The use of pulsed fluorescence detector to quantify free SO₂ in wines via the headspace

Abstract

Pulsed fluorescence SO₂ analyzers are widely used for atmospheric monitoring [1]. They are accurate, portable, sensitive and their price are reduced compared to advanced techniques like gas chromatography with sulfur chemiluminescence detection (GC-SCD). These instruments also allow for continuous and non-destructive measurements. In this study, we explored the potential of pulsed fluorescence detection to quantify free SO₂ in wine via the headspace (HS-PFD) and developed a method to minimize disturbances to SO₂ equilibrium.

Our approach involves injecting wine into a syringe, adding nitrogen to create a headspace, and gradually releasing the gas phase into the SO₂ analyzer. Molecular SO₂ levels are determined using a calibration curve based on model wine spiked with potassium metabisulfite, while free SO₂ is calculated considering temperature, pH, and alcohol by volume (ABV) with the equations described in [2]. The method demonstrated high sensitivity, with detection and quantification limits of 0.012 mg/L and 0.032 mg/L, respectively—equivalent to roughly 0.5 and 1.3 mg/L of free SO₂ in a wine with pH 3.5 and 10% ABV. The coefficients of determination of calibration curves ranged from 0.99 to 0.999, and the method’s precision, assessed across 18 wines measured in triplicate, yielded an average relative standard deviation of 3.5% (ranging from 1.2% to 8%).

We analyzed 81 Swiss commercial wines using HS-PFD and corrected iodometric titration (CIT). Results from both methods were comparable for white and rosé wines (white: N = 29, slope = 0.9, R² = 0.96; rosé: N = 9, slope = 0.98, R² = 0.95). However, HS-PFD consistently measured lower free SO₂ in red wines compared to CIT (N = 43, slope = 0.61, R² = 0.85). Further analysis of 20 selected wines using CIT, aeration-oxidation (A-O), HS-PFD and acidification-HS-PFD (Acid-HS-PFD) demonstrated that methods acidifying wine samples (CIT, A-O and Acid-HS-PFD) overestimate free SO₂ in red wines, as acidification released weakly bound SO₂. The amount of released SO₂ correlated positively with the total and SO₂-bleachable anthocyanin content of wines.

Our results align with previous literature [2,3,4,5] and demonstrate that the HS-PFD method delivers performance comparable to high-end analytical techniques like GC-SCD and GC-MS, but at a significantly lower cost. Future developments and automation possibilities will be discussed.

References

[1] Villanueva, F., Ródenas, M., Ruus, A., Saffell, J., Gabriel, M.F. (2022). Appl. Spectrosc. Rev., 57:7, 531-579.

[2] Jenkins, T.W., Howe, P.A., Sacks, G.L., Waterhouse, A.L. (2020). Am. J. Enol. Vitic., 71, 222–230.

[3] Coelho, J.M., Howe, P.A., Sacks, G.L. (2015). Am. J. Enol. Vitic. 66, 257–265.

[4] Carrascon, V., Ontañón, I., Bueno, M., Ferreira, V. (2017). J. Chromatogr. A, 1504, 27–34.

[5] Layton Ashmore, P., Valdez, F., Harbertson, J.F., Boulton R.B., Collins T.S. (2023). J. Chromatogr. A, 1695, 1-7.