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Peat & whisky

Picture of Peat & whisky

Peated whisky is a divisive area, and one of the most challenging concepts for many first time drinkers, but for a huge number of whisky fans peat is the stuff dreams are made of. Put simply peat is the top layer of a bog, which consists primarily of decaying plant fibers, the peat is harvested in fields and can be cut into blocks with a shovel. Peat is an ideal fuel and has therefore been used for centuries to heat and dry the barley for whisky production. Peat was generally used where there were no trees, coal or other sources of heat. Today peat is very deliberately used to produce particularly smoky single malts, and on occasion grains.

What is Peat?

Peat is a form of proto-coal created by rotting vegetation. While coal is a sedimentary rock form created by prehistoric vegetation, peat is an early form of the same fuel created by the decay of much newer vegetation. Peat is typically used as both fuel and as a soil enricher.

Why is peat used to make whisky?

The creation of whisky from barley requires malting to break down the starch into yeast digestible sugar. Halting germination before the grain starts to sprout and consume the created sugars requires heat, so whisky makers traditionally dried their grain with whatever fuel was available be it wood, coal, or peat. Nowadays electric powered hot air is typically used to produce unpeated spirit, a few distilleries still make use of peat however, in particular on the Scottish Isle of Islay where it’s peated style is something of a signature statement.

How is peat flavour measured?

Peat levels are measurements using a high-performance liquid chromatograph (HPLC). With a HPLC a chemists can analyze and quantify the composition of a substance calculating how many phenols are in the malted barley or have deposited on it. This value is given in phenolic parts per million (PPM). Often the PPM value of the barley is measured and not even the value of the New Makes let alone the final whisky. Because of this the PPM value says little about how many phenols and thus peat aromas are in the final whisky and how smoky it actually is as the production process and variation within can significantly reduce the final PPM.

What is PPM?

PPM is the calculation of the mass of a chemical or contaminate per unit volume of liquid, in this case a phenolic count measured in parts per million. PPM also can be expressed as milligrams per liter (mg/L). The PPM in the New Make can also be measured. However, as phenols are lost during the distillation process, as described below, this value is always lower than the PPM value measured in barley. Perhaps for this reason, almost all manufacturers prefer to give the higher value - even if it says little about the peat in the whisky.

Lost phenols during production

A significant proportion of the phenols are lost during the production of the whisky. Some get stuck to the bottom at the end of the mash, some are lost or change during fermentation and very many do not survive the second distillation. Since phenols are large molecules with a high boiling point, they are only released in vaporized form at the very end of the distillation process. This is the point at which the master distiller makes a cut to avoid unwanted substances in his distillate. If this cut is made later, more phenols get into the new make. The Master Distiller can therefore control the peat content in young whisky fairly precisely.

Even when stored in wooden barrels, phenols are steadily broken down in the following years. For example, anyone who has compared Laphroaig 10 years, 18 years and 25 years with each other will be able to confirm this. If the 10-year-old is still very medicinal, these aromas are less likely to be found in the older bottlings. The power of peat and smoke often decreases over the years, the whisky becomes softer. It changes, and with it the taste and intensity of the peat. Of course, the Laphroaig 25 years is still a smoky whisky, but the differences between the bottlings in terms of smoke and peat are still obvious.

Measuring the liquid with Knockdhu

Given the issues with measuring PPM based on barley the speyside Knockdhu distillery decided that a more meaningful way of communicating the smokiness of a whisky would be to measure the liquid in the bottle. For the anCnoc Rutter, Flaughter and Tushkar expressions the distillery opted to tell consumers about the PPM level after distillation and maturation rather than at peating. Unfortunately while more accurate this proved more confusing as well, the Flaughter was marketed at 14.8ppm which under the existing sytem implies a moderately peated whisky. In truth the Flaughter was much more heavily peated. Sadly, the concluded thatalthough more accurate, measuring PPM levels in the final liquid was too confusing for consumers and the distillery has since returned to the industry standard of listing PPM level of the barley instead of the whisky itself.

