Peat Explained: PPM, Islay Smoke and How It Fades with Age
Smoke is whisky's most divisive flavour. Here is where it comes from, what the ppm number really measures and why old peated malts taste softer.
MAN TARGET EditorsOctober 27, 2026 4 min read
Some men fall for peated whisky on the first sip; others never forgive it. Either way, smoke is the single loudest dial in the glass. Understanding where it comes from turns it from a gamble into a choice, and helps you decide whether you want a whisper of bonfire or a full harbour fire on a stormy night. The good news: the smoke is not random. It is a set of deliberate decisions made long before the cask.
From bog to kiln
Peat is partly decayed vegetation, mosses, sedges and heather, built up over thousands of years in waterlogged ground. Cut, dried and burned in a kiln, it releases smoke rich in phenolic compounds. Malted barley drying above it absorbs those compounds, most readily while the grain is still damp. The longer and denser the smoke, the more phenols the malt holds. Distillers then shape the result through how they distil and where they make the cut between heads, heart and tails.
Reading ppm
- PPM means parts per million of phenols, measured in the malted barley before it is mashed.
- A large share of those phenols is lost in fermentation, distillation and the cut, so the spirit carries far less than the malt.
- Many classic Islay malts are made from heavily peated barley, while Bruichladdich's Octomore 08.3 was made from barley peated to 309 ppm.
- PPM is a guide, not a promise: distillation style and cask can make a lower-ppm whisky taste smokier than a higher one.
Peat is not a flavour you add. It is a decision you make in the kiln and live with for decades.
Islay peat versus Highland peat
Peat is local, and so is its smoke. A 2006 study by Barry Harrison at Heriot-Watt University compared peat from Islay, Orkney and the mainland and found real differences in composition: Orkney's Hobbister Moor peat held more heather, while some mainland peat contained more woody material. Woodier peat, with more lignin, can produce different phenols. That may help explain why Islay smoke is often described as medicinal and maritime, while Orkney's reads more like heather and mainland peat more like a bonfire, though research continues.
How smoke ages
- Young peated malts show the smoke at full volume: ash, tar, iodine, embers.
- Over years in cask, the phenolic punch fades while fruit, oak and sweetness move forward.
- Old peated whiskies often show smoke as a soft, integrated thread rather than the headline.
- Taste a young and an older expression from the same distillery side by side, add a few drops of water and enjoy in moderation.
Key takeaways
- Peat smoke is absorbed by barley while it dries in the kiln.
- The ppm figure measures phenols in the malt, not in the bottle.
- Peat from different places may give different smoke; research is ongoing.
- Smoke is boldest in young whisky and softens with years in cask.
FAQ
What does ppm mean in whisky?
It is the concentration of phenols, in parts per million, measured in the malted barley. The finished whisky contains much less, because phenols are lost during production.
Does a higher ppm always mean a smokier whisky?
Not always. Distillation, the cut and the cask all influence how smoky the whisky tastes, so ppm is a useful guide rather than a precise measure.
Does peat smoke fade as whisky ages?
Yes. Phenolic intensity generally declines during maturation, so older peated whiskies tend to taste softer and more integrated than young ones.
See our rankings of peated malts, from gentle bonfire to full Islay storm.
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