Sunday, February 13, 2011

Peat

Pagoda-roof of a kiln
Heat is needed in kilning to dry the malts. Peat has been traditionally used as fuel for kilning in Scotland, especially in the areas where coal was not easily available, such as the islands, Campbeltown and nothern highlands. During the 20th century more affordable coal, gas and oil became more available and gradually the use of peat in kilning dimished. In 1940 it was common to use 25-50% peat for Lowland malts, 50-75% peat for Highland malts and usually 100% peat for Islay and Campbeltown malts. The rest of the fuel was usually coal or anthracite. Nowadays the Islay, Orkney and Campbeltown distilleries are famous for their peated malts, but also some mainland distilleries have experimented with peated malts.

Sphagnum bog
According to Encyclopedia Britannica peat is spongy material formed by the partial decomposition of organic matter in wetlands. Usually peat consist of decomposed Sphagnum and low growing plants such as heathers and different sedges (cotton grass, rushes, grasses). Wetlands can be formed by heavy rainfall or by a water basin filled by ground water. Scottish peatlands are usually formed by heavy rainfall and therefore contain more sphagnum moss and heather and less any woody vegetation. The surface layer of a peat bog is thin and aeriated moss and the deeper layers below 50cm are usually waterlogged. As the moss grows the deeper layers decompose. Because of the waterlogging there is less oxygen available and thus the decomposition of organic matter is slow and incomplete. The growing moss on top creates pressure on the deeper layers, producing thicker peat especially below the waterlevel.
 
Heather (calluna vulgaris)
Peatland plants consist mainly of cellulose, hemicellolose and lignin. The lignin in grasses contains all the monolignols; coniferyl, sinapyl and p-coumaryl-alcohols, as lignin in heather is mostly coniferyl-sinapyl-type (see previous blog). Sphagnum is very different in structure consisting of a sort of polyphenolic network. Sphagnum moss is therefore richer in p-hydroxyl-phenols instead of the usual lignin-derived compounds. Therefore burnt sphagnum releases more simple phenols and burnt wood more syringol and guaiacol-derivatives with slightly different smoky aromas. Wooden stemmed plants with more cellulose and hemicellulose decompose into simpler carbohydrates. Surface layer has proportionally more carbohydrates and less phenols and deeper layers have increased levels of potentially harmful nitrogen compounds and hydrogen sulphide (aroma of rotten eggs). Nitrogen compounds are probably produced by a range of fungi. Hydrogen sulphide is generated by bacteria in anaerobic conditions, usually below the waterlevel, from other sulphur compounds.

Anthracite coal
The smoky flavour of a peat reek is supposedly coming from simple phenols, such as phenol, its alcohol-derivatives and creosols, and to some extent from guaiacols, furans and pyrans. Syringyl-compounds are not thought to be of major significance in producing smoky aromas. Different carbonyl-compounds seem to soften the phenolic aromas. Without the carbonyls and guaiacols the phenols can taste ashy, sharp and hard, whereas together they produce aromas of smoked meat, savory "maggi" and burnt sugar. Large amounts of nitrogenated compounds give higher levels of pyridines and result in astringent, green and rubbery flavours. In addition the nitrosamines produced by the nitrogen oxides in malt are carcinogenic. The formation of nitrosamines can be blocked by sulphur oxides, which can be produced by burning sulphur-containing coal or rock sulphur with peat or by adding gaseous suphur dioxide to non-sulphurous gas. Paradoxally the rubbery, unpleasant "sulphury" odor (from nitrogenated compunds) in a whisky can result from not using enough sulphurous fuel in kilning. Also the right temperature in firing is important as more smoke and lignin-derived aromas are extracted and less nitrogen released if the peat burns without flames in realtively low temperatures.

Peat layers
There are considerable differences between peats from different origins. Islay peat is usually richer in phenols, guaiacol, vanillic compunds and nitrogens but poorer in carbohydrates than the peat from the mainland . This is probably because of the greater amount of Sphagnum and lesser amount of wooden stemmed plants in Islay bogs. Wooden plants, especially decideous plants contain grater amounts of syringol-based aromas compared to phenol-rich Sphagnum and relatively guaiacol-rich bog plants. Orkney peats are of an intermediate type as they contain more carbohydrates than Islay peats and more phenols than mainland peats from Tomintoul. The extraction depth is also important, as especially in Orkney there are great differences in peat composition as surface peat is closer to the mainland peat and deeper layers reseble Islay peat. The best extraction depth seems to be just above the water level near the surface. This is probably because of greater amount of carbohydrates and lesser amount of nitrosamines and hydrogensulphide in the surface layer. The drying of peats and the controlling of burning temperatures are also easier if the peat is not too thick.

