Jenny Savits, Enology Field Specialist
Sulfur dioxide is used in winemaking as a preservative to prevent oxidation and microbial spoilage. It exists in three forms; bisulfite (HSO3-), molecul
ar SO2, and sulfite (SO32). The equilibrium is pH dependent with the predominate form at wine pH being bisulfite. Most of the rest is molecular and very little, if any, remains in sulfite form. These forms make up what is termed as ‘free SO2’. Free SO2 can be lost through volatilization or binding, thus management is important. A target of 0.6-0.8 mg/L (ppm) molecular SO2 has been deemed sufficient to keep wine protected. Charts are commonly available to relate the desired free SO2 level to a given pH of the wine to hit the target molecular SO2.
When bisulfite forms binds with other molecules in wine (i.e. sugars, acids, anthocyanins, etc.), this is termed ‘bound SO2’. Once bound, the SO2 is no longer available to protect the wine. The total SO2 is achieved by adding the free and bound, and has a legal limit set 
by the TTB of 350 mg/L (ppm).
Regularly scheduled testing is necessary to manage SO2. Common methods in commercial wineries are aeration-oxidation (A/O), the Ripper method, or ations of Ripper using electrochemical sensors to identify the iodine titration endpoint. These methods all include steps to acidify and dilute the sample, which disrupts the SO2 equilibrium and leads to overestimation of the free and molecular SO2. Red wine is more susceptible than white and rosé due to the breaking of bonds loosely held between bisulfite and anthocyanin. In the wine, the bisulfite would be unavailable to protect the wine. During analysis that weak bond is broken and the bisulfite is counted as free, leading to an overestimation of the free SO2.
Researchers have developed a method to more accurately measure free SO2, namely in red wines, without disrupting the SO2 equilibrium. The Headspace Gas Detection Tube Method (HS-GDT) is lower in cost and simpler than other options (i.e. gas chromatography). A wine sample is drawn into a syringe with a known volume of headspace. Sample and headspace equilibrate for a period of time before the headspace is pushed through a gas detection tube affixed to the syringe. The detection tube is packed with color sensitive materials that change proportionally to the concentration of the SO2 pre
sent. The length of the amount of color change in the tube is measured in mm. Calculations are then used to determine molecular SO2 by Henry’s law coefficients. Free SO2 concentration can be determined using the molecular SO2 value, the pH, and the Henderson-Hasselbalch equation. A calculator spreadsheet tool is available to assist. Common sources of error include measuring the headspace volume incorrectly, leaking of the syringe apparatus, and releasing the headspace too quickly through the gas detection tube. Precision and accuracy are checked by preparing model wine and SO2 solutions.
To compare, SO2 values in red wine reported for the HS-GDT method ranged from 30-60% lower than commonly used methods to measure SO2. Currently, it’s recommended to target lower molecular SO2 (0.2-0.3 mg/ L) for dry reds if using the headspace method. Further investigation to validate these recommended targets is still needed. The method is not widely used as of yet, but research is mounting to suggest that standard methods used to measure SO2 are not accurately predicting the microbial stability of red wines (with substantial anthocyanin content) and are overestimating the protection the SO2 is providing.
Resources
Coelho JM, Howe, PA, and Sacks GL. 2015. A Headspace Gas Detection Tube Method to Measure SO2 in Wine without Disruption SO2 Equilibria. Am. J. Enol. Vitic. 66:257- 265. DOI: 10.5344/ajev.2015.14125
Howe PA, Worobo R, and Sacks GL. 2018. Conventional Measurements of Sulfur Dioxide (SO2) in Red Wine Overestimate SO2 Antimicrobial Activity. Am. J. Enol. Vitic. 69:210-220. DOI:10.5344/ajev.2018.17037
Dlubac G and Sacks G. Measuring Sulfur Dioxide (SO2) in Wine using a Headspace Gas Detection Tube Method. https://grapesandwine.cals.cornell.edu/extension/grape-and-wine-resources/