Peat Terroir

Arbroath maltings

In 1800's the peat used in maltings was commonly local and the kilning was done in almost every distillery. During the latter part of 1900's and the invention of industrial malting processes the maltings have been concentrated to bigger facilities, such as Diageo's Roseisle, Glen Ord and Port Ellen, Simpson's Tweed Valley, Baird's Abroath and Inverness, Greencore's Glen Esk and Buckie and Edrington's Tamdhu. Several distillery maltings are still operational, but only a few distilleries malt all of their barley on-site (Springbank, Glen Ord, Tamdhu). The use of peat has diminished as more economical fuels have been introduced, but some peat is still used to create smoky flavoured malts. The peat used in Scotch whisky maltings is sourced mostly from the north-east of Scotland, Islay and Orkney.

Peat is formed in waterlogged lands by partial degradation of organic matter. There are some differences between peat compositions based on different climate, vegetation, bog type and cutting depth.

Peatlands can be divided into bogs, fens, marshes ja swamps.  Bogs are formed by heavy rainfall (therefore called ombotrophic) and usually contain more sphagnum moss and less woody vegetation than the other peatland types, which are formed mostly by waterlogging from the ground water (minerotrophic). Fens (aka basin bogs or valley bogs) contain more sedges and grass. Marshes are treeless intermittently waterlogged areas and usually accumulate peat very slowly. Swamps are very minerotrophic and contain large amounts of wood and nutritients. Average peat contains 90% water and 10% dry matter, of which 92% is organic and 8% inorganic (practically ash). Organic matter consists mainly of residues of lignin and different carbohydrates, but there are considerable variations depending on the vegetation and the bog type. Ombotrophic peats are richer in phenols and aromatics, but due to poorer vegetation they lack carbohydrates, lignin and nitrogen. Western Scotland and the islands are especially abundant with blanket bogs. About 10% of Scotland in covered by blanket bogs and 1% by basin bogs.

Peat bog	Maltings	Location	Bog type
Glenmachrie	Laphroaig	Islay	basin
Gartbreck	Bowmore	Islay	basin
Castlehill	Port Ellen	Islay	blanket
Hobbister Hill	Highland Park	Orkney	blanket
St Fergus	various	Aberdeenshire	basin
Tomintoul	various	Speyside	basin
Machrihanish	Springbank	Campbeltown	blanket

The extraction depth is also important as the surface layers are usually rich in carbohydrates and poor in phenols and the deeper layers might have too much harmful nitrogen- and sulphur compounds due to anaerobic fungal and bacterial metabolism. On the other hand some fungi seem to produce vanillins such as acetovanilline from lignin, for some unknown reason especially so in island peats.

Location of peat bogs used in whisky production

Islay peat bogs (from left: Gartbreck, Glenmachrie, Castlehill)

It seems that the most significant factors in peaty aroma are the bog location and the cutting depth. The bog type and the vegetation play some role, but they are not that important for peaty flavour. The peats from Islay, for example, are very similar with each other as the peat from the basin bogs Glenmachrie (Laphroaig) and Gartbreck (Bowmore) cannot be identified by infrared spectroscopy. 

The Castlehill peat for the Port Ellen Maltings somewhat different from the basin bog peats from Islay, probably due to different microbiology of the blanket bog and a greater amount of woody material are therefore more lignin-derivatives and carbohydrates. 

Surprisingly, Hobbister Hill peat from a blanket bog is chemically more similar to the Islay basin bog peats (Gartbreck/Glenmachrie) than the blanket bog Castlehill peat. The local microbiology of peat bog might therefore have a greater impact on the peat composition, influencing both the nitrogenlevels and phenol-concentrations in the peat. Another explanation is that the bog types are overlapping, for example Hobbister Hill bog might have some basin bog properties (standing water bowls) especially in the deeper layers; this might explain the similarity of peats cut from the deeper layers of Hobbister Hill bog with the Islay basin bog peats. 

Tomintoul peat is different from the Island peats; a basin bog, as expected, contains great amounts of carbohydrates, but there is also lots of sphagnum moss, which for some reason does not result in the same amounts of phenols as in the islands. It could be speculated that this is due to different climate, microbiology or variety of Sphagnum in the western islands and the mainland.

St Fergus peat is rich in woody material and therefore rich in lignin derivatives, especially syringyl. There is however a great amount of phenol-compounds in the St Fergus peat, although it is a basin bog and the sphagnum-content is lower than in for example Tomintoul peat. The location of St Fergus bog is closer to the sea than of Tomintoul bog. Maybe the proximity of sea results in greater amounts of phenols in peat? Explanation might be for example lesser temperature fluctuations or differences in microbiology or drainage.

There are no scientific analyses available on Machrihanish (Springbank) peat, but since it is cut from a blanket bog and located near sea in the western Scotland, it probably is closer to the Islay and Orkney peats than the mainland peat.

Some breweries use artificial smoke flavourings in their malting processes, apparently common raw materials are extracts from burnt deciduous trees, such as beech or birch. These are likely to produce more aromas from syringyl, guaiacol and carbohydrates resulting in softer but not as phenolic aroma as from peats. I don't know whether Scotch whisky industry uses smoke flavourings in their maltings, but it is not prohibited in the Scotch whisky regulations. A black alder smoked malt could be an interesting experiment (a free hint for any progressive distillers reading this).

Anyway, there clearly is a peat terroir and even some local variations in the compositions. Also the cutting depth is important and it will be interesting to see wheter the peat composition will change as the limited amounts of peat for example from Islay are utilised and either the cutting depth or the cutting location changes.

REFERENCES AND FURTHER READING:

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Bryce JH et al (ed). Distilled spirits, production, technology and innovation. Nottingham Univ Press, 2008
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Guillén M et al. Carbohydrate and nitrogenated compounds in liquid smoke flavorings. J Agric Food Chem 2001; 49; 2395-2403 

Guillén M, Manzanos M. Study of the components of a solid smoke flavouring preparation. Food Chem 1996; 55; 3; 251-257

Guillén M, Manzanos M. Characterization of the components of a salty smoke falvouring preparation. Food Chem 1996; 58; 1-2; 97-102

Harrison B, Priest F. Composition of peats in the preparation of malt for scotch whisky production. J Agric Food Chem 2009; 57; 2385-2391

Harrison B et al. Differentation of peats used in the preparation of malt for scotch whisky production. J Inst Brew 2006; 112; 4 ; 333-339

Jefford A. Peat, smoke and spirit. Headline 2004

Jounela-Eriksson P. The aroma composition of distilled beverages and preceived aroma of whisky. Academic Press 1978

Kostyra E, Barylko-Pikielna N. Volatiles composition and flavour profile identity of smoke flavourings. Food Qual Pref 2006; 17; 85-95
Lehtonen M. Phenols in whisky.  Chromatographia 1982; 16; 201-203

Russell I (ed). Whisky, technology, production and marketing. Academic Press 2003

Swan JS, Howie D. Sensory and analytical studies of regional influence on the composition of Scotch malt whisky. Institute of brewing, 1983.

Valaer P. Scotch whisky. Industr Engineer Chem 1940; 32; 7; 935-943

Walker GM, Hughes PS. Distilled spirits, new horizons: energy, environment and enlightenment. Nottingham Univ Press, 2010