48 hrs ago I brewed a 1.067 white stout, pitched WLP002 English Ale yeast at 66F. Given the ambient temp in my garage this time of year (low to mid 60s), I set my thermostat controller to 65F at the low end of the yeast's optimum range (temp probe in a thermowell in the wort). The chest freezer kicked on and dropped it to 60F shortly after pitching, so I actually disconnected the freezer and put my heat wrap around the carboy thinking I might have to use it. I let it rise on its own though with fermentation starting and it was active and sitting at 67F last night. When I got home from work this evening it was very active and the probe was reading 74F, so I reconnected the chest freezer and set it to 70F to try and slowly bring it down. But again it went way down to 65F and now there is no airlock activity. Do you think this quick of a drop in temp is enough to stun the yeast and stall fermentation, or will it pick back up again without problems? I've never had a problem with fluctuating temps like this before.
You'll probably get a lot of esters, but the batch shouldn't be ruined. At this point I'd probably just bring it back up th o 69-70* FWIW (Not much) Do you have the freezer setup as a heating element by any chance?
I double checked the settings on the controller. It's right, set for cut in and cooling. Part of the problem may be this is only a 3 gallon batch so it may be more susceptible to temp changes.
I'm fairly sure the temperature of the wort/beer didn't drop 9 degrees in a couple of hours. The temp in the fridge can certainly change fast but even 3 gallons of liquid takes a while to change. Only if you are measuring true liquid temp with a thermowell can you trust your thermometer. Where is your probe placed? I routinely have my ferm fridge overshoot the set temp (too low) by a degree or two but the wort temp is largely unaffected. That said, a sudden drop in temperature can make the yeast go dormant . . . which is one of your symptoms. I try to move wort temps just a couple of degrees a day (at most). Consider raising the wort temp back up into the upper 60s to try and perk up the yeast. Also, if you have room, put a large bucket of water or even a 750ml bottle of water in your fridge and measure the temp of the water. This will approximate your wort temp before any increases due to fermentation.
That's a good point. A majority of the thermowell is above the level of the liquid because it's only a 3 gallon batch, so it's probably reading the temp of the headspace as much if not more than the wort itself. I'm going to work to get it back towards 68F or so.
Try filling the thermowell with water (if your temp probe is waterproof). The thermowell liquid should balance with the liquid of the wort and be less effected by air temp swings. I do this even for 5 gallon batches where Mr. Thermowell is deeply submerged . . . cover the probe with water and it should measure true wort temps. I still keep a large water bottle in the ferm fridge with a floating thermo as a "back-up" to my temp measurements.
So if the temp swing wasn't as drastic as I thought, would the yeast go dormant that quick? Or is this a matter of it cooling and causing contraction and temporary absorption of CO2?
Difficult to say. By the time you get it back to upper 60s you'll be coming up on 3 days of fermenting . . . which could be the time active fermentation would be naturally fizzling out. From here on your airlock will be an "iffy" indicator at best. You seem to have a pretty good grasp of what's happening . . . the only other thing I suggest is to ultimately raise the temp to the high end of the strain's range in an attempt to reduce any fusels (if present). You really want those yeasties to stay active during the conditioning phase. And you might consider taking a grav reading a little earlier than normal.
