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

A medical view on maturation of whisky

A fresh medical view on the maturation issue and a shameless rip-off from The Lancet.  Just in case someone does not know The Lancet, it is a highly respected English medical journal, established in 1823 and still one of the big three among the medical journals. Below are two articles first published on 11.1.1902 and 19.3.1904 discussing the definition and consumption of whisky.

Some interesting points are made concerning for example malt and grain whiskies
"There is a marked difference between the physiological action of spirits derived from different sources...The potentialities of these substances for good or evil depend upon the materials from which the spirit is produced."
and the quality of blended whiskies
"...age is essential in order to reduce spirits to state fit for consumption...Even genuine malt whisky must be matured, whereas grain spirit...featureless as regards flavour...age can have no advantegeous change [on grain spirit]."
and wine finishes
"Grain whisky... either has to be flavoured by adding a small proportion of full-flavoured malt whisky, or...storing it in sherry casks in which malt whisky has been ripened"
and the disappearing age-statements
"It seems to us that it avails nothing to the consumer to be sure of getting all-malt spirit when he asks for it if he cannot at the same time receive some guarantee also as to its age."
and the rise of NAS-whiskies
"The whole object of the Bill would be destroyed if...it were possible to supply true malt whisky so immature as to be injurious to health."
and the decline of properly aged quality single malts
"The detractors of all-malt whisky might be expected for their own purposes to supply...pure malt whisky...which though genuine could lay no claim to...age."
All this a bit over hundred years ago. Tradition?


References:
Lancet 1902;159;4089;109
Lancet 1904;163;4203;816-817

Rejuvenation

Scotch whiskies are predominantly matured in ex-bourbon casks. The previous filling with bourbon extracts much of the flavours, but there is still a lot left for the refills, too. The water-soluble extracts seem to exhaust faster than the lignin-derivatives or the lipid-solubles, although the filling strenght does influence extraction rates. For example oaklactones and soluble oaktannins, especially ellagitannins are largely depleted during the bourbon fill and the first Scotch fill. By extending the maturation period it is possible to create whiskies with considarable oak flavours, although the proportions of flavour compounds tend to differ from the first fill maturations (see table 1 below).

 

Table1. Extracts from ex-bourbon casks used for Scotch malt whisky. From left: 1st, 2nd and 3rd/more refill (modified from Piggott&Conner 2003)

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Lignin does break down in presence of alcohols, but most of the linkages are resistant to ethanol. Therefore lignin does not degrade completely even in the presence of high filling strenghts during long maturation, but the release of monoligninderivatives tends to slow down considerably leading to an exhausted cask. The relative absence of guaiacol ja syringyl compounds is thought to be a good marker for cask exhaustion. The guaiacol concentrations are probably quite similar to those of ferulic acid concentrations shown in table 1 and the amount of syringic acid is likely to represent the amount of total syringyl compounds (for information on lignin construction see previous blog 15.2.2011).

Exhausted casks can be rejuvenated by scraping the inside of the cask, removing the used wood and bringing new wood in contact with the spirit. Untoasted wood however gives green and raw aromas and scraped casks are usually toasted or charred again. Recharring might impart too much colour and change the aromas towards bourbon-style whiskies and therefore retoasting in apparently preferred in Scottish cooperages. Regenerated recharred casks differ considerably from ex-bourbon casks used for the first fillings with Scotch. In table 2 are presented the sensory profiles of 3 year old grain whiskies matured in refill bourbon casks compared to decharred recharred exhausted ex-bourbon casks. The recharred casks seem to produce more sweet and woody notes as refill casks tend to bring out the drier woody notes. In terms of flavour compounds the recharring promotes fast lignin breakdown and caramellisation of hemicellulose thus extracting considerably more guaiacols, isoeugenol and vanillin commonly associated with new wood barrels. Surpisingly the concentrations of oaklactones are only slightly greater in recharred casks compared to refills. Perhaps oaklactones are already extracted from the deeper layers of the wood exceeding the scraping depth in rejuvenation and the recharring affects primarily wood layers that are already poor in oaklactones. Another possible explanation is the lack of seasoning of rejuvenated casks after the scraping which might lead more green wood notes and less oaklactones. A recharred cask with more guaiacol and vanilla extractives is more likely to promote smoky and sweet flavours of the spirit than a refill cask, although the sweet notes differ from those of the first fills as they are probably caused more by caramellisation products and vanilla than oaklactones. The charred layer is likely to remove some of the off-flavours, especially some sulphury aromas, but as seen in table 2 toasting also considerably reduces suphury aromas, so the filtration by active carbon layer might not be the only explanation for this.

