My guess is that the zinc sulfate hydrate is decomposing at the autoclave temperature, typically 220 C (428 F).probably first to a lower hydrate, and then possibly to insoluble ZnO. The issue is temperature, not pressure. I don't think the zinc solution needs comprehensive sterilization. If you add it toward the end of the boil you may see satisfactory results. Don't add the cloudy slurry to the fermenter. ZnO is not at all toxic, but it could give rise to gushing.. In any case, insoluble compounds of zinc are not helpful to your yeast.
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Roger Barth
Emeritus Professor of Chemistry
West Chester University
Author (with Matthew Farber) of
Mastering Brewing Science, 2nd Edition
ISBN 978-1-119-78333-0
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Original Message:
Sent: 04-24-2026 14:50
From: Eddie Padilla
Subject: Autoclaving Zinc
Hello!
I wanted to know if anyone is observing their zinc solution form crystals/flakes after autoclaving. They appear very fine and translucent - almost like a snow globe.
For context, we mix 3g of zinc sulfate heptahydrate into 50 ml of distilled water in an autoclave-safe glass jar, shake it up real well, and steam autoclave for 40 min. This "charge" is added to the fermentor during our first KO.
Before, we "cooked" our solutions in a water bath (1 hour @ 185F), and no crystals/flakes would form. This leads us to believe that its pressure related.
Anyways, long story short, does anyone know what these flakes/crystals are, why they formed, and is it safe to add to our wort. Thanks!
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Eddie Padilla
Smog City Brewing Co
(310) 320-7664
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