Wine and mead
Potassium metabisulfite calculator.
Enter the pH, volume and molecular SO2 target to see the free SO2 you need and the potassium metabisulfite to add.
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How it works
Sulfur dioxide protects wine and mead from oxidation and spoilage microbes. Only part of it, the molecular SO2, is the active form, and how much of your free SO2 is molecular depends on pH. VinoEnology gives the recommended molecular SO2 as 0.5 mg/L for red wines, 0.8 mg/L for white wines and up to 1.5 mg/L for dessert wines. Wikipedia lists the pKa of sulfurous acid as about 1.81, so the molecular fraction at a given pH is 1 ÷ (1 + 10pH − 1.81), and the free SO2 target is the molecular target divided by that fraction. This is our own calculation from the published pKa, so treat it as an estimate.
Potassium metabisulfite is 57.6% SO2 by weight (VinoEnology). Grams to add = free SO2 to add (mg/L) × litres ÷ 1,000 ÷ 0.576. GotMead notes that 1 g of K-meta, about 1/8 teaspoon, per gallon provides 150 ppm.
Worked example. 5 US gallons (18.93 L) of white wine at pH 3.40 with no free SO2 and a molecular target of 0.8 ppm. The molecular fraction is 2.5%, so the free SO2 target is 0.8 ÷ 0.0251 = 32 ppm. Adding 32 ppm to 18.93 L needs 32 × 18.93 ÷ 1,000 ÷ 0.576 = 1.05 g of potassium metabisulfite. If the batch already has 15 ppm free SO2, the dose falls to 0.56 g.
Higher pH needs much more free SO2 for the same protection: at pH 3.8 the target for 0.8 ppm is about 79 ppm, which is well above the 50 ppm level where VinoEnology says SO2 can be tasted. Acidity adjustment is usually the answer in that case, not more sulfite.
Frequently asked questions
How much potassium metabisulfite per gallon of wine?
It depends on pH and your target. For white wine at pH 3.4 with no free SO2 the tool gives about 0.21 g per US gallon. GotMead notes 1 g per gallon would give 150 ppm.
What is molecular SO2?
It is the active, protective part of free SO2. VinoEnology recommends 0.5 mg/L for reds, 0.8 mg/L for whites and up to 1.5 mg/L for dessert wines.
Why does pH matter?
Wikipedia explains that lower pH shifts free SO2 toward the active molecular form. At higher pH you need far more free SO2 for the same protection.
Can I use this for sparkling mead or if I plan to add sorbate?
GotMead says not to add chemicals to a sparkling mead because it stops the priming sugar from fermenting. Sorbate must be used with proper SO2 for stabilizing, per VinoEnology.
Sources
Opened
- VinoEnology: SO2 addition and SO2 reduction: Potassium metabisulfite is 57.6% SO2; recommended molecular SO2 of 0.5, 0.8 and up to 1.5 mg/L; free SO2 over 50 mg/L can be tasted.
- Wikipedia: Sulfur dioxide: Infobox pKa about 1.81, and that free SO2 exists as molecular SO2 and bisulfite depending on pH.
- GotMead: Newbee Guide, Chapter 18, Bottling: 1/8 teaspoon (1 gram) of K-meta per gallon provides 150 ppm free SO2; no chemicals in sparkling mead; asthma caution.
- GotMead: Newbee Guide, Chapter 5, Terminology and Calculations: Honey and water must usually has a natural pH of about 3.7 to 4.6.
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Scope
Sulfite can cause severe allergic reactions in some people, particularly asthmatics (GotMead). Free SO2 should be measured, not assumed, and these doses are estimates that ignore binding in your batch. Follow local labelling and limits. Not professional or food-safety advice.