Long ago and far away, when distillery tours and live whisky tastings were common, a question could often be heard at events where peated whisky was served: “What’s the PPM of this one?” Curious whisky fans asking this question were searching for the fixed number of phenolic ‘parts per million’ (PPM) that’s often quoted by brand reps and industry figures which, in theory, determines on a sliding scale the level of peat in a whisky. These fans have made the assumption – encouraged by the whisky industry – that the higher the PPM number, the more ‘peaty’ aromas and flavours will be found in the spirit.
By this logic, more peat equals more phenols which therefore equals more smoky aromas and flavours in the glass. The light peat of Benromach, for example, pales next to the 40 PPM of a Laphroaig, while Laphroaig cowers in the face of 80-309 PPM whiskies released by Bruichladdich under its Octomore brand. However, as with so many aspects of whisky, the role of peat in the creation of aromas and flavours we find in our glasses has many facets, some which are still unknown. The fact that Octomore certainly is not twice (or more) as smoky as Laphroaig to nose and taste indicates that there are many different factors at play which combine to deliver the organoleptic qualities of peated whisky.
Peat being burned in the kiln at Laphroaig.
Peat (‘turf’ to the Irish) are mosses, grasses and plants decomposing without oxygen over tens of thousands of years. When extracted, it resembles a cross between mud and clay. Once the primary fuel source for Celtic tribes, today it is sometimes dried, broken into bricks and burned in the fireplaces of the Scottish Highlands and islands.
In the world of whisky, it is burned in a kiln oven to dry barley after malting. The smoke from the peat sticks to the barley through adsorption. That smoke contains the compounds, primarily phenols, that eventually infuse a glass of whisky with smoky, meaty, and medicinal aromas and flavours.
Now, the organic chemistry: phenols (or phenolics) are a group of chemical compounds by which one or more hydroxyl groups (an oxygen and hydrogen atom stuck together) are bonded to an aromatic hydrocarbon group (a jumble of bonded carbon and hydrogen atoms). The simplest phenolic compound also happens to be called phenol, or C6H5OH.
“You can divide the phenols found in whisky into roughly two groups, phenols and guaiacols” says Dr Barry Harrison, a senior scientist at the Scotch Whisky Research Institute. “The former tend to bring medicinal notes, and the latter are often smoky and sweet.”
For a 2009 study that he co-authored, Harrison collected peat extracted from different regions of Scotland at different depths, and identified the compounds that emerged from the samples. Of the 106 compounds detected, 46 were phenolics. Various peats contained different quantities of compounds – though Harrison notes that some are harder to perceive in the glass than others, “Though you’ll have different amounts of phenolic compounds present in the peat, the thresholds at which you’ll taste them can also vary massively for each of them.”
Harrison says even the most sensitive noses and palates would struggle to identify individual phenolic compounds, and that it’s basically impossible to know where the influence of one begins or ends: “We don’t quite yet know to what extent non-phenolic compounds in peat play a role in flavour,” he says. “It’s something we’re working on.”
Peats stacked at Hobbister Moor, source of Highland Park"s peat. Image by Søren Solkær.
As Harrison’s samples contained varying quantities of phenolic compounds, logic dictates that peats from different regions will provide different flavours. For example, Orkney-based distillery Highland Park often draws attention to the high concentration of heather used in its peat. Its reason for doing so is the claim that burning of Orkney peat creates carbohydrate-based phenolic compounds sourced from the heather’s broken-down lignins that lead to a sweeter, smoky profile.
Few people are better qualified to muse about peated whisky than Ardbeg distillery manager (and ex-Lagavulin manager) Colin Gordon, who replaced the recently retired Mickey Heads, and he thought, along with his colleagues, that there was something special about Islay peat: “I’m a romantic! The guys I know who worked at the Port Ellen maltings always preferred to burn Islay peat. Now I don’t have any scientific proof, but you did really notice it. Comparing peat from the mainland to Islay, the smoke from the Islay peat always looks richer and darker.”
Comparing peat from the mainland to Islay, the smoke from the Islay peat always looks richer...
However, it looks like Gordon’s romantic impulses are correct after all. Speaking at a virtual tasting covering the role of peat in whisky during this year’s Whisky Show: Old and Rare festival, Harrison’s colleague at the SWRI Frances Jack was emphatic that the region where peat is sourced definitely matters: “When we were looking at the different regions in the peat, the samples taken from each of the regions had to be standardised and we were looking at the same depth within the bog. You can just see that there’s big differences between the peats…so we know that depending on where you take your peat from it’s going to have an impact on the flavour.”
