ales are usually fermented between 68¡F and 72¡F. Lagers require colder fermentation temperatures, usually between 45¡F and 55¡F. Above these temperatures, yeast will give off unwanted fermentation by-products such as esters and fusel oils.
Esters lend a fruity smell to beer. In moderation, these molecules are an important part of the profile of some beer styles. For example, we expect that an IPA should have a fruity ale nose to it. In excess, however, esters become overwhelming and undesirable. Beers fermented too warm can have a distinctive banana smell that most beer drinkers find objectionable.
Fusel oils are alcohol molecules with more carbon atoms than ethanol, the primary alcohol in beer, wine and spirits. At low levels, these accepted in certain styles of beer such as barley wines or other high-gravity brews. However, they are usually thought to be objectionable in most styles of beers. At excessive levels, they can cause health problems.