Phenols & peat flavors

When peat is burned it creates, and releases large numbers of phenols. Phenols are aromatic alcohols and nitrogen-containing, ring-shaped compounds such as crysols, guiacoles, xylenols and eugenols. They have an oily consistency and are deposited on the barley husk. Each of these brings unique flavour notes to the resultant whisky. Depending on the phenol, these can be smoky, medicinal, spicy, meaty or sweet notes. The composition of the phenols in peat varies depending on the location of the field, climatic factors and the way the peat is burned consequently the taste of the peat in the whisky is not uniform. Peat may always look like brown earth, but not all peat is created equal and different whiskies will have different peat notes.

Sensory attributes associated with peated whiskies

  • Smoky
  • Ashy
  • Woody
  • Cedar
  • Musty-Dusty
  • Musty/Earthy
  • Burnt
  • Acrid
  • Pungent
  • Petroleum-like
  • Creosote/Tar

It’s worth noting that phenolic aromas in whisky not only come through smoking over peat, but also through the barrel. Phenolic structures are also formed when the oak barrel is charred which are similar to whisky medicinal notes and licorice aromas. For this reason it’s possible to identify smoky notes even in non-smoked whiskies.

The main phenolic compounds

Phenol, guaiacol and cresol are the most recognisable of the eight key phenolic compounds, though there are a far greater number to consider. Their impact on the whiskies flavour profile should not be considered in terms of concentration however. Guaiacol for example is amoung the most identifieable as its flavour threshold in water is estimated at 3 parts per billion (PPB). However Guaiacol is particularly soluable in alcohol and so the taste of guaiacol is enhanced by dilution.

Aromas of typical phenols in peated malt

Compound Aromas
Phenol Carbolic
2-methylphenol (o-cresol) musty, medicinally
3-methylphenol (m-cresol) woody, ethereal
4-methylphenol (p-cresol) medicinally
dimethylphenols (xylenols) medicinal, sweet
2-methoxyphenol (guaicol or creosol) medicinally, woody, smoky
4-ethylguaiacol smoky, fleshy
4-vinylguaiacol clove, spicy
4- (2-propenyl) guaiacol (eugenol) cinnamon, clove, spicy


Phenol itself is typically the most common and simplest phenol being a benzene ring with an appendage made up of an oxygen and a hydrogen atom. In other phenols, the benzene rings also carry additional molecular groups, such as methyl, ethyl or methoxy radicals or combinations thereof. In whisky there are over 20 such phenols, the most common and most relevant for the aroma are, besides phenol, guaiacol and its ethyl and vinyl derivatives as well as meta-, ortho- and para-cresol. All of these compounds are degradation products of the vegetable polymer lignin, which is contained in peat.


Along with Phenol cresols make up the bulk of the rich peat smoked aromas and flavours associated with Islay whiskies. Cresols are generally described as medicinal with odours of disinfectant and gum, however some cresol groups are also associated with sulphurous and even sewage like smells.


Guaiacol is generally considered to be smoky contributing both medicinal bandage notes amidst strong aromas of clove and earthy smoke. Ethyl guaiacol in contrast is associated with spicy and sweet aromas.

Factors influencing peat flavour

The source of the peat used for whisky making is one of the most important considerations as its composition will be dictated by this. The flavours depend on whether the peat is cut from the surface of the ground or from deep below, and whether it is island peat or peat from the mainland. If the peat comes from the surface of the ground, it tastes of smoke and bacon, campfires and blazing wood. Taken from the lower reaches of the soil, on the other hand, it tastes more like the Laphroaig notes of phenol and asphalt, of gasoline and of bandages. The difference between island and mainland peat is mainly due to its composition, which, it has been assumed, contains significantly more sand on the island than on the mainland and thus influences the consistency and thus the decomposition process.

Depending on how many methyl groups are ultimately attached to such a compound, there are also cresols and various guaiacols in addition to phenols. The latter are responsible for the spicy, sweet smoky aromas, as can be found above all in the moors on the Orkney Islands, on Hobbister Hill.

The classic medicinal Islay notes come mainly from the phenols and cresols from the Gartbrecks, Glenmachries and Castlehills moors on Islay and from the Machrihanish Moor in Campbeltown. Depending on the position of these methyl groups, the whisky can then tastes gummy, sulphurous or even like wastewater.