While the provenance of the peat is certainly a factor impacting the aromas and flavours that emerge, the methods used for whisky production also make a big difference and determine which phenolics will come through in the glass. It starts with the actual kilning of the peat itself. Each maltster has to deal with peats that contain varied moisture contents, and also must calculate how that peat should be burned when dealing with barley that will also vary in moisture content.
However, what really matters is that the barley gets smoked as efficiently as possible under the circumstances. Gordon illustrates how certain moisture conditions can lead to more peaty malt while emphasising that flavour considerations don’t yet come into play: “Looking at the grains that are going into the kiln, wetter grain generally leads to a higher phenol content. As you burn the peat, it sticks to the husk better. However, the length of time you burn the peat for and how you deal with its moisture content during kilning, doesn’t really matter for flavour. It’s about effective production.”
After this process, the measurement for phenols is taken from the malt (High Performance Liquid Chromatography, or HPLC, is considered to be the most reliable method for measuring phenols in whisky), and subsequently unpeated malt is usually mixed in with the peated malt to achieve the desired PPM figure. From this point on, phenols are constantly lost throughout the whisky-making process, starting with the milling of the barley. The phenols from the peat are located on the barley husk after smoking. If there’s a lot of husk waste (known as skinnings), or if there’s a lot of movement during milling, more phenols will be lost. Husks are also required during mashing; without them there will be drainage issues that can lead to the further loss of phenols, as well as the loss of other compounds necessary to create a good batch of spirit.
Phenols are also lost during mashing and fermentation. Some are left behind with the draff, while others are lost or transformed as water, barley and yeast combine to form new compounds. The distillation process and the cut points decided by each distillery also play a significant role. Heavier compounds, including phenols, are found primarily at the end of the distillation run when the spirit is extracted. The longer the hearts are run, the more ‘heavy’ phenolics incorporate themselves into the character of the new-make spirit that goes on to become whisky.
Keith Cruickshank, manager of Benromach Distillery in Speyside. Photo by Christopher Coates.
According to Benromach’s manager, Keith Cruikshank, this is how the Speyside operation brings more peated aromas and flavours to its heavily peated Contrasts: Peat Smoke release, compared to the usual softly peated house style of the distillery. “The skill of our dedicated distillers is especially critical when it comes to cutting the spirit, the moment when they consider the new spirit to be of the highest quality,” explains Keith. “As the Benromach core style is peated, the only thing we do differently for Peat Smoke is adjust the cut point from spirit to feints to create a smokier profile.”
Comparing the distillation processes at Ardbeg and Lagavulin also reveals why their smoky characters are so different from one another. Both distilleries cut onto their feints at a similar point, though Ardbeg’s malt is slightly more peated. Ardbeg’s Gordon describes his distillery’s profile as a mixture of soot and fruit, while he finds Lagavulin has a heavy, oily, and earthy character. The distillation processes are quite different. At Lagavulin, running a slow distillation combined with re-using a lot of foreshots and heavily phenolic feints (known as ‘charge’) from past distillations is important. Therein lies the tension that forms the core of Lagavulin, according to Gordon: “There’s a high charge in the spirit stills, above the man door. Because it’s high, even though there’s lots of reflux and you’re running those stills slowly and gently, that still leads to that really heavy, oily character. You’re still hammering everything through.”
At Ardbeg, on the other hand, Gordon illustrates how phenols combine with other elements to contribute to its fruity character: “We have a purifier there that means as your spirit is going over the swan neck into the lyne arm, there’s loads of reflux as it all gathers back and forth. This amplifies the fruity and estery notes you find in the whisky. There’s also a slightly shorter run of foreshots compared to Lagavulin.”
Then there is Octomore, literally the peatiest whisky in the world. Its origin and wide PPM range comes from the fact that Bruichladdich has asked its Inverness-based maltsters, Bairds Malts, not to mix in unpeated barley after the peated batch is put through an intense, slow cold-smoking process – making a consistent PPM level impossible to attain. However, the intense peat levels that go up to 309 PPM after malting are primarily tamed by Bruichladdich’s distillation process.
“We’ve got our tall stills, with these narrow necks, so you get a lot of reflux and contact with the copper. Also with the long fermentation we get these lovely fruity and floral flavours,” says Bruichladdich head distiller Adam Hannett. “Because of this, most other distilleries working with our malt would make a more phenolic whisky, but with us we’ve got a lighter, more elegant style, so we’re looking to achieve a balance that feels right.”
As a result, when considering the role of phenols in a dram, it actually is less about the amount that are squeezed into the malting process through kilning and more about how their loss is controlled throughout the remainder of the process. Because even the world’s top whisky scientists have not yet cracked all the mysteries of peat, whisky fans simply must trust the judgement and instincts of skilled producers like Cruikshank, Gordon, and Hannett, who are doing their best to create and share the full spectrum of smells and tastes of peated whisky.