Sulfur Dioxide Measurement and Management Chik Brenneman Anita Oberholster

Introduction What is sulfur dioxide? Why is it important Practical considerations for winemaking/Management during aging Measurement techniques What does it mean? Alternatives

Overview Chemical antioxidant and inhibitor of microbial activity Labeling requirements Industry wide trend for reduction in use

Chemical antioxidant Inhibitor of browning reactions in juice Polyphenol oxidase Cultivar, juice vs. must Typical additions of 30-50 mg/L Laccase is minimally affected Residual laccase activity in wine can cause browning Participates in binding reactions Aldehydes, e.g. acetaldehyde Anthocyanins

Inhibitor of microbial activity Juice – levels for enzyme inhibition are sufficient for inhibition or significant reduction in native flora In General Bacteria are sensitive Yeast are less sensitive Sensitivity varies by organism

Sulfur Dioxide Species Molecular (SO2) Bisulfite (HSO3-) Sulfite (HSO32-)

Sulfur Dioxide Species Molecular (SO2) Anti-microbial Peroxide scavenger Volatile

Sulfur Dioxide Species Bisulfite (HSO3-) Binding form (Hydroxy-sulfonates Acetaldehyde, acetaldehyde, glucose, quinones, anthocyanins

Sulfur Dioxide Species Sulfite (HSO32-) Anti-oxidant form Under wine conditions, this reaction is not favoreddue to the low concentrations present Peroxide scavenging of the molecular form causes retardation of acetaldehyde formation or browing

Relationship with pH pH Measure free H+ ions pH = -log [H+] Why is knowing the pH important? Microbial stability Effectiveness of SO2 Molecular SO2 is the form effective against microorganisms

Free and Total Sulfur Dioxide Total – Bound = Free Exist as a dynamic equilibrium Post fermentation goals to add and stabilize the equilibrium

Practical Considerations Understand the relationship with pH Volatility and sensory thresholds Balancing acidity independent of pH

Inhibitor of microbial activity

pH versus free sulfur dioxide The percentage of SO2 forms in solution at specific pH values* pH Molecular SO2 Bisulfite Sulfite Free SO2 for 0.825 mg/L molecular 2.7 10.50 89.5 0.00283 7.9 2.8 8.54 91.5 0.00364 9.7 2.9 6.90 93.1 0.00467 12.0 3.0 5.56 94.4 0.00596 14.8 3.1 4.47 95.5 0.00759 18.5 3.2 3.58 96.4 0.00964 23.1 3.3 2.87 97.1 0.0122 28.8 3.4 2.29 97.7 0.0155 36.0 3.5 1.83 98.2 0.0196 45.1 3.6 1.46 98.5 0.0248 56.5 3.7 1.16 98.8 0.0312 71.1 3.8 0.924 99.0 0.0394 89.3 3.9 0.736 99.2 0.0497 112.0 4.0 0.585 99.4 0.0627 141.0 *Boulton, et.al. Principles and Practices of Winemaking, 1985

Other Practical Considerations Cleaning Sanitation Segregation of blends

Sources of Sulfur Dioxide Potassium Metabisulfite Powder form Effervescent form Gas Concentration specific solutions

Calculating SO2 Additions KMBS comes as a powder, it is a salt of sulfur dioxide. 57% is ‘active’ Potency decreases over time (temperature/humidity) Example: add 50 ppm to 100 L (Desired addition (g/L)* volume (L))/0.576= g KMBS 0.050 (g/L)* 100 (L) = 8.6 g KMBS 0.576

Addition Strategy First additions for the prevention of and/or at the completion of the MLF 30 mg/L for whites, 50 mg/L for reds Test at 2-3 weeks as equilibrium is established Additional additions are based on free and total results Something is better than nothing

Sulfur dioxide analysis (free and bound) Ripper Iodine Titration Ripper Iodate Titration Aeration Oxidation Monier-Williams Fiastar FOSS

Measurement Methods Each method has its limitations Hydrolysis of the hydroxy-sulfonate can affect interpretation of the free form

Ripper Iodine Titration ReDox titration End point seen using starch indicator/redox electrode Useful for Free and Total SO2

Ripper Iodine Titration Limitations Iodine degradation Starch degradation Endpoint recognition Interfering compounds Polyphenols Ascorbate

Ripper Iodate Titration Iodate stability Iodide-Starch indicator Iodine produced in situ

Aeration-Oxidation (Vacuum aspiration) Air Aspiration/Nitrogen sweeping SO2 volatilized and bubbled through a peroxide trap Sulfuric acid titrated with NaOH Useful for Free and Total

Aeration-Oxidation (Vacuum aspiration) Limitations Carrier gas flow rates should be standardized Glassware/Condenser requirements Anti-foam is recommended Some bound SO2 can be released in the acidification step

Monier-Williams (AOAC sanctioned) Set up is similar to A-O Sample size consideration Reflux is 1.75 hours

Fiastar Colorimetric endpoint High volume throughput Up to 200 mg/L range of detection Cost $20-30K Maintenance is key Chemistry geek: Foss FIAStar Segmented-Flow analyzer Materials: - Foss FiaStar Unit(s) - Autosampler - Sample vials Analysis time: 5 minutes per sample, following 1 hour of start-up and calibration Accuracy:  2% v/v Hazards: None Cost: Approx. $15,000 per unit for FiaStar; $10,000 for autosampler unit Theory and Practice: Flow injection analysis works much like aeration/oxidation, but it is automated and performed on a microscale. Small amounts of sample are aspirated into small-bore tubing, with each sample separated by an air bubble. Pumps move the samples through a system of cartridges, where they undergo a series of reactions. The final reaction combines SO2 with a reagent to form a yellow dye, which is read photometrically and analyzed to calculate an SO2 reading. This method is very accurate, very expensive, and best used in large wineries or labs with high sample throughput. Flow injection analysis can be used for both free and bound SO2; free SO2 analysis requires purchase of one FIASTar unit, while total requires two.

FOSS FTIR scan of both the liquid and a gaseous sample. Secondary method Fast results FTIR support on line

Alternatives to sulfur dioxide Alternatives tend to address single variables Just say no (more than necessary) Reacting to ‘real’ number Clean fruit, good sanitation practices and some tolerance to higher volatile acidity

Alternatives to sulfur dioxide Lysozyme Velcorin Ascorbic acid Ozone Ultrasound UV light Low electrical current (LEC) Pressure

Summary Powerful tool in traditional winemaking Prize for a better substitute remains unclaimed Use lends to predicable stability in both opened and unopened bottles of wine

Questions?