Production

Phenolic fancies

Ian Wisniewski asks what does the term ‘ppm' stand for in relation to a peated malt, and what does this signify in terms of a malt's character?
By Ian Wisniewski
The choice of peated malts caters for every preference, from a mere whiff of smoke to a bonfire, which means that the peating level of each malt has now become a vital statistic. This level is expressed in terms of ppm (ie. parts per million), the standard measure used in chemistry, and denotes the level of phenolic compounds which provide the archetypal smoky, peaty notes.

The peating level quoted refers to the barley (once malted and peated), and not to the new make spirit or mature malt whisky. However, this level changes significantly during the production process, which means that knowing a malt’s peating level is only a starting point.

But first let’s recap on malting and peating. Malting begins with barley being steeped (soaked) in water and germinating. Any further growth is stopped by drying the malt, using heat produced by a kiln, and adding peat to the kiln creates smoke which is absorbed by the barley, essentially the husk. Adding a small amount of peat creates a lower peating level, while continuing to add peat increases the peating level correspondingly (the process taking up to about 24 hours).

A peating level around 10 ppm is considered light, and typically results in a malt whisky with a gentle waft of smoke. Around 25 ppm is a medium level, while 40-50 ppm or higher is considered heavily peated, and leads to a malt whisky with pronounced smoke.

Peating endows barley with phenolic compounds (ie. a phenolic ‘family’). This includes individual compounds such as phenol, cresol and guaiacol, which in turn comprise lighter and heavier elements.

“Phenol itself is the largest component, accounting for about 50 per cent of the total,” says Dr Bill Crilly, technical support manager, The Edrington Group. Phenol provides medicinal aromas, amongst others, while cresol, accounting for about 40 per cent of the total phenolics, provides aromas such as tar and asphalt. Guaiacol, at about
10 per cent, adds smoke.

“Phenol is the simplest of all the phenolic compounds, with cresol more complex, and guaiacol even more complex than cresol,” says Dr Bill Lumsden, Glenmorangie’s head of distilling and whisky creation.

The exact balance of phenolic compounds within the peating level depends on a number of factors, including the source of the peat. Island and coastal peat, for example, includes seaweed and a higher level of sand than inland peat. Meanwhile, the level of heather tends to be higher in peat from the north of Scotland than the south.

However, the extent to which certain components of peat contribute specific phenolic compounds is still not fully understood, and the subject of on-going research.

During the production process, mashing and distilling result in the peating level being reduced by up to 60-80 per cent.

Mashing sees the malt combined with hot water in a mash tun, to convert the starches within the malt into soluble sugars. As phenolic compounds are also soluble they leach from the malted barley.

This results in a sugary, phenolic liquid draining from the mash tun and proceeding to the washbacks (vessels in which fermentation occurs). Meanwhile, the draff (ie. residue barley) remaining in the mash tun retains a significant phenolic level.

Fermentation doesn’t affect the phenolic character, but with so many flavour compounds formed during this stage, including esters (fruity notes), there is a dramatic change from the cereal and phenolic character of the wort before fermentation, and a far broader range of characteristics after fermentation.

The role of the first distillation is to shape the character of the spirit, which is then refined by the second distillation. The low wines (ie. result of the first distillation) have a strength of 20-25% abv, and during the second distillation the strength rises to about 75-80%abv, before gradually declining.

The initial distillate, termed foreshots, is of an inappropriate quality and character. The strength at which to start collecting new make spirit (ie. for aging into malt whisky) varies among distilleries, but is typically around 75% abv, and the strength at which distillers stop collecting new make spirit is generally around 65% abv. This is known as the spirit cut, and this averages 70% abv, for example, is lighter and fruitier than a spirit cut with a lower average strength which is richer.

The remainder of the distillation run, known as feints, is collected separately from the new make spirit and combined with the foreshots, in order to be redistilled with the next batch of low wines.

“Lighter phenolics come through right from the start, with heavier, stronger phenols coming through as you continue, so there is a spectrum of phenolic compounds during distillation,” says Mickey Heads, distillery manager of Ardbeg.

Consequently, the spirit cut determines a distillery’s ‘house style,’ whether lighter or richer, which includes determining the phenolic character of the new make spirit.

‘The spirit cut is the defining factor, you could peat to 80 ppm and still get a new make spirit with light phenolics,’ says Stuart Robertson, Springbank’s distillery manager.

Russell Anderson, distillery manager at Highland Park adds:

“One way to collect more phenols is to extend the spirit cut. But then you must be very careful not to collect heavy, undesirable notes as part of the spirit collection.”

How the peating level is affected by aging is the subject of an on-going debate.

One theory is that peating levels reduce during aging, and consequently become mellower characteristics within a malt whisky’s flavour profile.

Another theory is that peating levels remain fairly constant during aging, but appear mellower due to the rising level of other characteristics, such as vanilla and fruit notes, which effectively ‘mask’ the phenolic character.

It’s a great topic to discuss while raising a glass.