One of the most valuable things I have learned since beginning work in the industry has been the importance of doing things today. Oxidation doesn't wait, Brett doesn't wait, and we as winemakers should not either. This should be a guiding principle for winemakers when it comes to the use of sulfites. I say sulfites, not sulfur dioxide, to emphasize something I run into in Oregon a lot. There is a general lack of knowledge about the role of sulfites and what is actually happening when we add sulfites to a wine.
One of my favorite litmus tests for new wineries and winemakers is asking about their sulfur program. I would love to report that I often learn new facts about how other wineries maintain sulfur levels, but it simply isn't true. While we may not have the capital and resources that some of the large California wineries have, I believe there is still no excuse to be lazy or ignorant when it comes to the knowledge and testing of sulfites. But astonishingly there are plenty of winemakers who seem to be.
If you take one thing away from this post, realize this. The effectiveness of sulfur dioxide is directly related to the concentration of sulfites and the pH of the wine. A good winemaker would never let a lab get away with reporting just a free sulfur number, the pH and free sulfur should always be reported together. Sulfites exist as three distinct compounds at wine pH (Ok, 2 unless you are majorly messing up). There is the very familiar molecular sulfur, SO2, bisulfite anion HSO3-, and the sulfite anion SO3=. At 0 pH it will exist as almost 100% molecular SO2. When the pH gets to ~4.5 the species is ~100% HSO3-. The behavior of HSO3-/SO3= is very similar.
A quick chemistry aside:
We chemists rely on changing the polarity of chemicals as a way to get them to elute from a solvent. The aeration-oxidation test for free sulfur takes advantage of this principle. By adding excess phosphoric acid the pH is driven down to maximize the amount of molecular SO2 present. Molecular SO2 is very volatile, while the deprotonated forms will not come out of water easily. In addition there is a very nice oxidation reaction that takes place between SO2 and H2O2, making the molecular form the most easy to capture and quantify. So what we are doing with aeration-oxidation is converting all of the sulfite species to molecular sulfur dioxide, eluting the mSO2 into peroxide, and then titrating. This is a very convenient way to measure the amount of molecular sulfur at an artificial (0) pH, but it tells us nothing about the amount of molecular sulfur at wine pH.
This is why the concerned winemaker should always ask for a pH. The most effective antimicrobial form of sulfite is molecular SO2. I have heard many different concentrations, but in general we keep wines in the cellar at 0.8 ppm molecular SO2. Bear in mind that we make a lot of cold climate Pinots, so our pH is usually below 3.55 or so. Our lowest is a Riesling clocking in at 2.95. To be adequately protected, the Riesling would be kept at 12 ppm free, and the 3.55 Pinot at 46 ppm free. This opens up a huge discussion on the winemaker's decisions around bottling, but that is a post for another day.
It is also worth nothing that pH can be a limiting factor for sulfur. A Cabernet with a pH of 3.80 simply will not drink well with 79 ppm free. However, that is no excuse to be lazy. One must lab trial and come up with a level that balances detectability with protection; having too much sulfur by 15 ppm is a lot easier to deal with then having to remedy early and excessive oxidation with the possibility of a host of spoilage organisms.
Chemicals are very valuable tools. SO2 is a wonder-chemical to a winemaker. Anti-oxidant, anti-oxidase, and anti-microbial; it is a perfect tool for the long and short term storage of wine. And when it is used correctly, the consumer will enjoy the wine exactly as it was intended. If there is anything valuable to be learned in this post, it is that fact. Good SO2 management makes good wine.