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Fermentation temperature fluctuation

Hey all,

So it’s pretty commonly understood that controlling the fermentation temperature is one of if not the single most important step for making good beer.

But yeast strains typically have a range of temperatures they work well in. For ales, that range tends to be in the 60s F. We all know that if the fermentation temperature is above the desired range off flavors will occur, and if the temperature is below the desired range the beer will be poorly attenuated.

I have been wondering though what effect, if any, a fermentation temperature that fluctuates WITHIN the desired temp range has on the final beer.

With the coming of fall I have no problem keeping my fermentation temps in the 60s, typically 62-68, but the temps certainly do fluctuate over the course of the day. Cooler at nights and in the morning, warmer as the day goes on…

Does anybody have any thoughts on the matter? Given an appropriate fermentation temp range, does fluctuation within that range affect anything?

Just so we’re speaking the same language, “fermentation temp” refers to the temp of the beer, not the ambient air temp. During the active phase of fermentation, beer temp can be 5-7 degrees higher than ambient temp, as yeast create heat exothermically as they ferment. If this was your meaning, my mistake.

The reality is, you are talking about a decent size thermal mass. If the temp of your room fluctuates a bit throughout the day, it will be difficult to move the temp of your fermenter (more so, the liquid inside it) with a small degree of ambient temp change. The other side of this argument is that while a 1* Fahrenheit difference doesn’t feel different to us (billions of cells), it can feel a hell of a lot different for a little yeastie boy (1 cell).

This is one of the thoughts behind ‘swamp cooler’, as you are adding liquid thermal mass that is ‘buffering’ temperature changes.

The reality is, temp SWINGS are almost as bad as, if not worse than fermenting too warm. This can cause yeast to drop out prematurely and leave a partially fermented beer, or stress the yeast and cause off flavors (acetaldehyde (green apple) and diacetyl (butter)) are the most common). If you are fermenting in a basement, place your fermenter in a tub of water (3-4" of water is all you need).

You want the temp to increase over the course of fermentation, to your point, to ensure full attenuation. For most of my ales, I use the following temp schedule (beer temp)

Days 1-3: 63-65*
Days 4-8: 65-67*
Remainder (usually another 10-14 days): 68-70*

+1 to most of that. Make sure we are talking fermentation temp and not ambiant.

Even a swamp cooler without any extra cooling element (ex. frozen water bottles) will make temp swings much more gradual and much less significant, regardless of ambiant temperature.

I tend to use frozen bottles from days 1-4, then not worry after that.

Great, this is just the info I was looking for.

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