Ask The Brewmasters

I've got some cans that are discolored. See attached picture. It may be a little hard to see, but the bottoms are almost like they're stained a light brown, and there's a sort of rainbow sheen. The rest of the can is just fine, it's only on the bottom. Any ideas on what this is?

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Colin Foy
Head Brewer
West Side Brewing
Cincinnati, OH
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Colin:

This looks to me to be simple dome staining.  There are a number of things that could lead to this.  How are these cans processed after they are filled and seamed?  

Rick

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Rick Brundage
Director Thermal Processing
ChemTreat Inc
Powhatan VA
(804) 935-2263
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This looks like can dome staining. It is often seen when cans remain inside the hot zones of a tunnel pasteurizer or can warmer during prolonged line stoppages. 
There is no protective lacquer at the base of the can. The aluminum alloy used in cans contains iron and manganese. If the temperature is high and if the pH of the water is outside the range of approximately  6.5-7.5 the aluminum will dissolve from the surface exposing iron and manganese oxides which impart a brown/black stain.
Can dome staining can be suppressed by strict control of the pH and the addition of specific corrosion inhibitors to the hot zones. If the problem persists I suggest that you talk with your chemical supplier. Also, whenever there is a major line stoppage empty the pasteurizer /warmer as soon as possible.

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Richard Rench
Oakville ON
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Just to follow up on this, these cans were produced by a contract brewer for us. They confirmed the stains were from pasteurization, which they'd never done to our beers before and which they did without notifying us or discussing it with us.

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Colin Foy
Head Brewer
West Side Brewing
Cincinnati, OH
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The quality of the water used for tunnel pasteurizing can cause staining like this.  Temperature, pH, total alkalinity, and mineral hardness can all impact can staining.

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Jason Weihbrecht
GM of Brewery Operations
Oskar Blues Brewery
Brevard NC
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Jason is correct in listing  a few of the more important parameters that may influence can staining. For a more detailed explanation please read chapter 17 in Brewery Packaging volume 2 from MBAA.  Did the dome staining occur during a "skip" where cans sat in the pasteurization zone for an extended time ?  We're other cans in the regenerative zones not affected? If so, selection of the right corrosion/deposit inhibitor will normally extend the window of forgiveness associated with can staining.
Jack Bland,  Consultant

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Jack Bland
Consultant
ChemTreat
2019 Pampatike rd
King William
Virginia
jackb@chemtreat.com
804-337-3416
(804) 337-3416
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Original Message:
Sent: 08-24-2022 14:35
From: Jason Weihbrecht
Subject: Can discoloration

The quality of the water used for tunnel pasteurizing can cause staining like this.  Temperature, pH, total alkalinity, and mineral hardness can all impact can staining.

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Jason Weihbrecht
GM of Brewery Operations
Oskar Blues Brewery
Brevard NC
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