What Is SO2?
Sulfur dioxide is the primary preservative used in wine. It acts as an antioxidant and antimicrobial agent to help stabilize wine through its duration. Chemically, sulfur dioxide is described as SO2. This indicates that the compound consists of one sulfur atom (S) and two atoms of oxygen (O).
Why Is SO2 Used In Wine?
SO2 is the only preservative that has the ability to act as an antioxidant (inhibiting oxidation and nonenzymatic browning), enzymatic browning inhibitor, and antimicrobial agent (against wine yeast and bacteria) without altering wine flavor when used properly.
How Is SO2 Used In Wine?
Sulfur dioxide is generally added to wine in the form of granular salts of metabisulfite or sulfite. The primary salt of choice used in the wine industry is potassium metabisulfite (KMBS). KMBS can be purchased at any local winery supply store.
Depending on the supplier, about 57.6 percent of KMBS will contribute to the sulfur dioxide concentration in wine. This conversion must be considered upon addition to juice or wine:
[(Desired concentration of SO2) / 0.576] x (volume of the wine) = (mass of KMBS to add to the wine)
For example, let's say you wanted to make an addition of 50 mg/L or ppm (milligrams per liter or parts per million) SO2 to a wine tank with a volume of 500 liters (L). To calculate the mass of KMBS granules needed to add to the wine tank:
[50 mg/L SO2 / 0.576] x 500 L = 43,403 mg or 43 g of KMBS
When the KMBS is added to the wine, the SO2 will dissolve into either a bound or free form. The bound forms are "inactive" and do not contribute to any antioxidant or antimicrobial properties in the wine. The free form of SO2 is the active form and contributes to the wine's stability. The two forms together (free plus bound) equals the total SO2 in the wine.
Did you Know?
If winemakers are making frequent SO2 additions to a wine, but see no change in the free SO2 concentration, the wine is most likely oxidized and contains a higher concentration of acetaldehyde, a component of oxidation. Acetaldehyde will readily bind to newly added SO2, and will prevent a change from being seen in the free SO2 concentration.
The SO2 concentration in wine is in constant equilibrium and will change, especially when the wine is moved. Monitor SO2 concentrations:
- After fermentation and/or malolactic fermentation
- Through oak aging
- During tank storage
- After product additions or cold/protein stability
- Prior to bottling
- After bottling
A good quality-control step is to test the free SO2 concentration every 2 to 3 weeks.
Caution: Sulfur dioxide is a hazardous material that can release toxic gases, especially when it comes in contact with acids or low-pH environments. Special handling and caution are advised. Be sure to consult the material safety data sheet (MSDS).
Preparation of a liquid sulfur dioxide stock solution (in water) is also a possibility for wine additions. Solutions of 5-10 percent are commonly recommended and convenient for making several SO2 additions to various wines at one time. Stock solutions should be prepared immediately prior to use, as they are not stable for long periods of time.
Based on the fact that KMBS is made up of 57.6 percent sulfur dioxide, the addition of 1 gram (g) of KMBS to 1 liter (L) of water will equal 0.576 grams of SO2 in solution. The concentration of this solution is 0.576 grams per liter (g/L) SO2.
How to Make a 10 Percent SO2 Stock Solution
Remember that you need about twice as much KMBS to get a desired amount of SO2 because KMBS contains 57.6 percent SO2. Therefore, to figure out the mass of KMBS needed to obtain 100 grams of SO2:
100 g SO2 / 0.576 = 174 g KMBS
Using a 1-liter volumetric container (e.g., cylinder, volumetric flask), add the appropriate amount of KMBS (for a 10 percent solution, this would equal about 174 grams KMBS). Then, fill the container with (unheated) distilled or RO (reverse osmosis) water to the 1-liter line, slowly dissolving the KMBS in the solution.
To determine the appropriate amount of SO2 to add to a tank, use the C1V1 = C2V2 equation, where C1 = the concentration of SO in the stock solution, V = the volume of stock solution to be added to the tank, C2 = the desired concentration of SO in the tank, and V = the volume of the tank. Keep in mind that all units must be the same between the concentrations and the volumes.
For example, a wine tank is 500 liters in volume and you are looking for a 100 mg/L concentration of SO2 in the tank. How much 10 percent (100 g/L) SO2 stock solution will you need to add to the wine tank (assuming this is the first 2 addition)?
First, convert 100 milligrams per liter to grams per liter (100 mg/L = 0.1 g/L).
Using the C1V1 = C2V2 equation, substitute in the information you know, and solve for V1 (the volume of stock solution needed to add to the wine tank):
C1V1 = C2V2
(100 g/L)(V1) = 0.1 g/L(500 L)
V1 = 0.5 L or 500 mL
You would then add 500 milliliters (mL) of the 10 percent stock solution to the wine tank to make the appropriate SO2 addition.
Although the SO2 concentration should theoretically come to equilibrium in the wine, it is recommended that wineries mix the wine well to dissolve the SO2 solution throughout the entire volume of the tank. It is advised to check the free SO2 concentration at least 24 hours after addition to ensure accuracy.
Know Your pH So You Know Your Molecular SO2 Concentration!
The free sulfur dioxide content is made up of three parts: molecular SO2, bisulfite (HSO3-) SO2, and sulfite (SO3-2-); see diagram on front. Because the sulfite form is quite negligible (less than 1 percent), the percentage of free SO2 in the sulfite form is not often calculated. The majority (over 90 percent) of free SO2 is composed of the bisulfite form, which acts as an antioxidant in wine. However, a small portion of the free SO2 will also exist as the molecular form. The molecular SO2 is the antimicrobial agent that is contributed by the free sulfur dioxide concentration. Molecular SO2 can be calculated using the following equation:
[Molecular SO2] = [Free SO2]/[1+10(pH-1.8)]
Winemakers are advised to aim for a molecular SO2 concentration between 0.5 and 0.8 mg/L, as these levels are inhibitive to most wine yeast and bacteria growth. The percentage of free SO2 that is in the molecular form is directly related to the pH of the wine. Therefore, in order to make accurate additions of sulfur dioxide, winemakers will also need to know the pH of the wine. Because the pH of wine is inconsistent among all wines, it is not advised to make standard concentrated SO2 additions to each wine. Base SO2 additions on a wine's pH.
As the pH gets lower (≤ 3.0), less SO2 needs to be added to the wine to retain the appropriate molecular levels. A wine that is closer to a pH of 4.0 will require much more SO2 to retain an appropriate molecular level. For example, at a pH of 3.0, a free SO2 of 13 mg/L is needed to obtain the 0.8 mg/L molecular SO2 concentration. At a pH of 4.0, a free SO2 of 125 mg/L is needed to obtain the 0.8 mg/L molecular SO2 concentration that will be inhibitive to yeast and bacteria growth. This relationship can be seen using the equation above.
At molecular levels below 0.5 mg/L, the SO is not providing enough antimicrobial protection to ensure wine stability. At molecular levels above 0.8 mg/L, the free SO can easily be sensed through a burning or irritating sensation in the nasal passageway.
- Monitoring the Winemaking Process from Grapes to Wine: Techniques and Concepts by Patrick Iland (ISBN: 978-0-9581605-6-8)
- Introduction to Wine Laboratory Practices and Procedures by Jean L. Jacobson (ISBN: 0-387-24377-1)
- Wine Analysis and Production by Bruce Zoecklein (ISBN: 0-8342-1701-5)