Aside from proper yeast management, temperature control plays the largest role in producing good quality beer. Even warm fermenting varieties such as ales require cooling lower than room temperature. Fermenting warn causes the yeast to produce more esters and fusel alcohols. The risk of spoilage also increases with heat. The temperature should be monitored during the fermentation process. Temperature variations in excess of 10 degrees should be avoided. Large thermal shifts can shock yeast cells and can cause them to go dormant. Brewers in northern climates may have the opposite problem during the winter months.
There are several methods that can be used to control the temperature of the beer. Evaporate cooling is simple and inexpensive. A large shallow pan can be filled with water. Dampen a t-shirt. Place the fermenter in the water and slip the damp t-shirt over it. A water bath can also be made by filling a clean trashcan with water and ice. Fermentation chillers are also available that will monitor the temperature. Lagers need colder temperatures and typically need refrigerators or freezers. For those who need to prevent their brew from becoming to cold, brewbelts can be purchased. These encircle the bucket and produce heat electrically.