Fusel oils are the double-faced Janus alcohols that can ruin or enhance whisky’s unique aromatics, body, and flavour complexity.
They are a group of long-chain alcohols with higher-than-water boiling points that manifest as oil after fermentation, and concentrate through distillation. Ethanol, the desirable alcohol in all spirit drinks, evaporates before water at 78.2˚C. The primary fusel alcohols, amyl (60–70 per cent of fusel oils), butanol, and propanol, are insolvent in water, carrying two extra carbon molecules. Odourless, colourless fusel oils have a fetid, offensive smell, and an acrid taste. Small doses make whisky unpalatable and injurious to drinkers’ health.
As the spirits industry arose, distillers found fusel oils contaminated cereal fermentations and distilled spirits, observing considerable variations in residual oil volumes between grain species. First described as phlegm in the 14th century, by the 18th century, British malt distillers called it grosser oil, essential oil, volatile oil, or empyreumatic oil, and in the 19th century, oil of whisky. Empyreumatic contamination included another offensive causation when wort particles were scalded in the still due to overheating and rapidly fired shallow pot stills. At the turn of the 19th century, the Scottish Lowland distilleries escalated their shallow still production to minimise tax, causing an acute problem as burnt wash and “oil raised and brought over” into the spirit. Supplied to London distillers, they redistilled and compounded the rectified spirit into gin and British brandy. In 1785, a Swedish chemist coined fuselöl, from Low German, meaning bad or inferior liquor, after distilling rye whisky. A French scientist identified the chemical composition of amyl alcohol as the primary oil in 1834, with fusel oil soon becoming the universal term.
During fermentation, yeast metabolises amino acids and proteins to synthesise fusel oils. Distillers discovered multiple pathways to remove or reduce its formation during fermentation by culturing yeast strains with lower fusel generation, aeration wort, avoiding low pH and high nitrogen environments, and prolonged, high-temperature ferments. Manual removal processes involved skimming off pot still oil residuum from the low wines, adding charcoal to the wash or low wines, filtering the distillate through charcoal, using chemical extractives, or masking with acids and flavourings like prune and sherry wine. One of the most popular methods was extra reflux distillations to rectify and extract the oil.
By the 19th century, cask maturation was understood to significantly change fusel oils, as the degrading oak cellulose released acids, along with oxidisation, converted fusel alcohols into flavoursome esters. Amyl and acid becomes amyl acetate, producing banana, pear, and apple flavours in the whisky. Over time, these or other flavours concentrated and evolved in the cask to mellow and increase complexity.
Malted barley has the lowest fusel content of all cereals, explaining why large second-use casks (ex-sherry or ex-bourbon) best serve malt whisky. The two grains with the highest concentration of fusel oils are corn and rye, necessitating different mashing methods, partiality for sour mash fermentation, and distilling on the grain with beer columns and doublers. This shows why Americans developed the smaller, charred new white oak barrels to rectify and filter the fusel content through the staves’ carbon membrane, with years of maturation improving the flavours and palatability of whisky.
At the turn of the 20th century, the world’s pre-eminent European scholar on whisky, Dr Philip Schidrowitz, expert witness to the 1905 Royal Commission on What is Whiskey, advised Dr Harvey Wiley, framer of the US 1906 Pure Food Act, on the unrealised benefits of fusel oils. Wiley reversed his previous ruling that whisky must be less than 0.025 per cent fusel parts per 100 ethyl alcohol to have unquantified amounts of fusel oils to be genuine whisky; otherwise, it is rectified or neutral spirit. This informed President Taft’s 1909 legal decision on defining straight whisky.