The make up of peat is different in different parts of Scotland and thereby influences the flavour of the whisky.
There are very few trees on the Orkney islands so there are no tree roots in the peat making it lighter and quicker to burn. Whiskies from this area, like Highland Park, tend to have a lighter smoky flavour than Islay malts.
Peat is found all over the British Isles and it is the use of peat in the Scottish whisky industry that helps to produce a drink that has a unique flavour within the world of spirits. Peat is earth that consists of grasses, moss, tree roots, dead animals and soil that has become tightly compacted over thousands of years.
Use in the whisky industry
Historically peat has been used in the whisky industry, especially as in many places it was the only consistent source of fuel. The process detailed below is time consuming and the use of peat dwindled as other fuels, such as coal and electricity, became more widely available. Now it is largely confined to the islands of Scotland - naturally they now have other fuel sources but many of the whiskies remain well known for their smoky, peaty flavours. Therefore, the distilleries have chosen to keep the practice so as to keep the distinctive flavours in their whiskies.
'Ginger' Willie McNeil from Bowmore distillery shows us how peat is traditionally cut
The peat is cut by hand using specialised tools and the resulting 'sod' is then left to dry in the open air for approximately two-three weeks. After this time, the peat is collected and then taken to the distillery. Most of the time the peat used is local to the distillery or cut from property owned by the distillery. The peat is burnt underneath the malted barley to stop its germination (see How is whisky made for further explanation). Peat is so tightly compacted and dense that it burns for a long time and with consistent heat and acrid smoke. This is also why it is still used as a domestic fuel in some areas of Scotland, especially the islands.
Peat is one reason why different distilleries have different characteristics in their whiskies. Peat smoke produces contains chemicals called phenols and these phenols are absorbed by the malted barley during the drying process in a kiln. The level of phenols are controlled by the length of time that the barley is exposed to the smoke, the amount of smoke produced and the type of peat used. The smoke that has been absorbed is then carried through the entire whisky making process and right in to your glass.
Once finished, the malt is taken away for mashing and the phenol level is measured. This level is known as the PPM – Phenol Parts per Million. A distillery will always have the same PPM for their malt and this value is also measured in the final spirit. Some is lost during distillation so the PPM is always lower at the end, roughly one third of the level of the original PPM in the malt. The PPM figure most commonly used is that of the malt. Most whisky has some smokiness but in most the PPM value is so low (eg. 1–5 PPM) that it is virtually undetectable. In smokier whiskies, it is easier to detect these levels as the PPM levels increase.
Here are some examples of PPM values of some well known distilleries
(the approximate PPM of their malt is in brackets in increasing value)