Fusel alcohols, also known as fusel oils, are higher-order alcohols (ie, alcohols with more than two carbon atoms) formed by fermentation and present in cider, mead, beer, wine, and spirits to varying degrees.
Excessive concentrations of these fractions may cause off flavours. Some beverages, such as whisky, traditional ales and ciders, are expected to have relatively high concentrations of fusel alcohols as part of the flavour profile. In other beverages, such as vodka and lagers, the presence of fusel alcohols is considered a fault. Very high concentrations—usually caused by incompetent distillation—can cause acute illness, including headaches, nausea, vomiting, clinical depression, or coma.
Fusel alcohols are formed when fermentation occurs:
- At higher temperatures;
- At lower pH; and
- When yeast activity is limited by low nitrogen content.
During distillation, fusel alcohols are concentrated in the "tails" at the end of the distillation run. They have an oily consistency, which is noticeable to the distiller, hence the other name "fusel oil". If desired, these heavier alcohols can be almost completely separated in a reflux still. Freeze distillation, on the other hand, does not remove fusel alcohols.
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