Free Sulfur (F-SO2)

Free Sulfur (F-SO2)Free SO2 is a measurement of SO2 that is unbound to other molecules. Its molecular form (SO2) is an antimicrobial preservative thats speciation in wine is highly pH, alcohol, and temperature dependent. The vast majority of Free SO2 in wine is bisulfite (HSO3), a powerful antioxidant in winemaking. The traditional calculation used to determine the speciation of sulfur in wine is the Henderson Hasselbalch equation, but recent studies have shown that

Free SO2 is a measurement of SO2 that is unbound to other molecules. Its molecular form (SO2) is an antimicrobial preservative that’s speciation in wine is highly pH, alcohol, and temperature dependent. The vast majority of Free SO2 in wine is bisulfite (HSO3), a powerful antioxidant in winemaking. 

The traditional calculation used to determine the speciation of sulfur in wine is the Henderson-Hasselbalch equation, but recent studies have shown that free anthocyanins in red wines complex with free SO2 converting most of it into its inactive bound form. Furthermore, the Henderson-Hasselbalch equation doesn't account for variables like temperature or ABV. 

While our lab uses the reference methodology for free sulfur determination in wine, aeration-oxidation (A-O), it’s worth noting that A-O doesn’t distinguish between anthocyanin-bound SO2 and truly free SO2. So, the following updated formula from Jenkins et al., 2020 can be used for molecular SO2 determination in white wines and accounts for pH, temperature, and ABV (click here). As for red wines, it is worth noting that Free SO2 may not always play the role we think it does in winemaking.

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