Pot still distillation

Wash still (bladnoch.co.uk)

Scotch malt whisky is distilled typically twice in copper pot stills. The first distillation is carried out in a wash still, usually slightly bigger than the spirit still used in the second run. Copper is used because of its good malleability and heat conduction but also for its catalytic properties and the ability to neutralize some of the sulphur compounds and off-notes. The stills can be heated either directly by burning gas, peat or coal or indirectly by steam coils running inside the still. If direct firing is used, there must be a rummager on the bottom of the still to prevent charring, at least in the wash still. The stills usually have a broad pot and a narrowing swan neck that turns to a lyne arm leading to a condenser, which can be a traditional worm, a shell&tube condenser or a plate heat exchanger. Various shapes of stills are used and it is believed that the bulbs and boiling balls etc increase the copper contact at the right spots to produce better spirit. Some distillers use a purifier in the spirit still, which is a pipe reversing some distillate from the lyne arm back to the swan neck producing more reflux.

Worm-tubs	Purifier	Direct firing
Balmenach	Ardbeg	Glenfarclas
Benrinnes	Glen Grant	Glenfiddich
Cragganmore	Glenlossie	Macallan
Dalwhinnie	Glen Spey	Springbank (wash) GlenGarioch (wash) Tobermory(wash)
Edradour	Strathmill
Glen Elgin	Talisker
Glenkinchie	Tormore
Mortlach
Oban
Royal Lochnagar
Talisker
Springbank (wash)

 
Basically the first distillation is a simple distillation of volatile compounds (such as alcohols), producing low wines of 20-25% abv from 6-9% abv wort. Wash is usually preheated to prevent excessive temperature differences and charring inside the still. The first distillation is usually deemed complete when the distillate is under 1% abv and about one third of the wort has boiled over to the wash safe. Wash distillation usually takes 5-8 hours to complete, depending on the size, shape, temperature and charge of the still. The residual is called pot ale.

The wash distillation is mixed with the foreshots and the feints from previous spirit distillations and distilled in the spirit still. The second distillation is a fractionated distillation; the most volatile compounds boil first and are called foreshots or heads, the second part is called the middle cut (spirit cut, heart), the third part is feints (tails) and the remaining liquid in the still is called spent lees.

Panek&Boucher 1989

Panek&Boucher 1989

The foreshots consist of the most volatile compounds, such as methanol, acetaldehyde (ethanal), some of the ethyl esters such as ethyl acetate and volatile sulphur compounds (see table of boiling points below). Along with them come some of the residues of the previous distillation, which are not high volatiles but soluble only in higher strenghts of alcohol, for example fatty acids and their esters. These compounds are therefore condensed into the still at the end of previous distillation and as the next distillation starts, they are made soluble again by the high alcohol strenght and swept up to the spirit safe along with the foreshots. The foreshots are either unpleasant in flavour or dangerous to health (or both) and therefore not included in the final spirit but returned to the next distillation. The long-chain fatty acids and other residues from the previous distillation tend to form a cloudy mixture when diluted to 80 sikes proof (45.7% abv).

	Boiling point C	Odor
acetone	56,5	nail polish remover
glycerol	290	sweet
acetic acid	118	vinegar
aldehydes
acetaldehyde	20,2	pungent fruity, green apple, metallic
furfural	161,7	almonds
alcohols
methanol	65	sweetened ethanol
ethanol	78	ethanol
1-propanol	97	fruity
2-propanol	82,5	fruity
butanol	118	banana, solvent
amyl alcohols	102-138,5	sharp, burning
2-phenyl ethanol	219	floral, rose
esters
ethyl acetate	77,1	pear,sweet
ethyl butyrate	121	pineapple
ethyl formate	54	rum, raspberry
hexyl acetate	171,5	fruit
sulphur compounds
hydrogen suphide	-60,3	rotten eggs
sulphur dioxide	-10	burnt sulphur
dimethyl sulphide	37	cabbage, vegetables
fatty acids
lauric acid	299	bay oil, soap
palmitic acid	351	waxy, creamy, soapy

