What Potassium Metabisulfite Actually Does
Potassium metabisulfite (K₂S₂O₅), commonly called KMS or Campden, releases sulfur dioxide (SO₂) when dissolved in wine or must. That SO₂ does two things: it inhibits spoilage microorganisms — wild yeast, acetic acid bacteria, and Brettanomyces — and it acts as an antioxidant, neutralizing free oxygen before it can damage aroma compounds and color.
But SO₂ exists in two forms in wine. Bound SO₂ has reacted with aldehydes, sugars, and other molecules and is essentially inert. Free SO₂ is what's actually protecting your wine. Of that free SO₂, only the molecular fraction — which depends on wine pH — is truly active antimicrobially. This is why pH matters so much when dosing: the lower the pH, the more protective power you get per milligram of added SO₂.
The Three Moments That Matter
1. At Crush
Adding SO₂ at crush inhibits wild yeast and oxidative enzymes (particularly laccase in botrytis-affected fruit) before inoculation. Typical rates: 25–50 mg/L for healthy fruit, 50–75 mg/L for fruit with mold or damage. Add directly to the crusher-destemmer or to the must immediately after crushing.
For red wines destined for MLF, keep crush additions low (25–30 mg/L). SO₂ is toxic to Oenococcus oeni — the bacteria that drive malolactic fermentation. A heavy crush addition can delay or prevent MLF from starting.
For whites and rosés, where you want to protect delicate aromatics from oxidation, 50 mg/L at crush followed by cold settling gives you clean juice without heavy browning.
2. During Aging and Racking
This is where most small-batch winemakers under-dose. Every racking exposes wine to oxygen. Every barrel is porous. Every tank has headspace. You need to test free SO₂ regularly — at minimum before every rack — and add back to your target range.
Target free SO₂ by pH:
- pH 3.2: 15–20 mg/L free SO₂
- pH 3.4: 20–30 mg/L free SO₂
- pH 3.6: 30–45 mg/L free SO₂
- pH 3.8: 45–60 mg/L free SO₂
These ranges target approximately 0.8 mg/L molecular SO₂ — the threshold that provides reliable antimicrobial protection. Higher pH wines need substantially more free SO₂ to achieve the same protection, which is one reason high-pH wines are more difficult to stabilize.
3. Before Bottling
The most critical addition of all. Once wine is in bottle, you can't adjust. Test free SO₂ within 48 hours of bottling and adjust to the top of your target range — the bottling process itself will bind some SO₂, and you want buffer heading into bottle aging.
Allow at least 24 hours after your final SO₂ addition before bottling to let it fully integrate and equilibrate. Then test again. Don't guess on bottling day.
How to Calculate and Add KMS
KMS powder is approximately 57% SO₂ by weight. The standard working solution is 10% KMS in water — 10 grams dissolved in 90 mL of water gives you 100 mL of solution that delivers roughly 57 mg SO₂ per mL added.
A simpler working formula: to raise free SO₂ by 10 mg/L in 100 liters of wine, add approximately 1.75 grams of KMS powder (or 3 mL of 10% solution). Scale linearly.
Always dissolve KMS in a small amount of water or wine before adding to the tank or barrel. Never add dry powder directly — it won't disperse evenly and you'll get hot spots.
Common Mistakes to Avoid
Dosing by volume without testing. "I add one Campden tablet per gallon" is not a SO₂ management strategy. Wine pH, existing free SO₂, and oxygen exposure all vary. Test, then dose.
Adding SO₂ while MLF is active. If your malic acid conversion is still in progress, SO₂ will kill or inhibit your bacteria. Wait until MLF is complete — confirmed by chromatography or enzyme assay — before adding SO₂ post-fermentation.
Forgetting to track bound SO₂. Total SO₂ at bottling is regulated in most markets (typically 150–350 mg/L depending on residual sugar and market). If you've been dosing aggressively through aging, your total SO₂ can be higher than you expect even when free SO₂ looks fine.
Tools That Make SO₂ Management Easier
Testing free SO₂ by aeration-oxidation (A/O) or Ripper titration gives you accurate, repeatable data. The Ripper method is faster but slightly less precise, especially in red wines — phenolics can interfere with the endpoint. The A/O method is the gold standard for finished wines.
Logging every SO₂ addition — date, volume, amount added, pre- and post-dose readings — gives you a clear record that's useful both for troubleshooting and for regulatory compliance if you're a licensed producer. A lot history that includes SO₂ data lets you spot trends: wines that bind SO₂ faster than expected, lots that consistently run high in total SO₂, or batches that are drifting out of your target range between racks.
Small winemakers who track this data consistently produce more stable wine and spend less time firefighting spoilage problems. That's the whole point.