The phenolic compounds, in particular phenol and cresol along with eugenol and guaiacol are the source of particularly strong, smoky and bitter flavours and aromas found within whisky. Despite the fact that you’ve likely never heard these words before these are substances you have encountered during whisky tastings as our sense of smell is sensitive enough to pick out some at as little as 0.1 ppm (or parts per million the equivalent to 1 phenolic molecule per million others).
Phenol & Cresol in Whisky
The first two compounds phenol and the various cresol compounds account for the bulk of the peat rich smoked aroma and flavour traditionally associated with the distinctive Islay style. Measured in PPM concentrations found in whisky range from (more highland style Islay) Bunnahabhain with only around 2 PPM to Ardbeg with approximately 54 PPM*. These compounds may be introduced either deliberately by drying the moist grain over peat fires, or by thermal degradation resulting from the heating process itself, while these chemicals can be found in water this concentration is too low to have a significant impact on the whisky in question.
*While the peat levels of the whisky is commonly measured by the PPM of the barley however this is not entirely representative as the level will dip by around one third in the new make spirit and continue to lower during the ageing or maturation process.
The three cresol ortho-Cresol meta-Cresol & para-Cresol, phenol and their alcohol derivatives collectively make up the bulk of the aromatic properties of whisky. The medicinal odour is primarily drawn from the meta-Cresol compound which has historically been used as an antiseptic.
A phenol derivative created by the charring of wood, Guaiacol is also responsible for the sweet smoke flavour found in whisky, its distinctive flavour is balanced against phenol and cresol by regulating the temperature and duration of the peating. Slower hotter peat fires will result in higher Guaiacol levels, while cooler and quicker drying will result in lower relative levels.
Guaiacol is an oily, yellowish liquid substance with the molecular formula C7H8O2 also known by the name 2-methoxyphenol. The solubility of the substance is low in water, but very well in organic solvents. Guaiacol is odourless being only identifiable via taste its flavour threshold in water is estimated at 3 ppb.
A base of Vanillin and Eugenol phenols, Guaiacol is also formed from various Lignin components such as the Ferulic acid (a cinnamic acid) gets into the whisky. The flavour character of Guaiacol is primarily classified as smoky, but it also imparts sweeter medicinal notes.
Eugenols occur in whisky and other spirits matured in oak barrels, imparting a strong strong and bitter flavours of clove and cinnamon.
Eugenol C10H12O2, scientifically described as 4-allyl-2-methoxyphenol, is one of the main components of the floral odour of some Eugenia- and Cassa species, and as a component of various essential oils. These include cloves, cinnamon and basil oil. The characteristic flavour of Eugenol is the unique scent of cloves.
A large number of compounds make up the