Chapter 8.1.3 - Temperature Factors
8.1.3 Temperature Factors
The third factor for a good fermentation is temperature. Yeast are greatly affected by temperature; too cold and they go dormant, too hot (more than 10F above the nominal range) and they indulge in an orgy of fermentation that often cannot be cleaned up by conditioning. High temperatures encourage the production of fusel alcohols - heavier alcohols that can have harsh solvent-like flavors. Many of these fusels esterify during secondary fermentation, but in large amounts these esters can dominate the beer's flavor. Excessively banana-tasting beers are one example of high esters due to high temperature fermentation.
High temperatures can also lead to excessive levels of diacetyl. A common mistake that homebrewers make is pitching the yeast when the wort has not been chilled enough, and is still relatively warm. If the wort is, e.g. 90F, when the yeast is pitched and slowly cools to room temperature during primary fermentation, more diacetyl will be produced in the early stages than the yeast can reabsorb during the conditioning stage. Furthermore, primary fermentation is an exothermic process. The internal temperature of the fermentor can be as much as 10F above ambient conditions, just due to yeast activity. This is one good reason to keep the fermentor environment in the proper temperature range; so that with a normal vigorous fermentation, the beer turns out as intended, even if it was warmer than the surroundings. However, if its mid-summer and you don't have a way to keep the fermentor cool, then the beer will not be very good - it will probably be phenolic, very fruity, and/or solvent-tasting.
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