time we add solids into our wine, such as oak (which, being porous, also brings some oxygen with it), tannins, specialized yeast products, fining agents, etc. we will have some amount of binding going on, lowering the free SO2.
Now that we have taken a closer look at the elements that can bind-up our free SO2 levels, we can focus on how to eliminate or at least minimize the impact they have on our wine. For each of these three problems, (essentially large losses of sulfite concentration) there is a corresponding course of action we can take to counteract potential ill effects.
· Aldehyde formation: Since aldehydes form when alcohol oxidizes, if we eliminate or limit the amount of oxygen the wine comes into contact with then we also effectively eliminate or limit the amount of aldehyde that gets formed in our wines. This can be accomplished by flushing any air spaces that the wine will occupy with inert gas. Examples of these “air spaces” include the headspaces of vessels, transfer lines, pump cavities, filter housings, etc.
· Spoilage Organisms: Good sanitization practices and being vigilant about keeping free SO2 at the required level will help to keep any microbial issues at bay. This not only keeps the wine from developing off flavors from unwanted microbial action, but also limits the total SO2 additions to a minimum, lessening the risk of a negative sensory impact from SO2.
· Introduction of solids: Finally, when adding any solids into the wine, realize that a small portion of the sulfite will become bound to the newly introduced element in the wine. Therefore, we will need to add a little bit more SO2 to compensate for this. After its initial impact on sulfite levels, your oak or other additives should not continue to adsorb portions of future SO2 additions.
Hopefully, the information in this section will help you to better understand how maintaining proper sulfite levels in a wine is about more than just doing a calculation and adding it to the wine. The actual quantity of sulfite needed to maintain the recommended free levels in a wine is never a fixed, “one size fits all” amount; it will be different for each of our wines. The way that individual elements in the wine interact and bind with the SO2 needs to be understood and taken into account if we hope to create the stable free SO2 level needed to protect our wines during élévage. However, remember that stable sulfite levels in a wine do not mean you can suddenly become negligent with your handling. As we have seen, improper handling of the wine will only cause the binding reactions we are hoping to avoid and as a result, the free SO2 will drop and force us to keep adding more and more sulfur into our wines. By understanding how the system works, you are now better to able to prevent this scenario.
For a complete explanation of how to calculate the exact amount of SO2 needed for our wines, along with further information on sulfur management, Please see section 10.7 of our Red Wine Making Manual