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If I put some beer in a secondary vessel for medium-term condition (e.g. lagering or dry hopping), do I need to put an airlock on it or can I just close it up with a stopper or put it in a keg and seal it? Presumably the yeast are not really generating any more CO2. (Note: I'm not talking about an actual secondary fermentation with fruit or something, where the yeast will clearly be active.)

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up vote 4 down vote accepted

You absolutely do not need an airlock for secondary, assuming you wait til fermentation is done. I've sealed a carboy with a stopper many times for a secondary, although these days I usually use foil. If the beer is still outgassing, you will have a bit more dissolved CO2 in it, but not enough to worry about the carboy exploding. A keg also works really well and can be vented by pulling the relief valve if necessary (although it almost never is). The best alternative, IMO, is to just not use a secondary.

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+1 on the foil and a rubber band. – Dale Jun 27 '12 at 21:50

The yeast may still be working, and even if they aren't, CO2 may still be coming out of solution from temperature changes or agitation. Glass carboys are not meant to hold pressure, and they fail in a very dramatic and possibly dangerous way. Use an airlock for safety.

A keg designed to hold pressure is a fine alternative. You can even keep it under pressure so when the lagering period is done, your beer is fully carbed and ready to drink.

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+1 said very well. Only stopper things that are meant to hold pressure. Period. – mdma Jun 27 '12 at 0:56
I seriously doubt the stopper would hold to the point of the carboy exploding, hehe. I could blow the stopper out of a tube, but doubt I could blow into a carboy and get it to explode! – Dale Jun 27 '12 at 21:53

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