Fast and accurate automated method for free sulfite analysis in wine

Fast and accurate automated method for free sulfite analysis in wine — summary

Importance of the topic

Sulfur dioxide (SO2) is widely used in winemaking as an antioxidant, antimicrobial agent, and enzymatic browning inhibitor. Regulatory limits and allergen labeling requirements make reliable quantification of free and total SO2 essential for quality control and compliance. Fast, automated, and low-reagent methods that deliver accurate free SO2 values support routine laboratory workflows in enology, QA/QC, and production monitoring.

Objectives and overview of the study

The study aimed to develop and validate an automated discrete photometric method for measuring free SO2 in wine using a Thermo Scientific Gallery discrete analyzer and system reagents. The method adapts the established p-rosaniline/formaldehyde reaction to a discrete analyzer format to provide rapid, low-consumable analysis with performance comparable to conventional Flow Injection Analysis (FIA).

Methodology

• Chemical principle: Sulfur dioxide in the sample reacts with p-rosaniline hydrochloride and formaldehyde to produce a chromophore measured photometrically.
• Operating conditions: Reaction is performed at 37 °C. Primary measurement wavelength is 575 nm with a side (reference) wavelength at 700/750 nm to correct for scatter and bubbles.
• Analytical sequence (per sample): 100 µL of R1 reagent + 10 µL sample + 10 µL water incubated 30 s for sample blanking; then 100 µL of R2 reagent added; after 75 s incubation the endpoint absorbance is read.
• Calibration: A 200 mg/L sodium metabisulfite stock standard is used. The analyzer automatically dilutes calibration points and applies a second-order calibration curve for concentration calculation.
• Measuring range: 2–100 mg/L free SO2, extendable via automated dilution.

Used instrumentation

• Thermo Scientific Gallery discrete photometric analyzer (discrete analyzer platform).
• System reagents: Thermo Scientific SO2 Free kit (P/N 984634).
• Reference method/instrument: Flow Injection Analysis (FIA) system using the same p-rosaniline/formaldehyde chemistry for comparator results.

Results and discussion

Sample set: 57 wines (35 red, 22 white) analyzed without pretreatment. Free SO2 concentrations ranged 0–46 mg/L (mean ~28 mg/L).

• Correlation with FIA: Combined coefficient of determination r2 = 0.9755. By type: white wines r2 = 0.9863; red wines r2 = 0.9803.
• Bias: Average positive bias observed vs FIA: +3 mg/L for red wines and +1 mg/L for white wines; individual sample biases generally within a few mg/L.
• Precision: For a red wine sample (N = 40) precision was 1.8% within-run and 2.1% between runs, consistent with accepted EBC/ASBC limits.
• Throughput: Analysis of 60 samples requires approximately 35 minutes, plus about 10 minutes for daily calibration and analyzer start-up.

Overall, the discrete photometric method delivers rapid, repeatable free SO2 results closely matching an established FIA reference method while using smaller reagent volumes.

Benefits and practical applications

• High throughput and automation reduce hands-on time for routine wine analysis laboratories.
• Low reagent consumption compared with FIA lowers operating costs and solvent/reagent waste, improving environmental footprint.
• Integrated bar-coded reagents and automated calibration simplify daily operation and traceability.
• Capability to run other analytes (sugars, acids, color, total SO2) on the same discrete analyzer enhances lab workflow consolidation.

Future trends and potential uses

• Further integration of discrete analyzers into multi-parameter wine quality panels could streamline routine enological testing.
• Method adaptation for broader matrices (e.g., other beverages and food matrices) and for lower detection limits via optimized incubation or optics.
• Continued miniaturization and reagent-saving chemistries to reduce waste and cost per analysis.
• Enhanced software algorithms for matrix compensation could reduce small systematic biases versus reference techniques across diverse wine styles.

Conclusion

The Thermo Scientific Gallery discrete photometric method for free SO2 provides a fast, precise, and accurate alternative to FIA-based determinations. It offers sufficient agreement with reference FIA results (r2 ≈ 0.98), low within- and between-run variability, and operational advantages including reduced reagent use and high throughput—making it well suited for routine enology and QC laboratories.

References

1. Jackowetz N; Li E; Mira de Orduña R. Sulphur Dioxide Content of Wines: the Role of Winemaking and Carbonyl Compounds. Research Focus, Cornell Viticulture and Enology Program, 2011–3.
2. Thermo Fisher Scientific. Automated Discrete Photometry. Product information (accessed November 3, 2014).
3. Cudiamat G. Continuous Flow Analysis & Discrete Analyzers. LC/GC, 2010 (accessed Dec 10, 2014).
4. Hicks M; Schenken J; Steinrauf M. Laboratory Instrumentation, 3rd Edition; J. B. Lippincott & Co., 1987; pp 284–292.