Controlling Fermentation Temperatures

If you look up any list of the top ten homebrewing errors, number one is always (seriously – always!) about cleanliness and sterilisation of your brew gear. However, not far after it you’ll find something about controlling your fermentation temperature.

This is primarily due to the fact that yeast operate best within a fairly limited range, usually just a few degrees. Below this they will become dormant and fail to properly ferment the malt sugars, above they become hyperactive, producing excessive fusel alcohols and diacetyl which can cause undesirable flavours in the final beer.

The optimum temperature varies considerably for each strain of yeast, with ale yeasts generally performing best between 18 C and 24 C, while lager yeasts tend to prefer much cooler temperatures, from around 7 C to 13 C.

So how can you control the temperature of your fermentation? The good news is that a number of techniques from the relatively inexpensive low-tech through to more involved methods are readily available.

For keeping your beer cool in summer, fermenters can either be half submerged in a bath of water – a technique recommended by John Palmer in his excellent How To Brew.

Alternatively, if you don’t have a spare bath lying around, then setting your fermenter in a shallower pan of water (just a couple of inches) and laying a towel over the top with the ends dipped in the water causes water to be sucked up into the towel where it evaporates regularly around the fermenter. This evaporation helps cool the fermenter (just like sweating does for humans). Use of a fan to blow cool air across the wet towel can considerably help this.

A more complex option is to build yourself a fermentation chiller, as per the Son of Fermentation Chiller instructions.

Alternatively, if you have a bit more cash available, you can buy a second hand fridge or chest freezer and attach a digital temperature controller to it to keep your beer bubbling away at your desired temperature.

For keeping beer warm in winter, a similarly large range of options abound, from beer belts (not generally recommended from what I’ve read) to heat mats and even warming wraps!

A temperature controlled freezer can also help to keep your beer warm, providing as it does an insulated box which will naturally warm up during your ale’s exothermic primary fermentation stage, and which should then retain the heat. To make things extra complicated, a heat source such as a light bulb could also be added to the thermostat to provide warmth if the temperature drops – obviously you need to be aware of any potential fire risks if going down this route but it may be worth considering in colder environments.

As I’ll be moving house soon and gaining a lovely new garage for my alcohol-based experimentations, I’m going to wait until I can find a suitable freezer and attempt the necessary electronics. I’ll post the details on here as soon as I have anything useful to share…!


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