Determination of Molecular and “Truly” Free Sulfur Dioxide in Wine: A Comparison of Headspace and Conventional Methods
Todd W. Jenkins, Patricia A. Howe, Gavin L. Sacks, and Andrew L. Waterhouse
Am J Enol Vitic. March 2020 : ajev.2020.19052; published ahead of print March 13, 2020 ; DOI: 10.5344/ajev.2020.19052
By Michelle Podolec
- Free sulfur dioxide SO2 forms (molecular SO2 and bisulfite) are related to the microbial and oxidative stability of a wine. However, most methods used for measuring in the wine industry overestimate the amount of SO2.
- “Truly” Free SO2 can be determined measuring the headspace concentration of SO2 but current methods for headspace SO2measurement are challenging to automate.
- Researchers developed a new, automatable method to measure “Truly” Free SO2 using headspace gas chromatography (HS-GC).
Winemakers have historically utilized sulfur dioxide (SO2) as a wine preservative. Researchers have discovered that when SO2 is added to wine, it exhibits five effects:
- Inhibits microbial growth, especially of spoilage organisms.
- Protects wine from oxygen exposure by consuming hydrogen peroxide and other oxidation byproducts.
- Limits browning of juices prior to fermentation by inhibiting polyphenol oxidase enzymes.
- Bleaches wine pigments.
- Binds to aldehydes and ketones associated with off odors in wine.
Sulfur dioxide (SO2) exists in free (molecular and bisulfite) and bound forms in solution. Free SO2 has protective antimicrobial qualities, antioxidants and enzyme-inhibition. Bound SO2 molecules do not exhibit these characteristics). Together, free and bound sulfites are added together to determine total sulfite concentration.
To be useful for winemakers, SO2 analyses must differentiate between free and bound SO2. Current methods for measuring SO2 include A-O (aeration-oxidation) and Ripper (iodometric titration). Both methods overestimate free SO2, especially in red wines, and thus measure “Apparently” Free SO2. This overestimation can be avoided, and the “Truly” Free SO2 determined, by measuring the headspace SO2 concentration in a wine sample. The Sacks lab has previously reported a method using colorimetric gas detection tubes (GDT) to measure “Truly” Free SO2, but this earlier method was challenging to automate.
Results and Discussion
Researchers built on previous approaches to measure volatile sulfur compounds by collecting a headspace (HS) sample followed by separation on a gas chromatography column and detection by a sulfur chemiluminescence detector (HS-GC-SCD or HS-GC for short).The optimized method required ~10 min per sample, and was easily automated.
The researchers evaluated a diverse group of 27 wines with two traditional “Apparently” Free SO2 methods (A-O and Ripper) and two methods for “Truly” Free SO2 (HS-GDT, and the new HS-GC method). As expected, A-O and Ripper consistently overestimated SO2 values than the headspace, especially in red wines. On average, “Apparently” Free SO2 values were over 2-fold higher than “Truly” Free SO2 in red wines, with certain wines having as much as 20 mg/L higher Apparent Free SO2. Wines with big discrepancies were more likely to have high anthocyanin concentrations.
Conclusions and Practical Considerations
Researchers developed a new analytical procedure for measuring “Truly” Free SO2 based on headspace gas chromatography (HS-GC). The method requires little sample preparation or reagents, requires ~10 min per sample, and is easily automatable. The work confirmed that the current methods for measuring “Apparently” Free SO2 in wines (A-O and Ripper method) consistently overestimate molecular and free SO2 in red wines. . Previous work has shown that “Truly” Free SO2 is a better predictor of microbial stability than “Apparently” Free SO2, and researchers hope that the new HS-GC method can be used by the wine industry to better predict wines at risk of spoilage.
Michelle Podolec is extension support specialist with the statewide viticulture extension program, based at Cornell AgriTech in Geneva, NY.