A common practice during the 20th century was to season the rejuvenated casks as well as some of the new casks with pajarete to create a sherry-cask profile. The first intentional wine-treatments were made in 1880s and the last larger scale treatments in the whisky industry were terminated prbably during the 1980s.

A recent develepment has been a treatment of exhausted casks with salt catalyst before recharring or retoasting; this "curing" increases the extraction from rejuvenated casks (tables 2&3 below). The salt solution probably increases the heat influence in deeper layers of wood especially during medium heating (toasting). It is also likely that salt increases the solubility of extractives into spirit, although the precise mechanism is not yet described.

Tables 2 and 3. Sensory profiles of 3 year old Scotch grain whiskies matured in different ex-bourbon casks (brutal modification from Reid KJG et al 2008, sorry...)

Rejuvenated casks are not particulary appreciated by malt whisky aficionados, but they probably are of good use in maturing young grain whiskies as they remove effectively some of the common off-notes associated with new make spirits. Also lighter style malt whiskies might benefit from a subtler cask influence lacking especially the overly sweet oaky notes.

REFERENCES

Boudet AM et al. Biochemistry and molecular biology of lignification. New Phytol 1995; 129; 203-236 

Conner JM et al. Changes in wood extractives from oak cask staves through maturation of scotch malt whisky. J Sci Food Agric 1993;62;169-174

Clyne J et al. The effect of cask charring on scotch whisky maturation. Int J Food Sci Tech 1993;28;69-81

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

Mosedale, JR. Effects of oak wood on the maturation of alcoholic beverages with particular reference to whisky. Forestry 1995; 68; 3; 203-230

Piggott JR, Conner M. Whiskies. In Fermented beverage production. Kluwer Acad 2003.

Piggott JR et al. Effects on scotch whisky composition and flavour of maturation in oak casks with varying histories. Int J Food Sci Tech 1993;28;303-318

Reid KJG et al. Understanding and enhancing cask performance. The Scotch Whisky Research Institute 2008.

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

Viriot C et al. Ellagitannins and lignins in aging spirits in oak barrels. J Agric Food Chem 1993; 41; 1872-1879

Cask variations

A range of different casks are used in Scotch whisky maturation. Before 1990 the casks were allowed to be of any wood, although only an odd chestnut cask has probably been used as most casks were made of oak. Now all casks have to be made of oak wood. Most casks are refill casks from bourbon industry made of Q.alba and to a lesser extent casks previously used to mature sherry (mostly Q.alba, sometimes Q.robur), port (Q.alba, Q.robur, Q.pyrenaica), wine (mostly Q.petraea), rum (mostly Q.alba) or even cognac (Q.robur). The first filling extracts much of the flavour compounds from the oak as most of free solubles are extracted during the first year of maturation. Spirits with higher filling strenghts reach deeper into the wood and extract more alcohol-soluble congeners, such as lactones, lipids, fatty acids, lignin-derivatives, phenolic acids and aromatic aldehydes. Lower strenght beverages and even fortificated wines extract more water-soluble components, such as tannins, glycerol and sugars. Esters are extracted practically evenly despite different ethanol concentrations. The breakdown of lignin continues as maturation is prolonged and more extractives are available at a considerably slower rate, higher alcohol-strenghts increasing the ligninolysis.

The high lactone-content explains the heavy sweet coconut aroma of bourbons, but the aroma is toned down in the refill casks as the lactones are largely depleted in the first fill with spirit. Wine extracts lactones too; a six month wine maturation decreases the extraction of oak lactones to about 30-40% in the second fill. The average oaklactone concentration in a new Q.petraea cask is about the same as in a wine-refill Q.alba and a bourbon-refill Q.alba is very likely to be much poorer in oaklactones. Trans-oaklactones extract themselves more easily during the first fill, so the cis-trans ratio is likely to be greater in refill casks, producing more spicy than sweet oak flavours. Q.robur is usually very poor in oaklactones even in a new wood cask, but Q.crispula (Japanese oak) is very rich in oaklactones, especially the cis-isomer, giving rise to spicy incence oak aromas, which are often further fortified with a sherry-treatment before filling in the whisky. Toasting increases the amount of lactones and although heavy charring diminishes the lactone-concentrations, the increased permeability through charred wood usually increases the total extraction into the spirit.

