Thanks, Bob, for joining fellow columnists and me for our first ever virtual WineMaker Harvest Boot Camp this past September! This question didn’t get answered in my Q&A session held at the end of my “Top 5 Harvest Mistakes to Avoid” talk (there were a number of others that didn’t get answered either that I’ll try to get to in future columns) so, considering the depth of this question, I wanted to cover it here.
SO2, otherwise known as sulfur dioxide, is added as an antioxidant and anti-microbial agent and has been used in winemaking since ancient times. Yeast cells even produce a small amount of sulfur dioxide themselves during the fermentation process, so in fact, there really is no such thing as a SO2-free wine, no matter how many folks touting “natural wine” or other kinds of vinous charlatanism would like us to believe otherwise.
So back to the question of when it’s opportune to first add sulfite to wine: I almost always add a little bit, say around 25 ppm total, at the crusher or juice stage for reds, whites, and rosés. This is to knock down the bacteria and mold populations that came in on the grapes, making room for my Saccharomyces, the good yeast we want, to take hold (they are not as sensitive to SO2). I make a KMBS (potassium metabisulfite) solution and just sprinkle it on the fruit.
For whites and rosés that I do not want to go through malolactic fermentation (MLF), I wait until the wine is settled down after primary fermentation is done (at least 24–48 hours). Next I’ll transfer to a new vessel and add about 30 ppm total to the wine. I’ll wait another 24 hours and make a another addition, making sure I’m measuring about 28–32 ppm free SO2. For reds, which I almost always put through the MLF process after pressing, I’ll perform a test for malic acid to verify that secondary fermentation is complete, then similarly, I rack and make the two SO2 additions to get my free SO2 around the same range.
Be sure to sample your lots monthly for VA (volatile acidity) and free SO2 and make little monthly adjustments to keep your wines 25–30 ppm free SO2 over the life of the wine until you bottle. That being said, make sure you’re keeping your containers topped up as full as possible. Having an untopped container is a recipe for oxidation, VA, and other microbial disasters, and no amount of sulfur dioxide additions will make up for high-oxygen ingress.