As mentioned earlier, fermentation is a critical process in brewing. You can have the perfect wort and the ideal yeast but if the fermentation doesn't go well due to temperature variations, the result will not be great beer.
The easiest way for the home brewer to control temperature is to modify a fridge (preferably a larder fridge, which does not have an ice box to get in the way) by adding a heater and a temperature control unit. There are numerous forum topics posted by members who have made their own fermentation fridges if you want details - do a search.
I can safely say that adding a fermentation fridge to my own brewing set up has made more difference to the beer I make than any other single factor, especially when it comes to producing consistently good beer. Yes, if you're lucky the temperature in your chosen brewing space will be just right, the weather won't change and your beer will be great. However, I've had many brews where this didn't happen and where I had no power to correct the situation with the result that the beer developed off flavours. The worst thing of all is a sudden rise in temperature due to hot weather, which can ruin an otherwise promising beer - if it gets cold you can raise the temperature fairly easily, but cooling your beer down is much more tricky.
If you don't have either the space or the inclination to use a fermentation fridge, there are alternatives. You could use a heating belt or heat mat, available from many home brew stockists. These simply go underneath or around the fermenter and maintain a sufficiently high temperature under cold ambient conditions. However because these devices can't provide cooling when it gets too warm, you will have to avoid making beer in the summer months unless you have a cellar or other stable, cool area available.
If you're one for DIY gadget building, it's also possible to create a cooling system that goes around the fermenter or even sits in the beer. These are simply pipes through which cool water is passed to prevent the wort getting too hot. While I've seen examples of these built by home brewers, in my view a fermentation fridge is easier to make and easier to run, so I don't really recommend them as a solution for the average brewer!
Historically, these devices, called attemperators, were used by commercial breweries, however more modern refrigeration methods have taken over in most cases.
These are two techniques that are invaluable to the brewer and which simply cannot be done without a brew fridge.
This is the process of reducing the temperature of the wort to below the working temperature of the yeast. This technique can be used to arrest fermentation early, and get a sweeter, more full-bodied beer. It is also useful for speeding up the clearing process, as yeast will settle more quickly when the beer is cold. Typically, the beer might be cooled to between 10 º C and 16 º C.
As the name suggests this is the production of lager by the traditional process of a prolonged secondary fermentation at very low temperatures (as low as 1 º C). Typically the beer would spend several weeks in secondary fermentation, then much longer in maturation. With a brew fridge you can get the temperature exactly where you want it for as long as you need it.
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