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Sulfite Question When Blending Wine

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Greg Ambrose — Pottstown, Pennsylvania asks,
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 at www.winemakermag.com/sulfitecalculator. 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?

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
Response by Alison Crowe.