Spicy tannins and vanillins are typical for Q.robur and Q.crispula, but their concentrations in Q.petraea seem to vary considerably, although some of the "odd" single results might be explained by hybridization with Q.robur or other oak species. Seasoning of the staves is preferred in Europe instead of kilning, probably due to more astringent and tanninic taste profiles of the European oaks compared to Q.alba. Tannins soften during seasoning by the effects of rain, bacteria, yeasts and oxidation. Vanillin, eugenol, furfural and cis-oaklactone are reported to decrease in colder climate and increase in hot climate seasoning, but this may be also partly explained by different microbes present in Australia compared to those in France and the USA.

Smoky and spicy guaiacols are created from lignin in toasting/charring and through ethanol-induced lignification during maturation. Higher temperatures increase the guaiacol-concentrations which are likely to enchance the smoky flavours in whisky. Especially 4-vinylguaiacol is very soluble in spirits and is usually depleted fast in the first fill. Guaiacols can be generated through ligninolysis and extracted into refills too at a slightly slower rate. Furfurals (caramel, burnt sugar) are also created by heating and quite rapidly extracted in the first fill.

The colours extracted from the oaks are different, as Q.alba and Q.crispula are usually red, Q.petraea is usually lighter, almost pink in colour and Q.robur extracts a strong yellow colour. Again, charring decreases the colour extractives in the surface layer but because of the increased permeability it usually adds to the typical red/orange colour typical of bourbon.

The East European Q.robur is usually a bit closer to Q.alba in terms of the flavour compounds as it contains usually more oaklactones, eugenols and vanillin but less tannins than Q.robur from France or Spain.

The cask absorbs some of the spirit or wine used in the first fill. A large bodega butt can gain up to 25kg in weight during several decades of solera maturation and it is very likely that such cask would impart quite a lot of sherry aromas to a refill spirit regardless of the oak species used. A barrel sized cask (usually 180l) soaks about 9 litres of 50% abv whisky during a bourbon maturation. A typical wine cask is used for 5 years and several refills and is likely to absorb several litres of wine. Some of the liquid used in the first fill are bound to enter the refill whisky, but the magnitude is not known. Recharring the cask before a refill burns most of the alcohols and probably flavours previously absorbed by the wood, but re- or decharring is apparently not an usual practice for first (scotch) fill bourbon casks. Rinsing of a cask is not likely to remove significant amounts of the previous liquid soaked into the wood. If a cask has been used in fermentation of wine, it is likely to lose some weight, probably because of the wood degradation by the yeasts. It is believed that the fermentation yeasts remove some of the bitter tannins and other undesired aromas from a wine cask, although not much wine is fermented in casks anymore.

The size of the cask affects the result too, larger casks having less wood surface and headspace per spirit volume. Spirit in smaller casks mature faster but suffer from greater oxidation per volume. Smaller casks impart probably more sugars, glycerol, eugenol and galloyl esters but less lignin-derivatives, vanillin and ellagitannins and this may result in relatively sweet and spicy aroma profile different from the profiles of the larger casks.

In conclusion, the new wood casks impart the most sweet oak, coconut, caramel and smoky aromas as the refills tend to be drier, subtler but still quite tanninic. The cask maturation is very rapid in first fill casks during the first months as the maturation in refills is usually more linear and predictable, although significantly slower. Different oaks have very different tasteprofiles and the strenght and the aromas of the first filling does affect the refill maturation of whisky. Seasoning softens the astringent aromas as kilning increases the sweet aromas but does not affect so much the tannins. Below is a table showing the main differences between the species considering whisky maturation.

