One of the multiple steps before distilling a malt whisky is malting the barley and one of the steps is drying the malt. For that, you need heat in kilning to dry the malts. Peat has been traditionally used as a cheap fuel for kilning (and heating homes) in Scotland in the past, especially in the regions where coal was hard to come by and expensive, like the islands, Campbeltown and the Northern Highlands. Now, the question is: what is peat?
Decomposed moss? Eww?
Peat is a kind of organic soil, a spongy material formed by the partial decomposition of organic matter. This matter mostly is plant material (usually Sphagnum and plants such as heather, cotton, grass, rushes…), in wetlands (swamps, bogs…). Its origins are due to botanical and geological processes, with contributions to peat deposits being attributable to animals, plants and diverse groups of microbial taxa. Peat is also the world’s largest terrestrial carbon store.
Peat forms in bogs or peatlands, that are a type of wetland with a high acid content. Wetlands also have different origins: some are elevated bogs due to heavy rainfall while others are the result of high groundwater level. Scottish wetlands are usually the former, and therefore contain mostly sphagnum moss and heather, and not much woody vegetation. The surface layer is made of thin and aerated moss. On the other hand, the deeper layers below half a metre are usually waterlogged. As the moss grows above, in the upper part called acrotelm, the deeper layers, the catotelm, see the moss decomposes. Because of the waterlogging, the oxygen is almost inaccessible in the soil (anaerobic conditions), and thus the decomposition of organic matter is slow and incomplete. Furthermore, the moss growing on top creates pressure on the lower and deeper layers, producing a thicker peat, especially below water level.
Flora and phenol… a
Sorry, this paragraph is not going to be the most fun to read, but bear with me. Plants in peatlands mostly are made of cellulose, hemicellulose and lignin. Grasses contain lignin, that contains all the monolignols. Sphagnum moss is richer in p-hydroxyl-phenols due to the structure of sphagnum which consists of a sort of polyphenolic network. Therefore, it makes burnt sphagnum release simpler phenols while burnt wood releases more derivatives from syringol and guaiacol which give out slightly different smoky aromas. Plants with more cellulose and hemicellulose, like wooden stemmed plants, decompose into simpler carbohydrates.
The surface layer has more carbohydrates and fewer phenols, proportionally, than deeper layers. These deeper layers have increased levels of potentially harmful hydrogen sulphides (the “aroma” of rotten eggs) and nitrogen compounds, that are probably produced by a range of fungi. As for hydrogen sulphide, it is generated by bacteria in anaerobic conditions, usually below water level, from other sulphur compounds.
Simple phenols like… phenol, but also its alcohol derivatives and creosols are supposedly the source of the smoky flavour of a peat reek. Different carbonyl compounds seem to soften the phenolic aromas, as without guaiacols and carbonyls, the phenols can taste ashy, sharp and hard whereas together, they give out aromas of smoked meat, burnt sugar and savoury Maggi sauce. Large quantities of nitrogenated compounds result in astringent, green and rubbery flavours due to the higher levels of pyridines. And the rubbery, unpleasant “sulphury” smell (from nitrogenated compounds) in a whisky can result from not using enough sulphurous fuel in kilning, which seems paradoxical!
I won’t go too far about peat origins today and won’t use the T word, as I’ll go in-depth in a future article about the differences in peat depending of their origins. To be brief, peat from Scotland’s mainland is usually richer in carbohydrates but poorer in phenols, guaiacol, vanillic compounds and nitrogen than Islay’s peat, because there is a greater amount of wooden stemmed plants but a lesser amount of sphagnum in mainland’s peat compared to Islay bogs. Orkney peats are in between mainland and Islay, as they contain more phenols than mainland peats like the one from Tomintoul, and more carbohydrates than Islay’s peat. And the extraction depth also has influence with the layers having great differences in peat composition.
Peat is usually hand-cut, though progress have allowed mechanical methods to excavate and spread peat. Traditionally, a farmer or labourer harvests peat by manually cutting think strips of peat using a large hoe (also called peat spade or cutter). It’s then cut in the form of blocks then stacked to dry, usually on the summer. Peat needs a good drying wind for about three weeks, which will enable the peat to lose 75% of its water content and shrink by about 25%. Once dry, a block of peat weighs between 0.75 and 2 pounds (0.33 kg to 0.9 kg). The mechanical method consists of a dredger or excavator to dig the peat from the drained bog. Then the peat is delivered to a macerator that will soften and separate its components by soaking it and extrude the peat pulp through a rectangular opening. The pulp is then cut into blocks which are, like the traditional method, spread to dry.
As you probably know, distilleries rely mostly on barley malted from industrial and commercial maltings. In days gone by, however, they had to malt their own barley. In short, malting the barley makes the starches within barleycorns soluble, so that the sugars may be converted to alcohol using yeast. Malting, in other words, tricks the barley, as distillery tour guides love to explain, into thinking spring has come. Barleycorns are then steeped in water and allowed to germinate before the process is halted by drying the barley in the kiln.
When peat is burnt to heat the kiln, it produces an especially aromatic smoke. Malt absorbs better these smoky flavours when hand dry (between 15% and 30% of moisture); therefore peat is burnt in the early stages of kilning. Grinding and composition of the malts also affects the aroma, as the husks are more inclined to absorb the phenols. How peated is a malt is usually specified by measuring the concentration of phenols with liquid chromatography measured in parts per million (ppm). A lightly peated malt has usually less than 5 ppm phenols, while a medium-peated malt is between 5 and 15 ppm, and a heavily peated malt between 15 and 50 ppm. However, some experimental peated malts like the one used from Bruichladdich’s Octomore range have well over 100 ppm phenols.
When a distillery talks about its peat level in ppm, they talk about the peat level in the malt and not in the new make. The phenol content and the strength of the smoky aroma are way higher in the malt and slightly higher in the wort than in the freshly distilled spirit. However, the phenol content of the spirit also depends heavily on the distillation and the cut. The foreshots contain barely any smoky flavour. Since they contain poisonous alcohols, they’re discarded anyway. The middle cut has very light and subtle smoky flavours though the end of the hearts is smokier. Finally, the tails, the last part of the distillation, are the part that is very smoky, about six times more than the middle cut. To produce heavily smoked whisky, it is then important to continue the middle cut as long as possible, but keeping in mind that too much feints (the tail) produces unwanted off flavours.
During maturation in the cask, phenols decrease over time, but the scientific explanation of this mechanism is not known. If you compare an old peated whisky and a young one from the same distillery and distilled about the same time, you’ll see that the young one is way peatier than the oldest one. A 25 ppm phenol content in new make is estimated to decrease to 10 ppm after 10 years, 8 ppm after 15 years and 6 ppm after 30 years of maturation in a cask.
Wow, still here? Thank you for bearing with me. Well, in fact, it was not that long and detailed an article, if you’re really interested, there are probably tons of books with way more detailed information about peat. Peat can be very divisive: some hate peat and some love it (I’m in the latter category). But there’s one thing to know: you can talk how rad peat is in a whisky club.