The middle cut (from bruichladdich.com)
Malt adsorbs the smoky flavours best when hand dry (15-30% moisture), therefore peat must be burned in the early stages of kilning. Also the grinding and composition of the malts affects the aroma, as the husks are more prone to absorb the phenols. Usually the malt is specified by measuring the phenol-content with high performance liquid chromatography (HPLC); lightly peated malt has <5 ppm phenols, medium peated 5-15 ppm, heavily peated 15-50 ppm and some experimental peated malts have well over 100 ppm phenols. The taste tresholds for different phenolic compounds vary greatly, for example 3 µg/l for guaiacol, 10-68 µg/l for creosols and 7100 µg/l for phenol. The phenol content and the strenght of the smoky aroma in the spirit is usually much lower than in the malts and little lower than in the wort, but it depends heavily on the distillation practice. The foreshots contain barely any smoky flavour, the middle cut is also quite subtle, but the last part of the cut is very smoky, about six-fold compared to the middle cut and about the same intensity as the wort. The tails (feints) has about third of the phenols in the last cut and twice of that in the middle cut. To produce heavily smoked malt it seems important to continue the middle cut as long as possible bearing in mind that too much feints produces unwanted off-flavours. None of the distillers have (yet) used only the last cut to produce very smoky spirit. In the table below are some phenol contents and middle cut alcohol contents. Possibly the more intense smoky flavour of the southern Islay whiskies (Ardbeg, Laphroaig, Lagavulin) is partly due to their longer middle cuts.

Lagavulin 1969 (from Whisky Exchange blog)
Phenols decrease during maturation, but the exact mechanism has not been decribed. It is estimated that 25 ppm phenol content in a new make becomes 10 ppm after 10 years, 8 ppm after 15 years and 6 ppm after 30years of cask maturation. Altough the synergetic nature of oak derived compounds (guaiacol, vanillin etc) can enhance the peaty flavours, it usually softens and diminishes during aging.



Phenol-levels of malts and new-makes in different distilleries and the ABV of the middle cut. (modified from Misako Udo: The Scottish Whisky Distilleries)
DISTILLERYMALT PHENOLS (ppm)NEW MAKE PHENOLS (ppm)MIDDLE CUT ABV
Ardbeg54 (42-70)24-2673-62.5
Bowmore20-258-1074-61.5
Bruichladdich3-4
76-64
Port Charlotte4020-25

Octomore129 (in 2003)46 (in 2003)
Brora7-40

Bunnahabhain1-2 (peated malt 38)

72-64
Caol Ila30-3512-1375-65
Highland Park35-40 (and unpeated malt used together)270 and then 2h40min
Lagavulin35-4016-1872-59
Laphroaig40-452572-60.5
Springbank

7-8 (formerly 15-20)68-63
Longrow55

Old Ballantruan (Tomintoul)55





REFERENCES, FURTHER READING:

Bozkurt  S et al. Peat as a potential analogue for the long-term evolution in landfills. Earth Sci Rev 2001; 53; 95-147
Bryce JH et al (ed). Distilled spirits, production, technology and innovation. Nottingham Univ Press, 2008
Da Porto C et al. A study on the composition of distillates obtained from smoked marc. Anal Chim Acta 2006; 563; 396-400
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 
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
Smith GD, Wallace G. Ardbeg, a peaty provenance. GW Publishing 2008  
Udo M: The Scottish Whisky Distilleries. Black & White 2006
Valaer P. Scotch whisky. Industr Engineer Chem 1940; 32; 7; 935-943
Voigt J et al. New highly aromatic products and distillates from smoked malt. In Distilled Spirits, Nottingham Univ Press 2010
Walker GM, Hughes PS. Distilled spirits, new horizons: energy, environment and enlightenment. Nottingham Univ Press, 2010

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