Peat is the flavour that divides whisky drinkers more than any other, and the one most often misunderstood. It is not an ingredient in the usual sense: nothing is added to the whisky. It is a fuel, burned under germinating barley to dry it, and the smoke from the burning leaves compounds on the grain that survive — some of them — through mashing, fermentation, distillation and decades in a cask to reach the glass as what we call smoke. This guide explains what peat is, how its flavour gets into whisky, what the number on the label measures, and why two whiskies with the same number can taste nothing alike.
The malting and mashing guide covers the kilning step in context; the how whisky is made guide places it in the whole process.
What peat is
Peat is partly decomposed plant matter — mosses, heather, grasses, sedges, in some places tree roots and seaweed — that has accumulated in waterlogged ground over thousands of years, too wet to rot fully and too cold to burn. It forms at around a millimetre a year; a bog a metre deep is a thousand years old. In Scotland and Ireland, where trees were scarce and peat was everywhere, it was the household fuel for centuries, cut in spring into bricks with a special spade, stacked to dry over the summer, and burned in the winter. Distilleries used it because it was what they had.
How it gets into whisky
Barley is malted by steeping it in water, letting it germinate for a few days to produce the enzymes that will convert its starch, and then drying it to stop the process. Drying is done in a kiln, and the fuel under the kiln floor can be anything. Burn coal, gas or hot air and the malt is unpeated. Burn peat, particularly in the early hours of drying when the grain is still damp and absorbent, and the smoke deposits phenolic compounds on the husk. The longer and the earlier the peat burns, the more phenols the malt absorbs.
The compounds are a family: phenol itself, cresols, guaiacol, syringol, xylenols and others, each with a slightly different aroma — medicinal, smoky, tarry, spicy, sweet. The mix depends on what the peat was made of and how hot it burned.
PPM: what the number measures
Peated malt is graded by its phenol content, measured in parts per million by weight. Unpeated malt is under 2 ppm. Lightly peated malt — the sort used for a whisper of smoke in a Highland whisky — is 5 to 15. Medium is 20 to 30. The classic Islay distilleries run 35 to 55. Bruichladdich’s Octomore series has pushed the figure past 300.
The number is real and useful to the distiller. It is misleading to the drinker, for three reasons.
It measures the malt, not the whisky. Phenols are lost at every subsequent step. Some wash out in mashing; some are consumed or transformed in fermentation; most of what remains is split by distillation, where the cut points decide how much comes over into the spirit. As a rough rule, only a third or so of the malt’s phenol content reaches the new-make spirit, and the proportion varies with the still and the stillman. Two distilleries buying 40 ppm malt can produce new-make at 10 ppm and 20 ppm. The fermentation and distillation guide explains where the losses happen.
It measures quantity, not character. Phenol content tells you how much smoke, not what kind. Peat from an Islay bog, full of seaweed and salt, gives iodine and brine; peat from a Highland moor, rich in heather and wood, gives a drier, sweeter, more bonfire-like smoke; Orkney’s heather peat is floral. Same number, different flavour.
It fades. Phenols decline during maturation, so a heavily peated whisky at twenty-five years tastes gentler than the same whisky at eight. A 50 ppm malt in a 30-year-old may be subtler than a 20 ppm malt in a 5-year-old. The maturation guide covers why.
Where the peat comes from
Most Scottish peated malt is produced by a handful of commercial maltings — Port Ellen on Islay, Glen Ord, and a few on the mainland — which buy peat from specific bogs and kiln to each distillery’s specification. A few distilleries still malt a portion of their own barley on traditional floors and burn their own peat: Laphroaig, Bowmore and Kilchoman on Islay, Springbank, Balvenie, Highland Park and Benriach elsewhere. Their house-malted portion is often what gives them a distinctive smoke, since the peat is local and the kilning is their own.
The source matters because peat is a record of what grew above it. Coastal peat contains marine vegetation and gives the medicinal, seaweedy character that Islay is known for. Inland peat from heather moorland is more floral and woody. Peat from areas that were once forested contains lignin from tree roots and gives a sweeter, more resinous smoke. Distillers who talk about “Islay peat” versus “mainland peat” are describing a genuine difference in chemistry, not marketing.
The vocabulary of smoke
Because peat is a family of compounds rather than one, tasting it means picking out which members are present. The flavour wheel guide lists the standard descriptors; the main ones are:
- Medicinal: iodine, TCP, bandages, hospital — cresols and phenol, strongest in coastal peat.
- Smoky: bonfire, woodsmoke, ash — guaiacol and syringol.
- Tarry: creosote, road tar, rubber — heavier phenols and sometimes sulphur alongside.
- Maritime: seaweed, brine, salt — partly peat, partly the cask’s seaside warehouse.
- Sweet smoke: bacon, barbecue, smoked ham — guaiacol with sugar from the cask.
- Earthy: damp soil, moss, mushroom — the bog itself.
A whisky that reads as “medicinal and maritime” is typically Islay; “woodsmoke and heather” is more likely Highland or island. Learning to nose the difference — the nosing guide explains how — tells you more about a peated whisky’s origin than the number ever will.
How to taste for it
Peat is easiest to learn by comparison. Pour an unpeated whisky and a lightly peated one from the same region side by side; the smoke in the second will be obvious against the first in a way it is not on its own. Then step up — medium, then heavy — and notice that the character changes as well as the intensity: a 20 ppm whisky is not simply half an Islay. Add water to a heavily peated whisky and the smoke will often recede and the fruit beneath it come forward, which is the fastest way to understand that peat sits on top of a spirit rather than replacing it.
Peat and the environment
Peat bogs store enormous quantities of carbon, and cutting them releases it. The whisky industry’s use is small against the total — most Scottish peat extraction has historically gone to horticulture and fuel — but it is not nothing, and the industry’s trade body has committed to responsible sourcing and bog restoration. Some distilleries now specify peat from managed sites; a few are experimenting with alternatives, such as smoking malt with wood, which gives a different and generally milder result. Whether “sustainable peat” is possible is a live question, and the answer will shape what peated whisky looks like in fifty years.
Why smoke isn’t the whole story
The thing to hold onto is that peat is one input among many, and the finished whisky is the product of all of them. Heavily peated spirit from tall stills with a high cut is light and citric despite the smoke; moderately peated spirit from worm tubs and a late cut is heavy and tarry. Bourbon casks let the smoke show; sherry casks wrap it in fruit. Age softens it; cask strength sharpens it. The number on the bag of malt is the beginning of a long chain of decisions, and the glass is the end.
