peat is, as defined by W. A. Kerr in 1905, “a spongy vegetable substance, composed generally of mosses and aquatic plants in different states of decomposition” that “is in the progressive stage from a vegetable substance to a mineral coal.” Because of the waterlogged conditions in peat bogs, there is insufficient oxygen for these organic deposits to be fully decomposed. Instead the plants—not just mosses but also heathers, grasses, and even trees—once dead are layered and pressed by the weight of subsequent generations and turned eventually into a soft, cakey substance without identifiable traces of its component plants, although those are still retained in upper layers of the bog. A peat bog will “grow” approximately 1 mm per year.
The physical makeup of peat will vary depending on the vegetation that has grown on that spot thousands of years before. Analysis of pollen and fibers from peats cut in different parts of Scotland indicates that each site can be defined by the composition of the peat. In general, peat from mainland Scotland has high levels of lignin from decayed woody material; that from the north, Orkney, and western isles has less lignin and more moss. Orkney has the highest levels of heather. Because of this, it is now believed that the physical makeup of the peat may give different aromatic characteristics to peated malt and, therefore, peated scotch whisky. Anecdotal evidence backs this up, suggesting that peat from the mainland gives off more wood smoke aromas; that from Orkney is more perfumed and heathery; while that from Islay mixes mineral, tarry, and medicinal elements.
In the seventeenth and eighteenth centuries, peat was used extensively in Scotland and in rural Ireland in the production of whisky, as there was no alternative fuel source close to hand for kilning the malted barley. In some locations, peat was also used to fire the stills. The nineteenth century saw the Irish move decisively away from peat as their distilleries consolidated in urban areas, while in Scotland its use became a choice, linked to style more than necessity.
The availability of different fuels for kilning means that today peat is used purely for its aromatic qualities, with barley malt kilned over the fuel giving whisky a distinctive aroma known as “peat reek.” This is produced by a series of flavor compounds grouped under the family of phenols. See phenols.
The green malt has a moisture content of between 15 percent and 30 percent. When the smoke from the kiln rises, the phenols adhere to the surface of the still damp barley, imbuing it with their character. It is only at this stage of the kilning process that the phenols have an effect. Once the moisture level of the malted barley drops below 15 percent, any smoke will simply pass across the bed; for that reason, peat is generally only used at the beginning of the kilning process. A peat fire used for kilning will not have any flame but will smolder instead.
A distiller will set a measurement of the phenolic parts per million (PPM) it requires in its malt, with 15 PPM considered light and anything over 30 considered heavy (Ardbeg, the iconic peated single-malt scotch whisky weighs in at 55 PPM, and some experimental bottlings from other distillers range into the 200s). The maltings will then assess how much peat needs to be burned and for how long. It is an inexact science, as the level of smoke will depend of the combustibility of the peat and even the direction and strength of the wind and how it impacts on the draw.
If a lower amount of peatiness is required, then heavily peated malt will be blended with unpeated to meet the correct PPM requirement.
It is mistakenly thought that PPM refers to the smokiness of a mature whisky. Instead it only refers to the malt. Phenols are lost in every part of the process from storage to distillation. Phenols have a high boiling temperature and therefore come across late in the second distillation. The distiller will decide how much of the phenolic section to retain by deciding when to cut from spirit to feints.
An early cut will miss some of the heavier, tarrier components; a later cut will include them. For example, Lagavulin and Caol Ila, both widely respected scotch single malts from Islay, use the same phenolic specification for their malted barley, but their perceived smokiness is very different because of different fermentation regimes, still shapes, and cut points.
Peatiness will also be reduced in maturation, either through evaporation, absorption by the wood of the barrel, or integration into the spirit.
Kerr, W. A. Peat and Its Products. Glasgow: Begg, Kennedy & Elder, 1905.
By: Dave Broom