The traditional way of making the first cut point is by mixing the foreshots with water in the spirit safe; as the spirit turns clear, the foreshots have mostly passed and the middle cut is collected into the spirit receiver. Some producers use timed foreshot runs and do not bother with the demisting test and therefore are are likely to cut the spirit a bit lower as a precaution and probably getting less of the highly volatile compounds into the spirit. The second cut can be determined by taste, abv or time according to the distillery practice. The feints are then run usually down to 1% abv and added to the next wash.

The feints are the last volatile fraction of spirit distillation. Feinty aromas increase slowly towards the end of distillation, developing from quite pleasant mushroom, cereal and popcorn aromas to leathery tobacco notes and further to ashy, fishy and even cheesy aromas not usually approved in whisky. The feints are usually rich in phenols and smoky aromas important for peaty whiskies. Therefore the second cut point must be determined with care to produce peated but not feinty spirit with off-notes. The feints are usually run quite fast to save time, but this makes some of the fat-solubles to adhere to the still. These compounds (mostly fatty acids) must be purged by an adequately long and slow run of foreshots in the next distillation, otherwise the whisky might become feinty no matter how early the second cut is made.

The spirit cut is usually about 72-65%, but there are considerable differences between distillers. To produce a very light aetheral whisky (richer in high volatiles) the distiller would be likely to use a tall still with a low charge run slowly in low temperatures for maximum reflux, a purifier, an ascending lyne arm, a purifier and a shell&tube condenser for maximum copper contact and cut short foreshots and long feints (earlier second cut) for maximum amount of high volatiles and less of the heavier aromas. To produce a peaty whisky the distiller must try to catch all the phenols from the latter part of the middle cut but also avoid excess feints.

The distillation is not only a simple process of separation, but the heat and the copper contact alter some of the flavour compounds, too.  Heat promotes Maillard reactions producing furfural and sulphur compounds, especially thiophenes and polysulphides, which at low concentrations add a pleasant roasted and complex flavour, but produce pungent and unpleasant cereal and sulphury notes at high concentrations. Heat also promotes aldehyde reduction to alcohols and acids and their conversion further to esters. Lignin-derived components such as coumaric and ferulic acids can evolve to more spicy guiaicols. Acrolein (peppery) can form from bacterial fermentation products in presence heat. Fusel oils, 2-phenethanol (rose, flower) and furfurals (caramel, burnt sugar) are formed in pot-stills, but not significantly in column still distillations and therefore are likely to be generated by heat differences during distillation.

Copper removes most of the sulphury, cereal, feinty and meaty aromas during distillation. Especially the copper contact of the first wash still distillation is important. Total removal of copper contact in the spirit still has surprisingly minimal effect on the mentioned off-notes.

References and further reading:

Adams AB. The distillation of alcohol. J Ind Eng Chem 1912; 8-14

Herstein KM. Chemistry and technology of wines and liquors. Van Nostrand Co 1935

Jounela-Eriksson P. The aroma composition of distilled beverages and preceived aroma of whisky. Academic Press 1978

Lea GH, Piggott JR. Fermented beverage production 2nd ed. Kluwer Acad 2003. 

Monica Lee KY et al. Origins of flavour in whiskies and a revised flavour wheel. J Inst Brew 2001;107;5;287-313

Piggott JR, Paterson A (ed). Understanding natural flavors. Blackie academic&professional 1994

Russell I (ed). Whisky, technology, production and marketing. Academic Press 2003

Udo M: The Scottish Whisky Distilleries. Black & White 2006

Walker GM, Hughes PS (ed). Distilled spirits, new horizons: energy, environment and enlightenment. Nottingham Univ Press, 2010

Webb AD (ed). Chemistry of winemaking. Am Chem Soc 1974