Regulating SO2 levels in winemaking processes

SO₂, sulfur dioxide, is a very important additive to the winemaking process because of its antioxidant and antimicrobial properties. Thus, it makes wine fresher, inhibits unwanted growth of bacteria and yeast, and improves wine’s storage length and shelf stability.

This makes it important for wine manufacturers to use precise methods for measuring SO₂ levels and infusing more SO₂ into wine. In the following, we will explain how to precisely measure and add SO₂ during winemaking processes.

How to measure SO₂ levels in wine

The two main ways to measure SO₂ levels in wine are the aeration-oxidation and Ripper methods. Generally, the more preferred way to measure both free and bound SO₂ in wine is using aeration-oxidation titration, which offers greater accuracy and precision but is slightly slower than the Ripper method.

Common main equipment needed to complete an aeration-oxidation titration test include a flask, impinge assembly, heating mantle, aspirator pump, flow controller, condenser with heat exchanger, and Buret assembly.

To perform the titration, SO₂ is aspirated for 10-15 minutes at a specific flow rate of 1 SLPM out of an unheated (for testing free SO₂) or heated (for testing bound SO₂) acidified wine sample, traveling as a gas through a condenser to an impinge filled with hydrogen peroxide solution which is then titrated with NaOH, determining total SO₂ levels.

Testing free SO₂

1. First fill the impinge with 10 ml of 1% hydrogen peroxide adjusted to a PH of 5.5-6.
2. Add three drops of SO₂ Indicator (Methyl Red + Methylene Blue in 50% Ethanol) to the hydrogen peroxide solution.
3. Attach the impinge to the rest of the aeration-oxidation apparatus, which includes the graham condenser and the flask. This can be configured to look slightly different based on equipment availability but should allow gas to travel from the flask through the condenser to the impinge without providing any alternative flow path.
4. Pipet 20 ml of wine into a 100 ml flask to act as the wine sample.
5. Add 10 ml of 25% phosphoric acid to the flask in order to acidify the wine sample.
6. Insert a bubbler and stopper into the flask and attach it to the AO apparatus so that gas from the flask will not be able to travel anywhere except through the condenser and impinge apparatus.
7. Next turn on the aspirator pump and set the flow controller to 1 SLPM for 10 minutes.
8. After turning off the pump, titrate the 1% hydrogen peroxide solution with a 0.01 N NaOH solution using the Buret assembly, recording the amount of .01 N NaOH needed to change indicator colors of the 1% hydrogen peroxide solution.
9. Lastly, determine the concentration of free SO₂ (in PPM) in the sample by calculating .01 N NaOH X titration volume X 1600.

Testing bound SO₂

1. After completing the free SO₂ titration, set the condenser through a heat exchanger which will help to further cool down the SO₂ gas as it travels to the impinge through the condenser.
2. Fill the impinge with 10 ml of 1% hydrogen peroxide adjusted to a PH of 5.5-6.
3. Add three drops of SO₂ Indicator (Methyl Red + Methylene Blue in 50% Ethanol) to the hydrogen peroxide solution.
4. Attach the impinge to the rest of the aeration-oxidation apparatus, which includes the graham condenser and the flask. This can be configured to look slightly differently based on equipment availability but should allow gas to flow from the flask through the condenser to the impinge without an alternative flow path.
5. Pipet 20 ml of wine into a 100 ml flask to act as the wine sample.
6. Add 10 ml of 25% phosphoric acid to the flask in order to acidify the wine sample.
7. Insert a bubbler and stopper into the flask and attach it to the AO apparatus so that gas from the flask will not be able to travel except through the condenser and impinge apparatus.
8. Place the flask into a heating mantle and turn it on so that the wine sample is heated but not boiling. Heating the sample releases the bound SO₂ to molecular SO₂, allowing it to escape as a gas measurable in the titration.
9. Next turn on the aspirator pump and set the flow controller to 1 SLPM for 15 minutes.
10. After turning off the pump, titrate the 1% hydrogen peroxide solution with a 0.01 N NaOH solution using the Buret assembly, recording the amount of .01 N NaOH needed to change indicator colors of the 1% hydrogen peroxide solution.
11. Lastly, determine the concentration of bound SO₂ (in PPM) in the sample by calculating .01 N NaOH X titration volume X 1600.

Calculating total SO₂

Calculate total SO₂ by adding together bound and free SO₂ from the separate titrations.

Importance of using flow controllers

If aspiration flow rates are too low, recorded SO₂ levels will be lower than actual sample concentrations and if aspiration flow rates are too high, recorded SO₂ levels will be higher than actual sample concentrations.

Alicat’s MCS-Series mass flow controllers offer a solution by controlling air and SO₂ flow up to a high accuracy of ±0.4% of reading + 0.2% of full scale with a control range of 1-100% of full scale, significantly reducing the chance of imprecise flow rates and allowing for better control during any aeration-oxidation titration.

How to add SO₂ to wine

SO₂ is generally added into wine during both prefermentation and postfermentation. Additionally, after bottling, wine barrels and corks are sterilized by SO₂ so that they can be reused for multiple batches.

Traditionally, the ancient Romans burned sulfur discs inside their wine barrels as a method of adding SO₂. Today, a more common method for adding SO₂ is to dissolve potassium or sodium metabisulfate (K₂S₂O5 or Na₂S₂O5) directly into wine. However, as adding additional minerals is not ideal, the best method of adding SO₂ for commercial wine makers today involves the precise injection of SO₂ gas.

By using gas flow controllers to precisely control rates of injection, SO₂ gas can be directly added to wine at very specific quantities. Because SO₂ gas is entirely free (molecular SO₂), it offers a maximization in the ratios of free to bound SO₂ relative to other methodologies.

However, since pure SO₂ gas is toxic, special precautions must be followed by wineries such as proper licensing, handling, and registration as well as the use of SO₂ resistant clothing, gloves, and respirator.

Injection of SO₂ gas in wine barrels

1. After following the steps for measuring free and bound SO₂ levels, take a PH measurement of a wine sample to determine how much additional free SO₂ is needed to fully protect the wine (reference chart here). Check that adding additional free SO₂ produces a new total SO₂ level which is still less than 200 PPM using.
2. Setup an SO₂ gas supply connected to a flow controller and barrel injection system.
3. Turn on the SO₂ gas source and inject SO₂ into a batch of wine by setting a the flow controller to a specific flow rate and time of operation relative to the total volume of wine and amount of SO₂ which you are adding.
4. After bottling, using the SO₂ gas source, inject additional SO₂ into empty corks and barrels at flow rates and times of operation relative to the sizes of containers and corks.

Alicat mass flow controllers customized for SO₂ injection

SO₂ is an extremely corrosive gas, and is fully compatible with our anti-corrosive MCS-Series and PCA valves. These devices are long-lasting and perform well in SO₂ injection processes.

Moreover, Alicat mass flow controllers include custom batching and totalizer settings to further automate the SO₂ injection process. For the largest winemaking operations, our devices are configurable with automated settings to reset the totalizer and to start the next batch, providing optimal repeatability, scalability, accuracy, precision, and quality.

Call, email, or chat with an applications engineer