Potassium metabisulfite goes into your wine at several specific points: at crush, after fermentation, at each racking, and at bottling. Each addition serves a different purpose and calls for a different dose. Getting the timing right protects your wine from spoilage organisms and oxidation without leaving harsh sulfur flavors in the finished bottle.
At Crush: Your First Addition
The first dose goes in immediately after you crush your grapes. The goal here is to knock back wild yeast, bacteria, and other spoilage organisms that ride in on the fruit skins, giving your chosen commercial yeast a clean environment to work in. For grapes in good condition, add 50 ppm of sulfur dioxide based on the total volume of the must. If your fruit shows visible mold or rot damage, increase that to 80 or even 100 ppm to counteract the heavier microbial load.
For white wines that won’t go through malolactic fermentation, a slightly higher starting dose of 80 ppm is common because whites are more vulnerable to oxidation during processing.
After adding sulfite at crush, let the must sit overnight before pitching your yeast. That window gives the sulfur dioxide time to do its work against unwanted organisms while dissipating enough that your commercial yeast strain, which is more sulfite-tolerant, can establish itself without trouble.
Between Fermentations
What you do next depends on whether you’re planning a malolactic fermentation. Malolactic bacteria are sensitive to sulfur dioxide, so if you want that secondary fermentation to happen, keep your addition very low after primary fermentation wraps up: 0 to 10 ppm at most. Some winemakers skip it entirely at this stage to give the bacteria every advantage.
If you’re not doing malolactic (common with many white wines and some fruit wines), you can move straight to a larger protective dose.
After Malolactic: The Big Addition
Once malolactic fermentation is confirmed complete, or once you’ve decided to skip it, this is the point for your largest single sulfite addition. Add 50 to 75 ppm of sulfur dioxide. This “big add” sets up your wine’s defenses heading into bulk aging, where it will sit for weeks or months exposed to slow oxygen exchange. From this point forward, you want to maintain at least 25 ppm of free sulfur dioxide in the wine at all times.
At Each Racking
Every time you rack your wine off its sediment, you introduce a small amount of oxygen. Add about 25 ppm of sulfur dioxide with each racking to replenish what gets consumed by that oxygen exposure and by ongoing chemical reactions in the wine. If you’re aging for several months and racking two or three times, these incremental doses keep your protection consistent without building up excessive total sulfite levels.
At Bottling
Your final addition comes just before bottling. A dose of 35 to 40 ppm ensures the wine has enough free sulfur dioxide to stay protected in the bottle during storage and aging. This is your last chance to adjust levels, so testing your free SO2 before this addition helps you dial in the right amount rather than guessing.
Why pH Changes Everything
The numbers above are general guidelines, but your wine’s pH determines how effective any given sulfite dose actually is. Only one specific chemical form of sulfur dioxide, the molecular form, actually kills bacteria and unwanted yeast. How much of your free SO2 exists in that active molecular form depends entirely on pH.
The relationship is exponential, not linear. At pH 4.0, you need ten times more free sulfur dioxide to achieve the same antimicrobial protection as you would at pH 3.0. This means a wine sitting at pH 3.8 or higher demands significantly more sulfite to stay safe than a crisp wine at pH 3.2. If your wine’s pH runs high, test and adjust your sulfite levels more frequently during aging, and consider adding slightly more at each stage.
How to Calculate the Right Amount
Potassium metabisulfite is only about 57.6% sulfur dioxide by weight, so you need more powder than you might expect to hit your target ppm. The simplified formula for home winemakers working in gallons:
Gallons of wine × desired ppm × 0.0066 = grams of potassium metabisulfite to add
For example, if you have 6 gallons of wine and want to add 50 ppm, that’s 6 × 50 × 0.0066 = about 2 grams. A precision kitchen scale that reads in tenths of a gram makes these small measurements much more reliable than volume-based estimates like “quarter teaspoon.”
Damaged or Moldy Fruit Needs More
Fruit with visible rot or botrytis carries a much higher population of spoilage organisms and produces enzymes that bind up sulfur dioxide, making it less available. Winemaking protocols from Laffort recommend scaling your crush-time addition based on contamination level: 50 ppm for low rot, 80 ppm for moderate, and 100 ppm for heavily affected fruit. The worse the fruit condition, the faster the sulfite gets consumed, so you may need to recheck levels sooner during early processing.
Staying Within Legal and Practical Limits
In the United States, finished wine sold commercially must stay within total sulfur dioxide limits set by federal regulation. For most table wines, the cap is 350 ppm total SO2, though well-made wines rarely come close to that number. Any wine containing 10 ppm or more of total sulfur dioxide must carry a “Contains Sulfites” label. Even wines made without any added sulfite can require this label, since fermentation itself produces small amounts of SO2 naturally.
For home winemakers, the practical ceiling is much lower than the legal one. If you follow the stage-by-stage additions outlined above, your total sulfite levels will typically land well under 200 ppm, which is the range where sulfite flavors remain undetectable to most people. Overshooting, especially by guessing rather than measuring, is what leads to that unpleasant burnt-match character. Testing your free SO2 with an inexpensive test kit before each addition keeps you in control and prevents stacking doses unnecessarily.