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The Science Behind Peat


20 May 2021

The Science Behind Peat


flavour production science

We're all drinking Oban 14 this month, which is an undeniably peaty dram, so we thought it might be worth taking the time to explore exactly how peat works in scotch whisky.

The peatiness of a whisky is measured in phenolic parts per million (pppm). By definition this means that a whisky of 1 pppm would contain 1 phenolic molecule for every 999,999 other molecules. A whisky with 1 pppm probably wouldn't come across very smoky to the average taster, however 20 pppm is more than sufficient to impart some serious smoke character. At the higher end of the spectrum, products like Octomore, produced by Bruichladdich, can come in at over 200 pppm! - it’s enough to make you cough just thinking about it.

Peat is possibly the only component of malt whisky production that is genuinely governed by terroir, but we are well aware that that statement may upset one or two people. The composition of peat smoke (aptly named ‘reek’) is made up various volatile compounds that give the familiar burnt mustiness of peat smoke. These compounds consist of number of phenols (coal smoke, pig odor), two furan aldehydes (sawdust and cereal) and series of alkanes (paraffin and other flammable gas), and aromatic hydrocarbons (diesel). Depending on the location, the surrounding plant matter, and the depth in the ground that the peat is cut, the compounds will differ. Ardmore in Speyside uses a carbon-rich peat that gives its whisky a light ‘sooty’ quality that’s in contrast to the more maritime and medicinal drams that you might find on Islay or Skye.

Peat smoke is imparted in to the barley during the malting process, at the stage where the germinated barley is being dried. The level of peatiness in the final product is nearly impossible to control during kiln drying, there are simply too many intangibles when trying to control smoke. These range from the exact moisture of the barley, to the make up of the peat itself, and how well the peat fire has been tended. A gauge can be taken by looking at the quality and density of the smoke, but that’s about it. When tending a kiln you will be looking for rich dark smoke rather than pale white smoke, a sign that it contains plenty of phenols. Fire is not your friend as many of the more delicate aromatics can be lost if the peat gets too hot. In a good kiln you should be able to touch the peat whilst its smoking without burning your hands. 

And then there’s the break point, which occurs about halfway through kilning (at roughly 18 hours) and marks the stage where the barley will accept no more phenolic compounds due to its ever decreasing moisture content. Pre-break there is plenty of moist air and the damp husk of the barley absorbs phenols like a sponge, but as the grain dries out so too does its capacity for trapping the peat characteristics. After the break point the air becomes drier and no amount of peat will increase the pppm any further.

Port Ellen Maltings on Islay always burn approximately 10 tons of peat for every 50 tons of barley and the results range from as low as 30 pppm, right up to 80 pppm. In commercial kilning, a sample of peated barley is taken after the kilning process and its pppm is measured. The batches are then blended with unpeated malt to reach the desired level of smokiness.

Of course the phenolic parts in peated barley wont all make it through to the bottle anyway. Despite being resilient little beasts the phenols have the trials of mashing, fermentation, distillation and ageing to contend with. Laphroaig peat their barley to 40 pppm, their new-make spirit is 25 pppm, but their finished whiskies are all below 10 pppm.

It’s also worth noting that the phenolic parts per million are only a guideline to how smoky a whisky will really be when you come to taste it. Although it’s possible to measure phenols accurately, the pppm does not account for other flavourful molecules that might reduce the sensation of smokiness or increase it.