Re-Thinking Free And Molecular Sulfur Dioxide Measurements In Wine
Data from thirteen years of wine industry laboratory proficiency testing were reviewed. Reproducibility was evaluated with Horwitz Ratios (HorRat); only alcohol, titratable acid, and total SO2 had acceptable values (mean HorRat < 2). Reproducibility and repeatability imprecision generally increased with analyte concentration, with notable exceptions of alcohol (both), volatile acidity (reproducibility), and total SO2 (repeatability). The methods or instruments for alcohol, titratable acidity, free and total SO2, and volatile acidity changed significantly over the time period. Results demonstrate the need for industry-wide improvement in analytical performance for some assays, and the potential benefit of adopting criteria guidelines for method performance. Accurate measurements of the major active sulfur dioxide species in wine (HSO3- and SO2) are important to studies of wine oxidation chemistry and microbial stability. These so-called "free SO2" forms are traditionally measured by either iodometric titration or by aeration-oxidation (A-O), or by comparable modern variants. These standard approaches require sample dilution and/or pH shifts, which perturb the equilibrium between bound and free forms, resulting in overestimation of free SO2 species and the reporting of "apparent" SO2. We describe a simple, non-perturbing headspace gas detection tube (HS-GDT) method for measurement of molecular and free sulfur dioxide (SO2) in wine using commercial industrial safety colorimetric tubes. A syringe is used to sample a wine and to create a closed headspace, which is expelled through the GDT after equilibrium is obtained. Henry's coefficients were constant over ethanol concentrations of 0-17% v/v. The HS-GDT method limit of detection in a model wine (pH 3.56, 12% v/v ethanol) was 0.21 mg/L molecular SO2, and was linear over 0.29-1.13 mg/L. Good agreement was observed between HS-GDT and A-O for white and blush wines, but molecular SO2 in red wines averaged 2-fold lower by HS-GDT. The difference in molecular SO2 values by A-O and HS-GDT was well correlated with estimated molecular SO2 decrease due to anthocyanin-bisulfite adduct formation (r2 = 0.936), supporting the hypothesis that dissolution of anthocyanin-bisulfite adducts occurs during A-O analysis. Future work on the relative impact of this molecular SO2 value relative to the commonly used "apparent" value on wine microbes is recommended.
Sulfites; Wine Analysis; Sulfur Dioxide
Dando,Robin; Worobo,Randy W.
Food Science and Technology
Ph. D., Food Science and Technology
Doctor of Philosophy
dissertation or thesis