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Did you know both lager and ale yeast can ferment at 100º F?  Do you know the difference between the flavor ale yeast makes at 70º and 100º?  Neither did I, so I looked into it to find out.

The first question might be, “So what IS the ideal temperature for ales and lagers?”  White Labs says the ideal temperature for ale yeast is between 68º and 72º F.  (20-22º C) Which brings me to the temperature range expected for lager yeast.  According to White Labs that range is 45º to 55º F. (7-13ºC.)

Another factor to consider is the raise in temperature caused by the fermentation process.  In the height of the fermentation process,  the temperature can be raised by as much as 15º F. (8.3º C.) in a 5 gallon batch.  In a room with the temperature of 72º, that could bring the wort temperature to 87º.

What happens if the yeast get too cold?  Too cold and the yeast slows down and goes dorment too early causing sluggish fermentation and if it goes dorment too early, you can get incomplete fermentation leaving a high concentration of unfermented sugar effecting alcohol content and flavor.

You ask, “what happens if it get too warm?”  On the slightly warm scale you get an increased amount of esters which produce a banana or a bubble gum flavor. Diacetyls is another flavor, that is the flavor and mouthfeel of butter similar to what you would get from movie theater popcorn or butterscotch flavors.  Let it get a little hotter and you start producing fusel alcohol that produces a solvent character to the beer.  You can sometimes loose some of that with prolonged aging but it never really goes away.

How to Keep it Cool!

So what is the best way to control fermentation temperatures?  Keep the fermentor about 10º less then the surrounding air.  A simple way to help cool your fermenting wort is the same way our bodies cool themselves, with simple evaporation. Since it would be insanitary to allow the wort itself to evaporate, place the fermenter in a tub or pan of water covered with a t-shirt or other material  that will wick water out of the pan and let it evaporate from the outer surface of the fermenter, cooling it in the process.  This works best when the air is circulated by a fan, allowing for increased evaporation of the water. Evaporative cooling can keep a fermenter approximately 5 to 15 degrees Fahrenheit  cooler than the surrounding air.

How to Warm it Up!

A garage or basement brewery may not be warm enough for ale or fermentation. If this is the case, it becomes necessary to invent ways to keep a fermentation warm without spending the money to keep the house or brewing area at the proper temperature.

The easiest method to warm a fermenter is to wrap it with a blanket or other insulating material to retain the heat produced by the fermentation. There is a very nice fermenter blanket available out there for little cost. You can monitor the temperature with a self-adhesive thermometer strip.

There is a heater available that wraps around the fermenter to keep it warm as well.

If a fermentor blanket is not enough, there are also ways to add heat to the system. An inexpensive submersible aquarium heater to warm water in a large tub that surrounds the carboy will also work. A self adhesive thermometer again can be used to monitor the temperature so you can adjust the aquarium heater to the desired temperature.

If you have an insulated box rigged with a cooler to keep fermentation cool, it can be rigged with a light bulb inside the box to keep it warm with control via temperature controller. Be careful to make sure the hot bulb will not contact any surface that could melt or start a fire.

For the sake of quality control, it is well worth the time and a small monetary investment to produce good quality homebrew!

heater link

How Temperature Affects  Fermentation

​Carboy Heater (linked of course)!