Fusel alcohols, also sometimes called fusel oils, or potato oil in Europe, are higher-order alcohols (that is, alcohols with more than two carbon atoms) formed by fermentation and present in cider, mead, beer, wine, and spirits to varying degrees.
The term fusel is German for "bad liquor".[1]
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The compounds involved are chiefly:
Excessive concentrations of these fractions may cause off flavours, sometimes described as "spicy", "hot", or "solvent-like". Some beverages, such as whisky, Siwucha, and 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 Korn, vodka, and lagers, the presence of fusel alcohols is considered a fault.
Fusel alcohols are formed when fermentation occurs:
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.
There is a popular belief that fusel alcohol contributes to hangover symptoms. One study indicated that fusel alcohol has no more significant undesirable health effects (headache, nausea, etc.) than ethanol, the primary active ingredient in all alcoholic beverages.[2] This study involved an animal model of hangover, and its significance in humans is unclear.