Can a Batch of Beer Ferment TWICE?!?!?

[ScrewBrew]

I've only been brewing for a year and only have a dozen or so batches under my belt so please excuese the newbieness. But my beer is fermenting.....AGAIN! WTF?

I've got a 5 gallon batch of Smashing Pumpkin Ale from Northern Brewer. I brewed it about 4 weeks ago. Things have been busy here lately with a child on the way so I haven't had a moment to transfer it over to the secondary. That was todays plan until.....

I open up my brew closet and....its fermenting! The airlock is bubling once every ten seconds, the beer looks milky, things are churning around...it looks just like it does towards the end of a standard fermentation.

It had been patiently waiting for me to transfer it into the secondary. It had cleared out marvelously and the FG dropped to where it should be. Then this happened...what gives?!?!?

 

[sudsmcgee]

What was the gravity, and what is it now?

 

[JonM]

Check the gravity, then check it again in a couple days. If it's stable, then it's done fermenting. Bubbling in the airlock can occur because of ambient temperature swings, changes in barometric pressure, etc. My batches continue to get little chunks of trub swirling up from the bottom long after fermentation is done.

 

[ScrewBrew]

Yeah....it wasn't the airlock activity that threw me off. It was the fact that my crystal clear brown ale turned back to chocolate milk. I'll check the gravity this afternoon. I can't remember what it was but I do remember that after two weeks in the primary there was no change after a number of readings a few days apart. Pure shananigans.

 

[Calder]

It can re-start. There is usually a good reason, such as it got cold, and then warmed back up again.

 

[deputyandy]

Most times i rack to secondary i get some activity, including a new yeast cake and even some airlock activity. I think the transferring acts to "rouse" the yeast and crank out a few more FG points. I've read that barrels used to be rolled around or taken for "walks" to rouse yeast in beers that have been aging. Makes sense to me.

 

[thegerm]

Beer in the fermentor can off gas CO2 once fermentation is done if the temp rises or the fermentor is disturbed.

Also, less benign, you could have a wild yeast or bacterial infection that can ferment it down to 1.000.

 

[tesilential]

Check the gravity now. I'm interested in how this turns out.

 

[tiltie]

Was the yeast Windsor? Windsor does this to me routinely (stops after a violent 24 h fermentation and re-starts after about 6 days). I suspect that there are really two yeast strains in there--one a very small subpopulation that is more attenuative than the main population. The subpopulation ferments the leftovers of the primary population. The subpopulation expansion is registered as a second wave of fermentation. The folks responding that your second fermentation is just CO2 venting don't realize that it is really a second fermentation--just as you describe--that requires an explanation.