Bubbling for over a month

[triskelion]

Just did a small batch pilsner smash and noticed it bubbling in the demijohns for a very long time. Did a gravity reading and it had definitely finished up but it was still bubbling like crazy, this is over a month in primary. I bottled and primed with dextrose today. I got a mentos and coke like reaction when i added the dextrose. It seems like there was a lot of dissolved co2 and the dextrose triggered its release. anyone else experience something like this? adding the dextrose seems to be the only thing that stopped it bubbling. On tasting it there's no off flavours so i don't suspect infection.
Only other odd thing i did was ferment at 18*C (64*F) after reading brulosophys article on warm fermenting lagers.

100% pilsner malt
1.040 OG
1.005 FG
3 additions of saaz
Mangrove jacks bohemian lager yeast

 

[RM-MN]

CO2 dissolves in the beer as it ferments and the amount that can be dissolved depends on the temperature. Fermenting cool allows more to be dissolved and it will still be oversaturated so the CO2 will come out of solution. That is one thing that makes the airlock bubble. You also mentioned that you made a small batch and had it in a demijohn. That should leave you with sufficient headspace that changes in atmospheric pressure can also make the airlock bubble.

Putting the dextrose directly into the beer will cause an explosion as the dextrose creates nucleation points for the CO2 to come out of solution. Always dissolve the dextrose in a bit of hot water before adding it to your beer. That will avoid the explosion and will mix better too.

 

[triskelion]

Those are fair points, i hadn't thought that headspace would come into it.
I brewed and bottled a stout at the same side and didn't have the same reaction though. Same headspace and temperature. I primed with less dextrose but not much less.
I'll do a larger batch if this one works out and see if it behaves differently.