There are two ways of assessing a peated malt. A more subjective approach is based on the intensity and range of smoky characteristics that appear in the aroma and on the palate. A more technical approach is to consider the peating level. This is expressed in terms of 'ppm' (parts per million), with 1ppm for example equivalent to 1 milligram in a litre. A peating level of around 10ppm is considered light, and typically results in a malt whisky with gentle wafts of smoke. Around 25ppm is a medium level, while malt whisky with a level of 40-50ppm, or higher, is considered heavily peated and generally delivers much more pronounced smoke.
The peating level quoted refers to the barley once malted and peated. During the peating process barley absorbs smoke created by burning peat. This smoke contains a range of phenolic compounds which provide the archetypal smoky, peaty notes. Achieving the required peating level depends on a relatively straightforward formula: burning peat and creating smoke for longer periods increases the peating level. There are eight key phenolic compounds, including phenol, guaiacol and cresol, together with a far greater number of minor phenolic compounds to take into account.
Each phenolic compound can contain a range of lighter and richer notes, and there's also a certain amount of 'duplication', as a number of phenolic compounds contribute smokiness and medicinal notes for example.
Phenolic compounds are measured in terms of parts per million, as well as billion (ppb) and trillion (ppt), and even a compound measured in ppt can have an impact. Moreover, it's not simply a numbers game, as a compound at a lower concentration can have a greater impact than a compound at a higher concentration. Phenol, for example, is typically present in the highest concentration, though it's not particularly active and its contribution (of medicinal aromas) is considered to be minor.
Cresol is far more active and includes aromas reminiscent of tar and asphalt.
Guaiacol, which is even more complex than cresol, contributes earthiness, smoke and medicinal notes, and makes a significant impact despite being present in relatively low concentrations.
The range and balance of phenolic compounds that peat contains is variable, and depends on a number of factors including the source. Island and coastal peat, for example, includes seaweed and a higher level of sand than inland peat. Meanwhile, the level of heather is typically higher in peat from the north of Scotland than the south. However, the extent to which particular components of peat contribute specific phenolic compounds is still being thoroughly researched.
Consequently, the peating level only provides an overall total, without indicating the level of individual phenolic compounds. Whether, and to what extent, there is any synergy between particular compounds, and the influence this might exert, remains to be confirmed.
Another vital aspect still being researched is the effect of ageing on phenolic compounds.
"I think there's a slight decline in phenol levels, they are volatile so dissipate through evaporation from the cask. Meanwhile, a lot of other flavours are also developing and evolving in the cask, and the lighter the peating level the more these other flavours can mask the phenols and so reduce their impact," says Gordon Motion, master whisky maker, Edrington Group.
Another factor is the type of cask used, typically Bourbon or sherry.
"You want the spirit and the cask to complement each other. Sherry casks add rich dried fruit and raisin notes, for example, which go very well with Ardbeg's rich smokiness, and create a really complex character," says Mickey Heads, Ardbeg's distillery manager.
The cask type should also be considered in conjunction with the 'fill,' ie. how many times the cask has been filled and used to age malt whisky (as each fill sees the cask's influence declining gradually).
"A first-fill Bourbon barrel gives a distinct vanilla character, and vanilla complements the peat very well, balancing the spice of the peat. Second and third fill casks contribute a lower level of flavour compounds, such as vanilla, compared to a first fill, which also means the phenolic notes will be more dominant. When putting together a bottling, it's a case of balancing different fills, as they all contribute an individual character," says Gordon Motion.
The importance of the spirit cut
The first distillation establishes the broader character of the resulting new-make spirit, which is selectively refined by the second distillation. The low wines (ie. result of the first distillation) have a strength of 20-25% ABV, and when re-distilled this rises to about 75-80% ABV before gradually declining. The strength at which distillers start collecting new-make spirit is typically around 75% ABV, and distillers generally stop collecting new-make spirit at around 60-65% ABV. This is termed the spirit cut. A spirit cut that averages 70% ABV, for example, is lighter and fruitier than a spirit cut with a lower average strength which is richer.
"Lighter phenolics come through at the start and gradually build, with richer phenolics coming through as the distillation process continues," says Mickey Heads, distillery manager, Ardbeg.