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Bottle caps turning black on inside

I noticed my bottle caps are turning black on the side exposed to the beer. The black discoloration is under the plastic coating on the caps ( I scraped the plastic off with a knife). The beer was bottled end of november 2021. Not terribly concerned as its on other side of plastic not touching beer. Just seems weird. The caps are from an overrun of caps from either phillies or pirates. Caps are like 5 years old…got a gazillion of em cause they were cheap. I bottle aged chimay blue and red clones for over a year (about 4 years ago) and never saw this before.

Anyone seen this happen?


Looks like a fungus. Me can not help you that much i keg only.

Agree with @wilcolandzaat , looks like caps have mold….I’d be a bit cautious as some of those are pathogenic. Never had that happen to me.

Time to get new caps I’d think. I wonder if the solution you “sanitized” the caps in was ok?

I agree it looks like mold. If it’s between the metal and the plastic only there shouldn’t be a problem but I would be kind of freaked out seeing it.

Sucks that you have so many at a bargain price and probably don’t trust them now.

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@mikem do you soak you caps in sanitizer before use? I’ve seen this occasionally on capped. bottled or jarred goods. I don’t think it’s a cause for alarm.

Good point. I notice mold on the lid of the bucket i have sanitizer in if getting old. I was figuring they got wet or damp in storage at some point. Probably why they were sold cheap.

they are submerged in sanitizer from less than a minute to hours. I will bottle 8 today and will use fresh sanitizer with short contact time on 4 and old hour soak on other 4 to see if different. I mix up sanitizer in 1/2 gallon increments so sanitizer goes quickly. Never had pH get too high and I discard when foaming drops or it looks too cloudy.

I’d pul off the plastic and investigate. I have mason jar lids that look like this and it’s rusty oxidation not mold

I believe he already did that …

Again probably not bubonic plague, however he asked. Mold loves moisture. Ask any Floridian.

My thoughts are- if it was caused by moisture from sanitizing or beer obviously there is contact between the now carbonated beer and whatever it is. Would make no sense that moisture would be able to get once but never again.

just throwing it out there but does mold need O2 ? Are they O2 absorbing caps? maybe some spores got under the cap and when the absorbed the O2 they grew. Im thinking once the bottles condition and the O2 is no longer present it wont affect the beer?

Did speak to a brew friend of mine he only bottles his brew. On every beer bottle cap he pulls of the plastic than sanatize them. He had mold before. But after removing plastic. He never had that issue again

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Remove the plastic than sanatize you caps. So the little mold creatures have no place to hide

squeegeethree I did some further investigation and the black stuff does not dissolve in water or break apart. Its hard. Looks like corrosion. The beer was 4 months old. I will find out if the soaking of caps in starsan is a bad move in 4 months. I have not observed it at two months.

The starsan is corrosive due to low pH. I had it eat up my old autosiphon rubber seals in an old version of NB autosiphons. I would always clean it, sanitize it and leave the starsan on it. Now I sanitize then rinse it off with water.

I wont remove the plastic as suggested because its real tough to get off…almost cut myself a few times. Thanks for the ideas.

I think yer onto sumptin’… I will wonder out loud… Will mold thrive in a low pH environment? Metal/tin do not like it… So, since the caps have soaked in an extremely low pH, I will suspect corrosion… not mold… " Hey! Can I get a rusty capped brew around here?"… Clowning, eh…
Sneezles61

I’ve found with my mason jars that the cap coating eventually fails to rust

Same here.

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