The compounds involved are chiefly:
Excessive concentrations of these fractions can cause off flavors, sometimes described as "spicy," "hot," or "solvent-like." Some beverages, such as whiskey, Siwucha and traditional ales and ciders, are expected to have relatively high concentrations of fusel alcohols as part of the flavor profile. In other beverages, such as vodka and lagers, notable presence of fusel alcohols is considered a fault. Very high concentrations - usually caused by incompetent distillation - can cause illness, including headaches, nausea, vomiting, clinical depression, or coma. Such a liquor may be referred to as rot-gut or rotgut.
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. In a reflux still they can be separated out almost completely if desired.