Monday, April 18, 2011

Five Gallons At A Time: Controlling Fermentation Temperature

If you ferment your beer in a refrigerator with an analog temperature controller, you may be tempted to submerge your controller's probe in water or rig up a system to base its feedback on your beer temperature. After all, it's the beer temperature that matters - not the ambient temperature - and measuring a liquid temperature will extend the life of the fridge by reducing the number of times it cycles on and off. Unfortunately, measuring a liquid instead of the ambient air will result in a loss of temperature control. Why? If you tell the fridge to turn off once your beer temperature drops below a certain temperature, say 68 degf, the air surrounding the beer will still be cold - probably in the upper 30s - and continue to cool your beer. Instead, you should control your air temperature to offset the heat generated by fermentation. For 5-gallon batches with healthy fermentations, ales tend to peak at 8-10 degf above ambient temperature while lagers peak at 2-4 degf above ambient. Your mileage may vary, so you should stick a thermometer strip on your fermentor to know for sure. Once your airlock begins to slow down, I recommend raising the ambient temperature to improve diacetyl reduction and ensure adequate attenuation. For ales, you can simply move the beer to a room temperature location. For lagers, I like to set my controller to the upper 50s and postpone lagering until the beer is fully fermented. Oh yeah, don't worry about cycling your fridge. It was designed to be opened frequently, thereby requiring a lot of cycling, and its internal temperature controller works the same way as your external controller.

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