Rapid determination of free sulfur dioxide in wine and cider by capillary electrophoresis

A novel method for the determination of "true" free sulfur dioxide (SO₂) in wine and cider was developed using capillary electrophoresis with direct ultraviolet-visible spectrophotometric detection (CE-UV/vis). Free SO₂ was measured in model solutions with different SO₂-binding agents present (α-ketoglutarate, pyruvate, acetaldehyde, glucose, fructose, and malvidin-3-glucoside) as well as a variety of white and red wines and ciders. The CE method was compared to three conventional methods for measuring free SO₂, the Ripper method, Aeration-Oxidation (AO), and pararosaniline by discrete analyzer (DA). While some statistically significant differences (p<0.05) were found between the four methods in unpigmented model solutions and samples, the values generally agreed. In the presence of anthocyanins in model solution and red wines, free SO₂ values found by CE were significantly lower than the other three methods (p<0.05). The difference in values found by Ripper and CE correlated strongly with anthocyanin content (R² = 0.8854) and even more strongly when accounting for polymeric pigments (R² = 0.9251). The results in red ciders differed from those in red wines, while the CE measured significantly lower free SO2 values than the other three methods, the difference in free SO₂ values measured by CE and Ripper correlated more closely with anthocyanin concentration (R² = 0.8802) than absorbance due to bleachable pigment (R² = 0.7770). The CE method was found to be rapid (4 min/injection), sensitive (LOD=0.5 mg/L, LOQ=1.6 mg/L free SO₂ in wine, 0.8 and 2.8 mg/L, respectively, in cider), robust, and repeatable (average RSD = 4.9%) and did not suffer from the issue of over-reporting free SO₂ in pigmented samples often observed with currently accepted methods.