By Maureen Moroney
In winemaking, the use of sulfur dioxide (SO2) is critical. We tend to talk a lot about free SO2 (FSO2) in particular, and not without good reason. The FSO2 and the pH of your wine determine how much SO2 is available in the active, molecular form to help protect the wine from oxidation and spoilage. FSO2 is also something we have to keep a close eye on, because it can be hard to predict how much will be lost, and at what rate, to binding or to aeration. Too much FSO2 can be perceptible to consumers, by masking the wine’s own fruity aromas and inhibiting its ability to undergo the cascade of oxygen-using reactions that happen when it “breathes,” or, in high enough concentrations, by contributing a sharp/bitter/metallic/chemical flavor or sensation.
But we talk a lot less often about Total Sulfur Dioxide.
Simply put, Total Sulfur Dioxide (TSO2) is the portion of SO2 that is free in the wine plus the portion that is bound to other chemicals in the wine such as aldehydes, pigments, or sugars. The TSO2 level is also regulated by the U.S. Alcohol and Tobacco Tax and Trade Bureau (TTB): The maximum allowable concentration for a bottled wine is 350 ppm (mg/L) of TSO2. Theoretically, in a perfect world, the TSO2 in any wine should mirror the total of all the SO2 additions that have been made to it over the course of its history. However, the measured TSO2 rarely matches the expected amount perfectly, due to imprecise additions, SO2 removed with lees and sediments, and any volatile SO2 lost to the air.
But if TSO2 levels are relatively predictable (compared to FSO2), and if FSO2 is more important for the overall protection of the wine, why should we care about TSO2?
Aside from the legal regulations, keeping track of a wine’s TSO2 level gives more context to FSO2 measurements. For example, an unexpectedly low Free SO2 measurement could be due to an SO2 addition that was improperly calculated, weighed out, or mixed in, or it could be due to a large percentage of that addition having been bound up to other components in the wine. Measuring the TSO2 will tell you whether all the SO2 you think is in the wine is actually there. Another insight that TSO2 can provide, in conjunction with FSO2, is a sense of how “clean” a wine is. A wine with high TSO2 has usually had many SO2 additions made over its lifetime, usually because the FSO2 keeps dropping. When we see a wine where the FSO2 is an unusually small percentage of the TSO2, it’s often an indication that there’s something making the wine chemically and/or microbially unstable. Finally, TSO2 can act as a sort of “cushion” for FSO2. When Free SO2 is lost, the chemical equilibrium in the wine can shift so that some of the bound SO2 may be released to its free state. This is why it’s a good idea to build a solid foundation of TSO2; the more Total SO2 you have, the more stable the Free SO2 tends to be.
Like so much in winemaking, maintaining the right sulfur dioxide levels can be a balancing act. FSO2 may be the showy, high-impact part, but it’s hard to understand and control it without an awareness of its sometimes-overlooked counterpart on the other side.