Peat, Phenols, and ppm: Why Scotch Tastes Smoky

My first taste of peaty whisky was Talisker 10, and like they say with other things, you never forget your first. I wasn’t sure if I liked it, it was like someone had put a bonfire in my whisky. Now, I love it and feel like smoky notes give a complexity to many whiskies, especially at an older age. Actually, old peaty whisky is probably my favourite style, it’s just a shame it’s so damn expensive…

But what the hell is peat, and why does it make our whiskies smoky?

Essentially, peat is a naturally sourced fuel. Small plants and vegetation that have died in bogs or moors become part of the earth as it decomposes and a new layer of flora grows again. In the earth though, that decaying vegetation is compressed and pushed further underground by the following layers of the life and death cycle above. Peat is harvested by simply shoveling it out of the ground. Peat burns well and originally would have used as heating and cooking fuel before coal arrived in more remote parts of Scotland. The important thing for whisky though was that it was used to dry barley after it had been malted. Distillers found that the barley picked up the smoky flavour from the burning peat, and that intense smoky flavour came through into the whisky.

You do occasionally get people telling you that peaty flavour is coming from the water. It makes sense I guess, if you run a tap in the toilet at Kilchoman Distillery, it comes out brown with the peat sediment in it. Peat is in the water, water goes in the whisky. However, if you smell a piece of peat, without burning it, it barely smells of anything.

Understanding Phenol

Burnt peat smalls and tastes smoky because of the presence of Phenol? So what is this phenol that makes our whiskies smoky?

Phenol is a cyclic aromatic compound, but also describes a class of aromatic compounds. The main ones in whisky are Phenol, Cresols, Xylenol, and Guaiacol. There are a tonne of others, and not all are Phenols. Some don’t appear to have any effect on flavour but can affect the volatility of some of the other flavour chemicals. In essence: It’s complicated.

Phenol itself gives you carbolic and antiseptic flavours, Cresols give the whisky medicinal, bandage-like flavour that’s very distinctive in Laphroaig. Xylenol can be extracted from coal tar and has a similar note, while Guaiacol gives a wood smoke type of flavour.

Depending on where the peat is cut from can change the proportion of different Phenols that come through into the whisky too. Highland Park for instance is famous for having a more heathery style of smoke to its whisky and this is often attributed to there being much more heather decomposing and becoming part of the peat than on Islay or mainland Scotland.

You may have noticed that some peaty whiskies use a measurement to declare how peaty they are. The term ppm, which stands for parts per million, is the measurement, but perhaps should be written as Pppm: Phenolic parts per million. A whisky that is 40ppm would be made up of 0.004% phenols. Check my math on that, I’m rubbish at math. [It checks out. -Ed.]

The human nose can detect 1ppm of most Phenols normally, but that can depend on the person, some being more sensitive to the presence of Phenols than others. Also depends on the specific Phenol, because evolution-wise is it pretty advantageous to be able to detect wood smoke, so we are genetically more sensitive to Guaiacol.

Phenol Beyond Peat

Other things that can effect the amount of smoky flavour in a whisky are the production process and maturation, as Phenols are lost at almost every stage of the process. One important aspect though is the cut from spirit to feints, as the Phenols are more concentrated towards the end of the cut. The longer the cut, the more Phenols are going to get through into the spirit.

Note that a lot of Phenol is lost along the way. Laphroaig usually have its barley peated to 40ppm but only 25ppm comes through into the spirit. After 10 years of aging, this drops to 10ppm. This is because these phenolic compounds are only partially oxidised and within the barrel they are allowed to fully oxidise, becoming more complex aromatic compounds. Laphroaig is known for becoming more tropical and fruity at older ages.

So at what point do they measure ppm? Is it the spirit? The final whisky? Neither: It’s actually at the point the barley is malted. Most distilleries specify a ppm level for their barley and let the maltsters do the rest. They can then burn a specific amount of peat for a certain length of time to get it to that exact peating level, and it then gets to the distillery as a very consistent product.

How do they measure ppm? The usual way is High Pressure Liquid Chromatography which is a way of chemically analysing the components of whatever you’re considering. Maltsters and distillers split the category of peated whisky into a handful of sub-categories; lightly peated (2-10ppm), medium peated (11-29ppm) and heavily peated (30-55ppm). Recently another category has emerged: the super heavily peated whisky, which can be anything upwards of 55ppm. Bruichladdich’s Octomore is famous for creating this new category but Ardbeg’s Supernova whiskies (no longer produced) were also at 100ppm. The original Octomores were 80ppm but are now regularly 167ppm and up. Bruichladdich have been setting the bar higher and higher though, and a new record has been set with the Octomore 8.3 at an almost monstrous 310ppm.

Below is a small list of some of the whiskies using peat to give you an idea of the scale of ppm levels. (Numbers are approximate and can vary from release to release):

Bunnahabhain: 2ppm
Bruichladdich: 4ppm
Benromach: 10ppm
Ardmore: 15ppm
Springbank: 20ppm
Talisker: 22ppm
Bowmore: 25ppm
Caol Ila: 32ppm
Lagavulin: 37ppm
Port Charlotte: 40ppm
Laphroaig: 45ppm
Ardbeg: 55ppm
Benromach ‘Peat Smoke:’ 67ppm
Laphroaig Floor Malted: 80ppm
Ardbeg Supernova: 100ppm
Octomore 6.1: 167ppm
Octomore 6.3: 258ppm
Octomore 8.3: 309.8ppm

Tasting Peat and Phenol

Does this mean that Octomore 8.3 is going to taste 8 times peatier than Laphroaig? No, not really. Because nature likes to screw with you, human perception of phenols levels off at around 60ppm (remember, that’s in the barley; what winds up in the whisky is around 25ppm), and when a whisky goes well above this it can can actually taste less peaty than something much lower. The 10 year old cask strength Laphroaig, or some cask strength Ardmores taste peatier to me than some of the Octomores, which I find go beyond overt peat and into a new realm of minerality and meaty complexity. In the end, ppm is a reasonably useless marketing term, as there are a lot of other variables to how peaty your whisky is going to taste.

Peating whisky is a complex science and art. It’s going to bring smoky flavours into your whisky, which you might love or hate, but which you have to admit is distinctive either way. Whether it’s a lightly peated Benromach, floor malted Springbank, or a monster Octomore, peat brings character into a lot of different whiskies and really spreads out the spectrum of flavour you can experience from whisky. Remember that all peat is not created equal. My best friend thought he hated peaty whisky after having a Laphroaig, but now Ardmore is his favourite dram. Even if you think you don’t like peated whisky, I’d recommend picking up a few bottles, trying some different types, and seeing if it grows on you.