Q: I have two wines made from grapes and stored in oak barrels. During aging I have monitored for free SO₂ routinely (every 3-4 months) and topped off with the appropriate sulfite necessary based on the pH and the calculations from the sulfite calculator. Herein lies my question: I would like to blend these wines prior to bottling. If the free SO₂ is appropriate for each wine separately, will I need to retest and adjust again after they are blended? Or can I assume that the combined wine will have the correct free SO₂ based on the fact each separate wine was adjusted properly?
— Greg Ambrose • Pottstown, Pennsylvania

A: You bring up a very good question. For the compound you’re talking about, sulfur dioxide, you’ll probably come pretty close to what you would predict based on knowing the volume and the current free SO₂. This is just like alcohol, residual sugar, or titratable acidity and can be predicted based on a simple volume to analysis algebraic equation. The one analysis number that never seems to behave this way is pH. This is due to the “buffering capacity” of pH and how it will change based on so many other components in the wine. Because of this, when it’s blended and encounters so many new components, it may respond in slightly unpredictable ways (say, change from a 3.65 to a 3.62, if you were blending two different wines with the same pH together) and will be impossible to calculate as a true algebraic average.

This is where it does get a little tricky with free SO₂ and why I say you’ll likely get pretty close . . . and not exact. Free SO₂ is pH-dependent so I would expect to get a little FSO₂ shift up or down depending on how much the pH of your blend moved. For this reason, it’s always best to re-check your FSO₂ after a major move or blend and adjust accordingly.

Want a quickie “cheat” or can’t send your sample to a lab? In my experience moving and blending wine around over the years, you should be safe (or safer) if you add about 10 ppm SO₂ to the wine after you’ve moved it. If you’ve come down a bit you’ll bump it back up, and 10 ppm over “ideal” range isn’t a deal breaker. Ideally we measure volatile acidity (VA) and FSO₂ every month so we can bump the FSO₂ back up when we top monthly. At the very least, you’ll want to measure FSO₂ as you approach bottling.