Conventional Measurements of Sulfur Dioxide (SO₂) in Red Wine Overestimate SO₂ Antimicrobial Activity

Abstract

Conventional approaches to measuring sulfur dioxide (SO₂) in wine, such as aeration-oxidation, iodometric titration, and flow-injection analysis, are known to overestimate molecular SO₂, particularly in red wine because of the dissolution of weak anthocyanin-bisulfite complexes during analysis. Methods for determining molecular SO₂ without perturbing anthocyanin-bisulfite complexes and other weak adducts exist, e.g., headspace gas-detection tube measurements. However, it is unclear whether SO₂ values achieved through conventional methods (“Molecular SO₂”) or nonperturbing methods (molecular SO₂) provide a better measure of antimicrobial activity. In our work, white and pseudo-red wines were prepared with varying additions of SO₂; the red wine was produced by spiking the white wine with an anthocyanin extract. “Molecular SO₂” and molecular SO₂ concentration in white wines were well correlated, but “Molecular SO₂” was significantly higher in red wines. Wines were inoculated with Saccharomyces cerevisiae strain EC1118 (Lallemand), and viability and culturability were evaluated at regular intervals. Both culturable and viable cell counts decreased significantly for treatments with 0.5 to 2.0 mg/L molecular SO₂ in the white and red wines, and for 0.5 to 2.0 mg/L “Molecular SO₂” in the white wine, but concentrations >2.0 mg/L “Molecular SO₂” were necessary to decrease cell counts in the red wine. These results indicate that anthocyanin-bisulfite complexes have negligible antimicrobial activity, and that conventional approaches to measuring “Molecular SO₂” are poorly suited to predicting the microbial stability of red wines.