Measuring and Adding SO2
Practically all wine is made with Sulfur Dioxide, SO2, added repeatedly in small doses to prevent spoilage. Still, wine can be made sulfur-free with very strict sanitation and processing techniques – but this not common (Frey Vineyards is an example, http://www.freywine.com/). Sulfur degrades certain types of enzymes which spoil the wine by oxidizing phenols; this is its role as antioxidant. A specific form of SO2 also kills bacteria and non-Saccharamyces yeasts by entering through their cell walls. There are 4 different instances when SO2 is added:
- Right after grape sorting and before cold soak - if fermentation will be done with non-indigenous yeasts. The purpose then is to kill off all indigenous yeasts first
- After malolactic fermentation has finished.
- During cellaring whenever barrels are topped up or racked.
- Just before bottling
On this page we describe how we calculate the amount of SO2 we add in each of the four instances and how we measure the concentration of SO2 in the wine before and after.
Different forms of SO2
The story is convoluted because only a portion of the “Total SO2” added is effective; that portion is called “Molecular SO2”, however you can only measure and control “Free SO2” (see Laboratory Tests): Total SO2 can be divided into Bound SO2 and Free SO2.
- Bound SO2: When adding SO2, a good portion of it becomes immediately bound to sugars, acetaldehydes and phenolic compounds (called “Bound SO2”) and becomes ineffective; the remainder is called “Free SO2”
- Free SO2 exists in 3 forms as Molecular SO2 (SO2), Bisulfite (HSO3-) and Sulfite (SO3=). An excellent explanation of how Free SO2 demonstrates itself is given in Yair Margalit’s Wine Concepts in Wine Chemistry 3rd edition, page 315-319. The graphic below shows on the left the relative percentage of each form in a solution depending on its pH. In very acidic environments (pH near zero). Molecular SO2 dominates. As the pH rises, Bisulfite replaces Molecular SO2 until the pH reaches around 4.5 and Molecular SO2 vanishes. When the pH rises further Bisulfate is increasingly replaced by Sulfite. Thus in wine which normally has a pH between 3 and 4, Free SO2 is a mixture of mostly Bisulfite (HSO3-) and a small portion of molecular (SO2).
Here is what matters: In wine the amount of Molecular SO2 in Free SO2 is relatively small. Example: if you want to have 0.8 ppm of Molecular SO2 you need to provide 22ppm of Free SO2 when the pH is 3.2; whereas you need 43 ppm of Free SO2 when the pH is 3.5. This is akin to breathing when you climb a very tall mountain: the higher you climb (in terms of pH) , the more air you have to breathe to get your necessary oxygen (Molecular SO2). The problem is: wine with a pH above 3.65 requires too high a Sulfur addition, thus one either depresses the pH by acidifying the wine or swithches to an entirely different regime.
How much SO2 to add?
The accepted practice in the wine industry is to target a top Molecular SO2 - level of around 0.5 – 0.7 ppm for red wine. The idea is to add as little SO2 as possible while still preventing spoilage.
- The upper bound is given by law (which sets a limit of 350ppm Total SO2) and, more importantly, by sensory degradation: Total SO2 above 100ppm can create a chemical taste that covers up fruitiness; Molecular SO2 above 0.7ppm has a burnt match smell (sulfur).
- The lower bound is given by the effectiveness of Molecular SO2 as antioxidant and bacteria- and yeast - killer.
These boundaries create a challenge in wines with very low acidity (high pH): the amount of Total SO2 to be added to create enough Molecular SO2 becomes too large. The solution to this challenge is to artificially increase the wine’s acidity.
The compound most often used to add SO2 to wine is not SO2 itself but a powder, Potassium Metabisulfite K2S2O5, “KMBS” which contains 57.4% SO2 . To calculate “A” the amount of KMBS (in grams) required you need to know:
- The volume of wine to be treated (gallons): V
- The current level of “free SO2” in the wine (in ppm) – see measuring “free SO2”: S
- The pH level of the wine: p
- The target level of “molecular SO2” (e.g. 0.5 ppm): M
The formula to calculate the estimated required amount of KMBS (in grams) is (derived in Margalit’s book) is:
A= 0.0065712 * V * [ M * (1+10(p-1.81)) – S ]
This formula provides a good approximation of the balance between Bisulfite (HSO3-) and Molecular SO2 (SO2) in a liquid with a pH in the range between 1.5 and 4.5. The chart on the right shows the relationship. The constant 0.006572 = 3.785 / 1000 / 0.574 comes from converting gallons (3.785) into liters, ppm [i.e. mg/L into g/L (1000) and the fraction of Total SO2 in KMBS (57.4%). Note the two markers on the curve: The amount of Molecular SO2 in Free SO2 is relatively small; i.e. the share of molecular SO2 is only 3.9% at a pH of 3.2, and it drops to 2% at a pH of 3.5. In other words, at normal pH levels in wine (between 3 and 4) we will see only a small amount free and potentially active Molecular SO2
So here are the guidelines we follow for SO2 addition in practice:
- In the years we use industrial yeasts for fermentation, we kill off all natural yeasts before cold soak or fermentation by adding an amount of KMBS which creates a molecular SO2 level of 0.5 ppm. When we use the naturally occurring indigenous yeasts we do not add KMBS upfront.
- After malolactic fermentation has finished we top up with KMBS to create a Molecular SO2 level of 0.45ppm.
- Whenever we top up or rack a barrel (every ~3 months) we measure the pH of the wine and the Free SO2 level and then we compute the corresponding Molecular SO2 level. If Molecular SO2 has fallen below 0.35 ppm we top up with KMBS to target a Molecular SO2 level of 0.45 ppm.
- Before bottling we top up with KMBS to a target Molecular SO2 level of 0.5 ppm
Further Resouces: Good websites:
- From distributor of SO2 measurement tools
- Easy to read article
- Excellent lecture
- Sulphite calculator from fermsoft.com
Measuring Free SO2
We measure Free SO2 by the "Aeration - Oxidation" method. First the SO2 is removed by a stream of air passing through the sample solution and the SO2 gas is trapped in a hydrogen peroxide solution which oxidizes it to sulfuric acid. Then the amount of sulfuric acid created is measured by titration with NaOH. The process takes 4 steps::
Step 1: Put the following into the round bubble flask (and close)
- 10 milliliter of 25% Phosphoric Acid (use rubber ball at end of pipette!)
- 20 milliliter of wine to be analyzed
Step 2: Fill the straight flask into which the vapor goes with
- 3% Hydrogen Peroxide up to the line and
- Add 3 drops of green indicator solution (in fridge)
Step 3: Set timer of pump to 10 minutes, turn on and put volume control so that indicator shows midlevel volume. Wait till done. Liquid in straight flask should turn purple
Step 4: Fill titrator with 0.01 Sodium Hydroxide up to an easily identifiable Starting point
- Put straight flask onto stirring table with stirrer inside and turn stirrer on
- Slowly drip Sodium Hydroxide into purple liquid and stop when it turns green
Result: Free SO2 [in ppm] = (Startpoint [in milliliter] – Endpoint [in milliliter]) * 16