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Phenolic Compounds in Whisky: Guaiacol and Eugenol

How phenols, cresols, guaiacol and eugenol shape the smoky, medicinal and spicy flavours found in single malt whisky, and what label ppm really means.

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The smoky, medicinal, clove-like and barbecue notes in whisky, especially in peated malts and whiskies from heavily charred casks, come largely from one family of chemical compounds: phenols.

They matter at very low concentrations, because the nose is highly sensitive to many of them. Some arrive with peat smoke during malting; others come from oak during maturation. The result is that two whiskies can both be called smoky while offering rather different accounts of the fire.

Phenols and cresols: peat smoke and medicinal notes

The smoke in peated Scotch starts at the maltings. When germinated barley is dried in a kiln over burning peat, the smoke leaves phenolic compounds on the damp grain.

The main compounds behind the sharper, medicinal side of peat smoke are:

Malt ppm against the glass

Peat levels are quoted as parts per million (ppm) of phenols, measured on the malted barley, not in the whisky. Ardbeg publishes 50 to 55 ppm for its malt, and Laphroaig 45 to 50 ppm for the share it floor-malts itself; Octomore releases have ranged from 81 ppm to over 130 ppm.

The figure on the malt is not what reaches the glass. It is a measurement taken at the start of production, before mashing, fermentation, distillation and the cask have had their say. Phenols are lost in mashing, fermentation and the distillation cut. Torabhaig, one of the few producers to publish both figures, reports 78.4 ppm in the grain and 19.7 ppm in the glass for Cnoc Na Moine: about three quarters of the measured phenols never reach the bottle. Phenols also soften slowly in the cask, which is why an older peated whisky often tastes less smoky than a young one from the same distillery. Our guide to peat and ppm explains why.

Torabhaig publishes the phenol level both in the grain and in the finished whisky. Cnoc Na Moine: 78.4 ppm in the grain, 19.7 ppm in the glass. Allt Gleann: 77 ppm in the grain, 17 ppm in the glass. Roughly three quarters of the measured phenols do not reach the glass.

Guaiacol: sweet, savoury smoke

While phenol and the cresols give the sharper, medicinal side of smoke, guaiacol (2-methoxyphenol) and its relative 4-methylguaiacol give smokier, more savoury notes: wood smoke, smoked bacon, roasted coffee and toasted oak.

Guaiacol reaches whisky by two routes:

  1. Peat smoke: Guaiacol and related methoxyphenols form when the lignin in peat breaks down in the heat of the kiln fire. In general, lignin gives off more methoxyphenols such as guaiacol at lower burning temperatures, and more simple phenol and cresols as the heat rises, so how the peat burns affects the mix.
  2. Cask toasting and charring: When coopers toast and char a cask, the heat breaks down the lignin in the oak and forms guaiacol and other phenols. That is why unpeated whiskies from heavily charred casks, bourbon included, can still have a smoky note.

Eugenol: clove and warm spice

Eugenol (4-allyl-2-methoxyphenol) is the compound behind the smell of cloves, and it gives whisky clove, allspice and warm-spice notes.

Most of the eugenol in whisky comes from the oak, formed as the wood’s lignin breaks down during seasoning and toasting and then drawn out during maturation. Some is also found among the phenols in peat smoke. European oak is often reported to give more eugenol than American oak, but studies differ, and the toasting of the cask matters as much as the species. Together with vanillin, another lignin product, eugenol is part of the spiced, fruitcake-like profile that sherry-cask malts are known for.

Summary of key whisky phenols

On smaller screens, scroll sideways to see every column.

CompoundMain sourceAromaWhere you find it
Phenol and cresolsPeat smokeMedicinal, antiseptic, tarry, damp earthPeated Islay malts (Laphroaig, Ardbeg, Caol Ila)
GuaiacolPeat smoke and charred oakWood smoke, roasted coffee, bacon, barbecuePeated malts and heavily charred casks
EugenolMostly oak; some in peat smokeClove, allspice, warm spiceMany oak-matured whiskies, often sherry-cask malts
VanillinLignin breakdown in toasted oakVanilla, cream, custardFirst-fill bourbon casks and virgin oak

Comparing peated and spiced whiskies

To compare these compounds in the glass, try a peated malt alongside a sherry-cask whisky:

About the author

John Fegan

· Editor

Hailing from the land of haggis and deep-fried Mars bars, John was always going to find his way to whisky. He has run Uisge Beatha since 2015 and Whiskipedia.com since 2020.