Ask The Brewmasters

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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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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I agree with Roger on all points.

If you were, for whatever reason, set on maIntaining a sterile zinc solution, you could use a filter flask like this, rather than autoclaving:

https://www.weberscientific.com/corning-disposable-sterile-filter-systems

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Jeffrey Muston
Principal
Muston Consulting
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Hi Eddie,

Along the lines Matthew mentioned. It's not completely a saturation issue; in the autoclave the zinc salt changes its hydration state, and when it cools, it rebuilds the heptahydrate crystals. This creates hyperlocalized supersaturation zones wherever there are nucleation sites, like tiny scratches in the glass. This can leave little flakes even though the solution is still well below its solubility limit. A quick swirl with a small amount of heat should dissolve them again. If you don't add these crystals to your wort, you are leaving behind some of the zinc you calculated for. 

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Kory Davis
QC and Propagation Specialist
The Brewing Science Institute
Woodland Park, Co
719-482-4895 ext. 3
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Thank you everyone for your inputs! Lot's to consider here. 

What we found as Roger pointed out was we were superheating our solution, decomposing the zinc. We reduced temp and time of autoclave and eliminated the crystalline formation. Once again, thank you all! 

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Eddie Padilla
Smog City Brewing Co
(310) 320-7664
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Just a word of caution - if you add the zinc to the end of your boil, a large percentage will bind to proteins in the trub and you won't deliver the expected amount to your fermenter. The best Zn addition point is in-line to the FV. 

FWIW, we also autoclave our zinc solution (~18%) and do not see crystallization. We hold at 15 PSI at 121°C for 20 min. 

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Matt Linske
Quality Lab Manager
Creature Comforts Brewing Co.
Athens, GA 30601
m.linske@ccbeerco.com
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