How much wort?
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Zinc you're aiming to add.
mg/L and ppm are the same thing here. Around 0.5 mg/L is a common all-round figure for a clean, complete fermentation.
Into the boil, or at pitching?
A decision you make before brewing, and it sets the dose. The boil addition is the usual reference: it meets the hot and cold break, where up to about half binds to the trub and drops out. Add the same zinc to the fermenter at pitching, after the break has settled, and almost none is lost — so you only need about half as much.
Count what the malt brings.
The addition
Food-grade zinc sulfateDose from a stock solution.
At homebrew scale the addition is only tens of milligrams — fiddly to weigh accurately. Far easier: weigh out a larger, comfortable amount of zinc sulfate once, dissolve it in water, and dose a measured volume with a syringe. Dissolve N g in 1 L and every millilitre carries N mg of salt — so the dose is just the weight you need divided by the strength.
Build the stock from whichever salt you'll dose — the two differ in zinc content. Stir until clear and keep it capped; it's stable for weeks. Pick a strength that lands the dose in an easy-to-read range, a few millilitres to a few tens. A concentrated zinc stock is more hazardous than the wort, so label it clearly. At commercial scale, where the addition is already several grams, it's usually simpler to weigh it straight.
How it's worked out
Zinc is an essential yeast nutrient — it drives enzyme activity through alcohol dehydrogenase and supports healthy growth and attenuation. A deficient wort shows up as a lagging, incomplete fermentation. Malt supplies only about 0.1 mg/L, while a clean, complete ferment generally wants 0.2–0.6 mg/L in the wort, so a small supplement is common — usually food-grade zinc sulfate.
The elemental zinc you need is simply Zn (mg) = addition (mg/L) × volume (L). Each compound is only part zinc by weight: zinc's atomic mass is 65.38 g/mol, so for the heptahydrate (287.56 g/mol) zinc is 65.38 ÷ 287.56 = 22.7%, and for the monohydrate (179.47 g/mol) it's 65.38 ÷ 179.47 = 36.4%. The salt to weigh out is the elemental zinc divided by that fraction: grams = Zn(mg) ÷ %Zn ÷ 1000. Worked example — 30 hL at 0.5 mg/L is 1,500 mg Zn, which is 6.6 g of heptahydrate or 4.1 g of monohydrate.
The target above is the zinc addition rate, taken as a boil addition — the common reference. Where you add it sets the dose. A boil addition meets the hot and cold break, and up to ~50% settles out with the trub, so the boil figure already carries that allowance. Add the same zinc at pitching, after the break has dropped, and almost none is lost — so you need only the share that would have survived, about half. Boil dose = target; pitching dose = target × (1 − loss). Counting malt's ~0.1 mg/L subtracts first, so the addition is (target − malt) for the boil, times (1 − loss) at pitching.
Zinc is toxic to yeast in excess. The healthy band is narrow, so weigh additions on an accurate scale rather than eyeballing — these are gram-and-fractions-of-a-gram quantities. Use food-grade material, and add it to the kettle so it disperses fully. Figures here are estimates; trust your own fermentation performance over any single number.
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