Ever wondered what happens when you ferment two beers in the same fermentation fridge?
Since you can only control hot and cold from one thermal probe you’d expect there to be some variation. The chart below shows the effect. The temperature is being regulated by a probe inserted into a thermowell in the Porter (the blue line) and the target temperature is 20 degrees C.
The purple line shows the temperature of a New World IPA (Grapefruit Juice) which is in a second fermenter stacked on top of the Porter. The target temperature for the IPA is also 20 C.
As you can see there are temperature variations of +- 1 degree either side of the target for the IPA. Within the tolerance levels for the yeast – but not very stable.
So what’s the lesson?
A double ferment is fine as long as you accept that you will see variations of +- 1 degree C during the process and that that temperature variation will depend entirely on the fermentation progress of the primary beer. If you’re fermenting something fairly meaty where you expect that there will be a lot of exothermic activity then expect the secondary beer to start off a good deal cooler than the target temperature because your cold inputs will be trying to keep the primary beer temperature down. Far better to do a double ferment of beers that have very similar profiles.