Regulating Fermentation Temperatures

I’ve had several people ask me recently about keeping fermentation temperatures down during the summer. First it’s probably necessary to detail exactly what “optimal” fermentation temperatures actually are. Every yeast strain out there has a published optimum temperature range; the data are available directly from White Labs and WYeast, as well as at most homebrew shops. In general, the lower end of the temperature range is fairly optimal and the upper end is far too high. (The exceptions to this rule are Belgian yeast strains, which may be ramped from a “normal” ale fermentation temperature to 25°C (77°F) or more over a period of a few days, in order to accentuate their ester profiles.)

That said, the vast majority of ale fermentations should take place in the range 15-21°C (59-70°F). For lagers, the range is somewhat narrower; about 8-11°C (46-52°F). It’s important to recognize that these are the temperatures of the beer itself; a vigorous fermentation can be several degrees above the ambient temperature. The key, then, to getting reliable results from fermentation is to start at or slightly below the desired temperature and apply sufficient cooling to keep it there. The most reliable way to do that is to use a thermostat to control a refrigerator or freezer, and simply place the fermenter inside. In my opinion, though, the up-front costs aren’t really justified unless you already have a spare refrigerator or freezer. If you have a kegerator, however, good news: serving temps overlap lager fermentation temps pretty nicely. If you don’t have a kegerator or can’t spare the room in it, though, there are still some simple options.

What I like to do is place the fermenter in a cooler (which also happens to be my mash tun) partially filled with tap water. The water temperature will remain a degree or two lower than the air temperature due to evaporation. If that isn’t enough cooling, a towel or t-shirt wrapped around the fermenter and dangled below the waterline will wick moisture further up the fermenter and reduce the temperature another degree or two. A fan blowing across the system helps even more. Finally, if the temperature drop still isn’t large enough, as in the case of a lager, cold packs or frozen water bottles can be dumped in the water and rotated every few hours. By doing this I’ve conducted lager fermentations when ambient temps are well above 80°F.

In the image below a wet towel and fan are being used without any ice. As you can see, the temperature of the water is about 69°F and the top of the towel is about 64°F, nicely bracketing the desired temperature for an ale. The ambient temperature was 73°F.

Using a towel and fan to keep fermentation temperatures down.