Finding Ideal SO2 Levels
Q: After reading a good deal of literature (including your book, The Winemaker’s Answer Book) there seems to be a plethora of opinions regarding the amount of SO2 required at different stages and varying pH levels. I find much consistency with the “general” rules for maintaining adequate levels of SO2, but when searching for more specific information, I find that there is a wider range of thought that doesn’t necessarily coalesce. For example, in one book the author suggests that free SO2 levels should never exceed 50 ppm unless “you are trying to address some specific problem.” In a different book, the author provides a table showing the relationship between pH and required free SO2 levels (the higher the pH, the higher free SO2 required). For example, a wine with a pH of 3.7 would require free SO2 levels of 63 ppm, clearly above the first recommendation of 50 ppm. And, in several books, there is a recommendation to reduce free SO2 levels at bottling to 25–30 ppm, with no mention of adjustments for wines with a higher pH.
In trying to simplify matters, I researched the use of molecular SO2 measurements in lieu of free SO2 levels. If I understand correctly, molecular SO2 takes the wine’s pH into account, so any pH scaling requirements are already accounted for in a molecular SO2 target. Most books recommend 0.8 ppm for the proper SO2 level, but don’t mention reducing levels at bottling. Others recommend 0.5 ppm molecular SO2 for red wines and 0.8 ppm for whites. While reading the latest issue of WineMaker, I noted that the Syrah recipe called for bringing the molecular SO2 level to 0.8 ppm (the white wine level) and maintaining it through bottling. While I’m aware that different opinions are more often in play than not, the lack of clarity is giving me a major headache. At this point, my best option for red wines seems to be maintaining a post-MLF molecular SO2 level of 0.5 ppm through bottling. Can you help me understand?
— Steve Wise • Pleasanton, California
A: You do an excellent job of outlining one of the major conundrums we all experience in the winemaking world. How much SO2 do we need to add to our wines to keep them safe? How much is too much? To be very honest, it’s something even commercial winemakers do “by feel.” While I wish we could all hue to a rule like, “Maintain 0.8 ppm molecular SO2 until bottling,” the reality is that, even for “normal” pH wines (reds 3.55-3.65 for example) that’s simply too much SO2 to have to add. You’ll adversely affect the aroma and quality of your wine, and while microbial happiness is always a goal we strive to achieve, you simply can’t control your bugs by SO2 alone. We have to learn to live with a certain amount of risk in return for having a wine we can smell without burning our nose hairs off!
Your approach of maintaining 0.5 ppm molecular is quite reasonable, and may be a rule of thumb that you find works for you and your wines. Myself, I tend to keep all of my reds around a pH of 3.65-ish and find that maintaining FSO2 between 26-30 ppm until bottling is usually just fine. I keep wines sound this way in tank and barrel (with good topping) for about 12-18 months before I bottle. However, if my pH levels are higher for some reason, I may add 5 ppm SO2 more and I tend to watch my VAs and monitor for film yeasts a little bit more. In a quick non-scientific poll of my winemaking buddies here in Napa and around the state, we all agree that trying to achieve a molecular SO2 of 0.8 ppm is a nonstarter. We all tend to use a variation of my approach and find that it works for us.
Related Links:
For more in-depth information on SO2, read WineMaker magazine’s Winter 2000 article “Solving the Sulfite Puzzle”
