The tang of peat smoke is a defining flavour of many whiskies, but the way in which sodden vegetable matter from the bottom of a bog becomes a key element of a sophisticated whisky is a complex blend of science and technology.
Douglas Murray, from Diageo’s Technical Centre for Europe in Menstrie, explains that peat is essentially a coal-like substance formed when mosses, grasses or trees decompose under waterlogged conditions.
In Scotland, there are three main areas where peat is gathered – Aberdeenshire, lslay and the far north of the country. Each imparts a unique influence on whisky.
“Peat from different areas has a different molecular structure, depending on what the original vegetation was,” says Douglas. “You get different peat from sphagnum moss or heather or grass and the different peat brings subtly different flavours to the whisky.
“However, whisky flavours are complicated and the differences introduced by using one kind of peat over another are just one of many variables.”
FROM SOURCE TO SMOKE
The peat is dug from so-called peat-banks, with the deeper, older layers being used for the greater consistency they provide. It is then dried out.
A burning issue
The actual peating process happens as the barley is being dried in a kiln after germination. “At specific times during that drying process, the malt is most susceptible to absorbing other things,” says Douglas. “At those times, we use a separate side burner that keeps the peat smouldering rather than burning, to create an intense smoke, which is fed into the air ducts feeding the kiln. The smoke coats the malt and is absorbed into it.”
No smoke without fire
The smokiness that is imparted is determined mainly by how intense the smoke within the kiln is rather than how long it is kept there for. “In the malting, we assess the ‘smokiness’ by measuring a range of organic compounds called phenols that come from the peat smoke,” says Douglas. “These are not the only smoky compounds, but they are indicative of what the overall smokiness will be. The initial test is to dissolve a sample in a solvent, which the phenols will colour. That gives us an indication of overall smokiness.”
FROM LAB TO GLASS
In the laboratory, the different compounds are separated using techniques such as high pressure liquid chromatography or gas chromatography analysis. “Here, we are measuring not just the total amount of phenolic compounds, but the level of different compounds which are naturally present in peat,” says Douglas. “These compounds include ortho and para-cresol, eugenol and guaiacol that are associated with medicinal, smoky and bacon-type aromas.”
The next step
Between a third and a half of the original phenol content makes its way through distillation to the final spirit, and the different phenols come through the distillation process at different times in the spirit run. “That means that the particular spirit cut point at an individual distillery will influence the recovery of these smoky compounds and the perceived smokiness in the spirit,” says Douglas.
The fab four
In the finished malt and malt whisky, phenols are measured in parts per million. Diageo has four categories of ‘smokiness: zero (no peat); ‘light peat’ which is about one or two parts per million of smokiness; an intermediate level which is between 10 and 30 ppm, and heavily peated, for Islay whiskies which have 30ppm or more.
From The Scotch Malt Whisky Society’s 12 unique flavour profiles, the three shades of green identify lightly peated, peated and heavily peated styles of whisky. Visit www.smws.com/flavours to find out more and refine your search for some phenol-heavy drams