Determination of Molecular and “Truly” Free Sulfur Dioxide in Wine: A Comparison of Headspace and Conventional Methods

• Todd W. Jenkins ^[1] ; Patricia A. Howe ^[1] ; Gavin L. Sacks ^[2] ; Andrew L. Waterhouse ^[1]
  1. [1] University of California System

     University of California System

     Estados Unidos

     
  2. [2] Cornell University

     Cornell University

     City of Ithaca, Estados Unidos

     
• Localización: American Journal of Enology and Viticulture, ISSN 0002-9254, Vol. 71, Nº 3, 2020, págs. 222-230
• Idioma: inglés
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• Resumen

  • Conventional methods such as Ripper titration and aeration-oxidation (A-O) are widely used for the analysis of sulfur dioxide (SO2) in wine. However, the free SO2 reported by these procedures is overestimated due to dissociation of weakly bound SO2 forms during the analysis, particularly from anthocyanin-bisulfite complexes. “Truly” free SO2 in wine can be determined from the headspace SO2 concentration of an equilibrated wine sample. A headspace SO2 method based on gas detection tubes (HS-GDT) was recently described but is not readily automated. While solid phase microextraction (SPME) yielded poor precision in our experiments, our new method, based on static headspace gas chromatography and sulfur chemiluminescence detection (HS-GC-SCD), is readily automated and achieves high precision (<5%) and low limits of detection (0.033 mg/L molecular SO2, or ~1 mg/L free SO2 in wine at pH 3.5). A-O, Ripper, HS-GC-SCD, and HS-GDT methods were compared on a diverse set of wine samples. Results from HS-GC were correlated with those from the HS-GDT method (r2 = 0.92) and achieved higher precision (relative standard deviation = 3.7%). HS-GC was highly correlated with A-O in white wines (r2 = 0.85, slope = 0.90) but had weaker correlation for red wines (r2 = 0.71, slope = 0.44). The flexibility of GC for other procedures as well as its stability and low operating costs per sample make it an attractive option, and headspace methods have been shown to be better for predicting microbial stability in red wines.