Method Which Corrects for pH of the Wine
The level of free sulfur dioxide (SO2) in wine is measured in terms of parts per million (ppm). The portion of the measured concentration that is active is greatly affected by the pH of the wine. The active portion is referred to as molecular SO2. There are two common target values, one at 0.8 ppm molecular which will inhibit Malo-lactic bacteria and the other at 0.5 ppm molecular which allows Malo-lactic fermentation to proceed. The pH of the wine or must has a great effect on the portion of free SO2 which is in molecular form. Since the portion of molecular SO2 decreases as the pH of the must or wine increases, accurate additions will require that you can also measure the pH of your must or wine. Use the following steps and tables to guide you in additions of bisulfite to your wine.
- Measure the existing SO2 in the wine. The method I am quoting refers to using the vacuum aspiration method.
- Prepare a bisulfate stock solution by mixing 4 ounces of potassium metabisulfite into 1 gallon of cold water. Please note that this solution concentration is 3.2% which is different than the UCD table which is based on a solution of 10% strength. Shake and stir well but do not sniff-it’s bad on the nasal passages. Keep this solution fresh so don’t store over 2 months and use fresh potassium metabisulfite not over 6 months because it also deteriorates.
- Accurately measure the pH of your wine.
- From Table A below, determine the ppm free SO2 required for the pH measured in the wine. For example, if I want 0.8 ppm molecular SO2 and the pH of my wine was 3.2…I would need add 21 ppm free SO2. If the wine were of higher pH say at 3.4 then I would need to add 32 ppm free SO2 to attain 0.8 ppm molecular SO2. If you measured the amount of SO2 in your wine as indicated in step1, subtract this value from the amount of free SO2 you will need to add which was determined from Table A. The difference is the amount you will need to add to your wine.
- Armed with the knowledge of the ppm free SO2 that you will need to add to your wine, go to Table B to find out how much stock bisulfite solution to add to your wine. Note that the amount to add has been adjusted upward because some of the added SO2 will combine with components in the wine and should not be considered free. These calculations assumed that 2/3 of the addition will remain in a free state. Stir well!
- For the hyperactive winemakers, measure the new SO2 level in 3 or 4 days and adjust again, if necessary. Measure and adjust, if necessary, after every cellar operation and before bottling.
- Add the smallest amount of SO2 to prevent deterioration of the wine. It has been recommended that the pH of the wine be lowered to at least 3.50 to keep total additions of SO2 to a minimum: 100 pm SO2 has been suggested as a target value. To lower the pH of the must or wine, refer to additions of Tartaric acid.
|Table Contains Free ppm SO2 for Target Molecular Values of:|
|pH of Wine||>0.5 ppm Molecular||0.8 ppm Molecular|
|2.90||7 ppm||11 ppm|
|2.95||7 ppm||12 ppm|
|3.00||8 ppm||13 ppm|
|3.10||10 ppm||16 ppm|
|3.15||12 ppm||19 ppm|
|3.20||13 ppm||21 ppm|
|3.25||15 ppm||23 ppm|
|3.30||16 ppm||26 ppm|
|3.35||18 ppm||29 ppm|
|3.40||20 ppm||32 ppm|
|3.45||23 ppm||37 ppm|
|3.50||25 ppm||40 ppm|
|3.55||29 ppm||46 ppm|
|3.60||31 ppm||50 ppm|
|3.65||36 ppm||57 ppm|
|3.70||39 ppm||63 ppm|
|3.75||45 ppm||72 ppm|
|3.80||49 ppm||79 ppm|
|3.85||57 ppm||91 ppm|
|3.90||62 ppm||99 ppm|
|3.95||71 ppm||114 ppm|
|4.00||78 ppm||125 ppm|
|Amount of Bisulfate Stock Solution (step 2) to Add to:|
|Desired Free||SO2 Adjusted SO2||5 Gallons||10 Gallons||15 Gallons|
|5 ppm||7.5 ppm||5/8 tablespoon||1-1/4 tablespoons||2 tablespoons|
|10 ppm||15 ppm||1-1/4 tablespoons||2-1/2 tablespoons||3-3/4 tablespoons|
|15 ppm||23 ppm||1-7/8 tablespoons||3-3/4 tablespoons||5-3/8 tablespoons|
|20 ppm||30 ppm||2-1/2 tablespoons||5 tablespoons||1/2 cup|
|25 ppm||38 ppm||3-1/8 tablespoons||6-1/4 tablespoons||5/8 cup|
|30 ppm||45 ppm||3-3/4 tablespoons||7-1/2 tablespoons||3/4 cup|
|40 ppm||60 ppm||5 tablespoons||5/8 cup||1 cup|
|50 ppm||75 ppm||5-1/4 tablespoons||3/4 cup||1-1/4 cups|
|60 ppm||90 ppm||7-1/2 tablespoons||1 cup||1-1/2 cups|
|70 ppm||105 ppm||8-3/4 tablespoons||1-1/8 cups||1-3/4 cups|
|80 ppm||120 ppm||5/8 cup||1-1/4 cups||2 cups|
|90 ppm||135 ppm||3/4 cup||1-1/2 cups||2-1/4 cups|
|100 ppm||150 ppm||7/8 cup||1-3/4 cups||2-1/2 cups|
UCD Table for Additions of Potassium Metabisulfite
The following table has been reprinted from the UC publication on making table wine. It does not refer to pH adjustments but it can be used as a quick reference. Please note that this table is based on a 10% solution which is different than the solution used for the tables that take pH into account.
|Desired final SO2 concentration (ppm)* of Must or Wine|
|(gal) (Add ml of 10% SO2 stock solution)|
* The volumes indicated assume 100 percent purity of the potassium metabisulfite (K2S2O5) and full strength of the stock solution.
For a 10% SO2 stock solution:
Make 2 ounces of bisulfate powder up to 591 ml. Use room temperature water, agitate until dissolved, then refrigerate.
Note: It is more convenient to mix up 750 ml of stock solution rather than 591 ml because you can use a wine bottle in which to both mix and store the stock solution. To use a wine bottle, add 72 grams (or 4 Tablespoons and 1 teaspoon) of Potassium metabisulfite to an empty wine bottle. Fill the wine bottle with water, then agitate until the powder is dissolved. This will provide you with the 10% SO2 stock solution with which to make additions as per the table above.