The age of the peat

Peat is incredibly slow growing, as each bog grows by only around 1mm per year. Consequently a bog of 3 metres depth will be approximately 3,000 years old. Thus it should come as no surprise that the phenols created from peat drawn from the top will differ from those further down.

Where the peat is found

As Peat is essentially the compressed form of decomposed organic plant matter, essentially young coal there is no shortage of countries where peat can be found. In Scotland peat covers approximately 23% of the country, primarily in the Highlands and Islands. However it is also found throughout Ireland, northern England, Scandinavia, and parts of Russia as well as in the U.S. Within Scotland peat is divided into five different regions, from which the aromatic profile of peat results; these are the moors on Islay, St. Fergus in Aberdeenshire, peat from Campbeltown, Hobbister Hill on Orkney and Tomintoul on Speyside. Ultimately, of course, it also depends on the type of vegetation prevalent because thistles, heather, sand and fern simply taste different.

Peat from Islay has a far higher proportion of algae, which may cause more marine-esque smoke. In the Speyside and other Highland regions there are more components of forests and their vegetation, i.e. wood, roots, moss and the like. Highland Park in the Orkneys cuts its peat from Hobbister Hill, which is said to contain a high concentration of heather. Because peat bears the fingerprint of every region some have posited that peat may consititute a form of terroir.

Kilning versus smoking & temperatures

The composition of the phenols in the exhaust air from a peat fire have a lot to do with the origin and age of the peat but the combustion conditions also have a significant role to play. Phenols are mainly formed at temperatures of 200 to 850 degrees, exactly what temperature prevails determines which phenols are formed preferentially. Phenol and cresols are formed at higher temperatures, whereas guaiacols tend to be formed at lower temperatures. Maintaining low temperatures during kilning requires a very moist peat fire. When moist peat is used there is only white smoke instead of flames and a low combustion temperature resulting in more guaiacol for a more smoky aroma. In contrast a higher combustion temperatures means more phenolic medicinal taste.

Traditionally kilning was conducted at the distillery, for thus reason the iconic pegoda roof’s have become a symbol for malt whisky being found on the road signs through Speyside in Scotland to denote the route of the Malt Whisky Trail. However this is no longer common practice and only a handful of distilleries still undertake on site malting and kilning, and even then this can only supply a small percentage of their annual need. This has led to the creation of specialist maltings, many of which dwarf the massive Port Ellen Maltings, all providing grain smoked to the respective distilleries specification. Peat is no longer the soul source of heat being applied either, where traditionally peat was the sole source of fuel industrial maltings will use air from coal and heat exchangers to halt germination, and then only use peat to smoke the grain thereafter. This development has dramatically reduced the amount of peat required to malt barley.

Peat smoke from whisky casks (ex-peated casks)

There is another possibility to bring the peat smoke aromas into the whisky. By maturing in barrels that previously contained heavily smoky whisky, the aromas can also get into the new whisky. This method is comparatively new, but is becoming increasingly popular. Examples are the Glenrothes Peated Cask Reserve, Glenlivet Nadurra Peated Batch and the Penderyn Celt.

Peat aromas and water

The water source for a single malt has been cherished for generations and many distilleries guard it like the apple of their eye. They often buy the land around the source in order to effectively protect it from pollution. Distilleries also like to speak of their pure spring water being ideal for whisky production, however every now and then we come across the references to peaty water. Contrary to the obvious assumption, this water does not provide any peaty or smoky aromas in the whisky as it is mineralic rather than phenolic. According to whisky expert Charles Maclean, water has the greatest impact in fermentation, where it affects bacteria and yeast growth. Peaty water has a lower pH value than unpeated water, which can under certain circumstances have a positive effect on fermentation.

Peatland exhaustion

In natural peatlands, the annual rate of biomass production exceeds the rate of decomposition and there is a constant supply of pear being created, so the whisky exchange April fools gag was just that. In fact while this proto-coal isn’t quite renewable, it’s not quite a fossil fuel either, if whisky production was the only threat to peatlands we would likely never run out of peat. Unfortunately the greatest threat to peat supplies actually comes not from enthusiastic whisky makers but primarily in the form of domestic gardeners as each year we are digging up 106 million cubic feet of what’s left for use in horticulture (Defra, 2010).

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