Since you can’t measure your free and total SO₂, let’s do some numbers to see what kind of a potential problem you might be facing. First off, let’s talk about your bottle-rinsing solution of 3 tablespoons per 4 liters of water. 3 tablespoons of KMBS (potassium metabisulfite) powder is equivalent to about 3 x 13 g (approximate weight of a tablespoon of KMBS), or 39 grams. Divide by 4 and your concentration is 9.75 g/L, or 0.975 g/100 mL, or 0.975%. That’s a pretty strong solution and let’s say that about 1 mL of that solution hung around in your wine bottle, which would contribute 0.00975 g of KMBS powder per bottle. Now, KMBS is only 57.6% sulfur dioxide, so then the full contribution is 0.0056 g, or 5.62 mg of total SO₂ per 750 mL bottle.
Let’s turn that into a language we’re more familiar with when talking about sulfur dioxide levels: ppm, or mg/L. If your bottles are 750 mL bottles that’s 0.75 L or 7.49 mg sulfur dioxide per liter of wine, 7.49 ppm. That’s not a huge addition, but if you already had a high level going into bottling, it certainly will add to it. As a point of reference, I typically like to bottle with a free (not total, which we’ve just calculated) SO₂ level of around 25–30 ppm, depending on the pH and the amount of SO₂ already in the wine. My total SO₂ at bottling (depending on the age of the wine and how many additions I’ve had to make over a wine’s lifetime) tend to hover around 75–125 ppm. As another reference point, the legal limit of total SO₂ in commercial wine in the U.S. is 350 ppm.
Now let’s look at how much SO₂ you added in your last addition, or 9 mLs of a 10% solution at your last racking. Your carboy is 20-L volume. A 10% solution can also be written as 10 g/100 mL, or 100 g/L. Since I don’t know whether you’re talking about a 10% solution as KMBS (which is 57.6% SO₂) or as SO₂, it’s hard to do the next step, but I’m going to assume you’ve got a 10% solution as KMBS. So, to 20 L of wine you added 9 mL of a 10 g/100 mL solution. In 1 of the mLs of your solution there are 0.1 g of KMBS, so you just added 0.9 g total, or 0.52 g of SO₂. In mg/L terms, you added 520 mg/20 L, or 26 ppm total as sulfur dioxide (not “free,” just “total” SO₂).
One of the challenges here is that I don’t know what the free and total SO₂ level of your wine was before that addition. If you’d been keeping your wine at about 25 ppm free SO₂, say for a year or so, and then added a total hit of 26 ppm on top of that, plus the 7.49 ppm leftover from your rinsing solution, it’s not too much of a stretch to think you could be looking at a free SO₂ of between 40–50 ppm, depending on the underlying chemistry of your wine. And indeed, that’s enough “extra” free SO₂ to where I could see you thinking the wine was a little hard to handle at the moment. You’re even in higher territory if you were talking about your 10% solution being “as sulfur dioxide,” in which case you’d be adding 900 mg/20 L, or 45 ppm total as sulfur dioxide.
So what can one do if one’s added too much sulfur dioxide to a wine? Since they’ve been all bottled up it’s a little hard to affect any kind of change, unless you want to open them all up and decant them . . . which you could do. My recommendation is to send a sample of the wine to be tested at your closest wine lab. Ask your local fermentation supply shop for contacts if you don’t have one already. I’d like you to actually see your free and total SO₂ numbers before you embark on any treatment because in this case sometimes the cure can be worse than the disease. If indeed you do have unnaturally high levels of free SO₂, how high it is will dictate my advice to you. If your free SO₂ is around 35–45 ppm and your total SO₂ is under 200 ppm, my advice is that you do nothing and just wait. The free SO₂ level will go down over time, so check on how a bottle tastes in another month or two. You might be pleasantly surprised.
If your free SO₂ is over 50 ppm and your total level is over 200 ppm, it’s unlikely that the wine will return to “drinkable” anytime soon. In which case, the best choice is to open all the bottles, decant (under a blanket of CO2 gas or argon to protect it from oxygen), and blend the over-sulfited wine with wine that has lower levels. This is a great solution . . . but only if you happen to have extra wine lying around that has lower levels of free SO₂. You may know that one of my mantras is “never blend a loser” but in this case the wine may be fine, it just needs its free SO₂ levels balanced out a bit. Before you blend, however, do send your prospective blenders out to a lab to measure their free and total SO₂ to see if the math even works out. It’s a good idea to have the free SO₂ going to bottle never be above 40 ppm, and ideally keep it between 25–35 ppm, depending on the pH of the wine (high pH wines need more SO₂).
There are some people who say that over-sulfited wine can be treated with hydrogen peroxide, but as I’ve never done it and as hydrogen peroxide is a powerful oxidizer and can utterly ruin your wine, I don’t recommend it. Instead, I’d go with one of the approaches I’ve discussed. You don’t want to create an even bigger problem by trying to solve the first one when it’s possible that all you need to do is wait.
Since the earlier calculations don’t imply gross over-sulfiting here, I will mention one other possible sulfur-stink problem winemakers encounter . . . hydrogen sulfide (rotten egg odor). You could try a “penny test” or if you have any copper sulfate solution, pipette a drop of that into a glass with 50 mLs of wine to see if it helps. If either of those tests resolve the sulfur situation, you will then need to do a proper bench trial to determine what’s the smallest addition needed to solve the problem, then add that amount to the batch. For more instructions on this, check out my “Wine Wizard” column in the June-July 2021 issue or at https://winemakermag.com/wine-wizard/properly-adding-copper