Peat is partially decayed vegetation and organic matter that can accumulate over thousands of years to create large peat bogs hundreds of feet thick. In North America, peat bogs are known as muskegs, and when chunks of peat are dried, it can be burned for fuel. In areas with fewer trees to burn, peat has long been used has a source of heat. Peat has long been used to malt barley for making whisky, and in doing so, the peat smoke gets absorbed by the grains. The grains then pass the smoke flavor, known as peat reek, on to the whisky. It turns out that every peat bog has a distinct chemical signature with a specific aroma and flavor profile determined by the organic matter composition and climactic conditions of the place in which it formed. For instance, in areas that get a lot of rain, peat tends to accumulate a large quantity of sphagnum moss, which contains phenolic compounds that impart a medicinal character to peated Scotch whiskies. The more moss in the peat, the more medicinal taste in the whisky. In Eastern Scotland, where the climate is drier, the peat has less moss in it, so whiskies from that region lack the overtly medicinal quality of Scotch from the western part of the country. The western islands of Scotland, such as Islay, and the Northern Highlands, also contain peat with maritime influences, so the whiskies made there exude aromas of iodine or smoked, oily fish. And in the Orkney Islands, the peat composition includes a high quantity of heather, which imparts a delicate, floral, potpourri like aroma to the local whisky.
Apart from the composition of the peat itself, the temperature at which it is burned also influences the flavor profile of whisky. Excessive combustion temperatures can degrade some of the volatile peat phenolics, impacting the flavor of the whisky. For instance, one of the phenolic compounds called guaiacol, imparts the aromas of kippers, smoked bacon and aged cheese to whisky. Cresol, another phenol, imparts medicinal flavors reminiscent of disinfectant and antiseptic. When peat is burned at higher temperatures, by stoking the fire or using drier peat, the amount of cresol increases and the amount of guaiacol decreases. In addition to choosing what kind of peat to burn, whisky makers can also alter the aromas in their whiskies by changing the combustion temperature to, say, increase the smoked fish aromas and decrease the medicinal aromas. The fermentation time and temperature as well as the amount of aging also influence the total quantity of phenolic compounds, and the perceptible flavors, that remain in the bottled whisky. Read more here at Forbes.