Concluding our three-part exploration of flavour compounds in whisky [make sure you read the previous pieces on esters and aldehydes – Ed.], we’ll be focusing in this piece on the big smoky flavours provided by phenols. For anyone who loves smoke in their whisky, phenols are largely responsible for these excellent aromas.  

Phenols make their way into whisky when barley is dried using peat smoke. Peat is plant matter (mostly sphagnum mosses) that has decomposed for thousands of years in an environment with no oxygen. The biological ingredients of the peat (and as a result, the flavours they impart to the whisky) can vary a little depending on where the peat was cut.

Highly combustible, peat was the fuel source for ancient Celtic tribes and Scots throughout the centuries. When peat is burned in a kiln for making whisky, it releases smoke impregnated with phenolic compounds that are adsorbed [this is not a typo, adsorption is a different thing – Ed.] by the barley, which is then mashed, brewed and distilled.

Phenolic compounds in whisky are subdivided into phenol, cresols, and guaiacols. Phenol (which is a specific compound that also belongs in the overall group of ‘phenol’ compounds) and cresols make up the majority of phenolics found in whisky, and are responsible for the medicinal aromas you can get in a bottle of Laphroaig, for example. This applies especially to meta-cresol (m-cresol) which is an antiseptic! Guaiacols also provide smoky and meaty elements, but some, especially eugenol, provide rich spicy aromas (euganol is what gives cloves their unique flavour).

Here’s a list of most of the phenols found in whisky:

Phenolic compounds and their aromas (sourced from Ian Buxton and Paul Hughes’ book The Science and Commerce of Whisky)

Compound Flavour
Phenol Carbolic
2-Methylphenol (o-cresol) Musty, medicinal
3-Methylphenol (m-cresol) Woody, ethereal
4-Methylphenol (p-cresol) Medicinal
Dimethylphenols (Xylenols) Medicinal, sweet
2-Methoxiphenol (Guaiacol or creosol) Medicinal, woody, smoky
4-Ethylguaiacol Smoky, meaty
4-Vinylguaiacol Spicy, clove
4-(2-propenyl) guaiacol (Eugenol) Cinnamon, cloves, spicy

 

Phenolics in whisky are measured by PPM (Phenolic Parts per Million), and it’s the first number any whisky geek wants to know when they drink a smoky whisky, under the assumption that a higher number means a peatier malt. However, there are a few problems with this system.

First is that PPM almost always measures the phenol levels on the malted barley before it undergoes mashing and distillation. Phenols can be lost during any part of the whiskymaking process, and PPM also depends on the spirit cut that is chosen by distillers that then becomes the new make spirit put into casks (many phenols are lost in the feints, or tails). Therefore it is more useful to know PPM levels of the new make spirit itself, a number which not many distilleries disclose easily. Furthermore, although new make spirit contains roughly half as many phenolic compounds as peated barley, the PPM is also further reduced by varying amounts during maturation, with more compounds lost to the angels the longer the whisky is in the cask. It’s therefore very hard to know what the exact PPM of a whisky actually is, especially when it’s older!  

The second problem with PPM is that varying methods of measuring it can give different numbers for the same whisky. UV/VIS spectroscopy, HPLC (High Performance Liquid Chromatographic Technique), gas chromatography/mass spectrometry (GC/MS), and gas chromatography/time of flight mass spectrometry (GC/TFMS) are all viable ways of measuring PPM, but each can give a different figure (the last two are probably the most accurate methods). In other words, a PPM measurement for whisky can only ever be an extremely educated guess.

What it all comes down to, then, is that the best indicator to measure smoky and meaty aromas is simply your own judgment! Drinking more smoky whisky will help train your senses to identify the flavours and smoky elements that you enjoy the most. Out of all the methods for detecting peat mentioned above, this one is the most fun if maybe the least scientific, so happy drinking!

Sources:

  1. http://whiskyscience.blogspot.co.uk/2011/02/peat.html
  2. http://uiscebeatha.co.uk/eugenol-guaiacol-whisky/#hide1
  3. https://onlinelibrary.wiley.com/doi/pdf/10.1002/j.2050-0416.1983.tb04150.x
  4. http://www.whisky-news.com/En/reports/Peat_phenol_ppm.pdf
  5. https://books.google.co.uk/books?id=cmsoDwAAQBAJ&dq=Ian+buxton+whisky+book&source=gbs_navlinks_s