	Q.alba	Q.robur	Q.petraea	Q.crispula
ring width	+++	+++	+	?
oaklactones	+++	+	+(++)	+++
-cis	(+)	(+)	++	+++
-trans	+++	+	+	++
eugenol	+(+)	++	+	?
vanillin	+(+)	++	+(++)	+++
furfural	+	+++	++	+++
tannins	(+)	+++	+	++(+)
roburins	(+)	+++	+	?
phenolics	+	+++	++	?
colour	red	yellow	pink	red
tyloses	+++	+	++	(+)

	seasoning	kilning	toasting	charring
oaklactones	+	(+)	++	+++
eugenol	-	+	+++	++(+)
vanillin	++	+	+++	++(+)
furfural	++	+	-(+)	+
tannins	---	-	---	-
roburins	---	-	--	-
guaiacols	(-)	(+)	++	++
colour	-	(+)	+	++

References:

Cutzach I et al. Identification of volatile compounds with a toasty aroma in heated oak used in barrelmaking. J agric food chem 1997;45;2217-2224

Doussot F, De Jeso B, Quideau S, Pardon P. Extractives content in cooperage oak wood during natural seasoning and toasting; influence of tree species, geographic location and single-tree effects. J Agric Food Chem 2002; 50; 5955-5961
Gallagher et al. Whisky losses during aging. Ind Eng Chem 1942; 8; 992-995

Garde-Cerdàn T et al. Effects of composition, storage time, geographic origin and oak type on the accumulation of some volatile oak compounds and ethylphenols in wines. Food Chem 2010; 122; 1076-1082

González Gordon M, Sherry. Cassell Ltd 1972
Gomez Plaza E et al. The effect of successive uses of oak barrels on the extraction of oak related volatile compounds from wine. Int J Food Sci Tech 2004;39;1069-1078

Gougeon RD et al. Expressing Forest Origins in the Chemical Composition of Cooperage Oak Woods and Corresponding Wines by Using FTICR-MS. Chem. Eur. J. 2009, 15, 600 – 611

Gougeon RD et al. The chemodiversity of wines can reveal a metabologeography expression of cooperage oak wood. PNAS 2009; 106; 23; 9174-9179 Mangas J et al. Volatiles in distillates of cider aged in american oak wood. J Agric Chem 1996; 44; 268-273

Martinez J et al. Effect of the seasoning method on the chemical composition of oak heartwood to cooperage. J agric food chem 2008;56;3089-3096

Monica Lee KY, Paterson A, Piggott JR. Origins of flavour in whiskies and a revised flavour wheel. Review. J instit brew 2001; 107; 5; 287-313

Mosedale JR, Puech JL. Wood maturation of distilled beverages. Trends in Food Sci Technol 1998; 9; 95-101

Mosedale, JR. Effects of oak wood on the maturation of alcoholic beverages with particular reference to whisky. Forestry 1995; 68; 3; 203-230
Mosedale, JR. Variation of the flavour and extractives of european oak wood from two french forests. J Sci Food Agric 1996; 70; 273-287

Mosedale JR, Charrier B, Crouch N, Janin G, Savill PS. Variation in the composition and content of ellagitannins in the heartwood of european oaks. Ann Sci For 1996; 53; 1005-1018

Perez Coello MS et al. Analysis of volatile components of oak wood. J Chromatograph 1997;778;427-434

Prida A, Puech JL. Influence of geographical origin and botanical species on the content of extractives in american, french and east european oak woods. J Agric Food Chem 2006;54;8115-8126

Prida A, Ducousso A, Petit RJ, Nepveu G, Puech JL. Variation in wood volatile compounds in a mixed oak stand: strong species and spatial differentiation in whisky-lactone content. Ann For Sci 2007; 64; 313-320

Prida A et al. Relation between chemical composition of oak wood used in cooperage and sensory perception of model extracts. J Sci Food Agric 2009;89;765-773

Spillman PJ, Sefton MA, Gawel R. The effect of oak wood source, location of seasoning and coopering on the composition of volatile compounds in oak-matured wines. Aust J Grape Wine Res 2004; 10; 216-226

Viriot C et al. Ellagitannins and lignins in aging of spirits in oak barrels. J Agric Food Chem 1993;41;1872-2879
Withers SJ et al. Comparison of Scotch malt whisky maturation in oak miniature casks and american standard barrels. J Inst Brew 1995;101;359-364

Oaky flavours

Hundreds of different flavour compounds have been identified in whisky. The synthesis and degradation and synergistic properties of these compounds is still poorly understood as there are so many aspects contributing to the result of cask maturation.

Oak cask affects whisky by extracting wood compounds that influence the taste directly or together with the spirit compounds or by removing or changing some compounds from the raw spirit. Cask also allows evaporation and oxidization of spirit and the volatile flavour compunds through the headspace of the cask or by a lesser extent through the surrounding air through the wood or the bunghole.

Oak wood consists of cellulose (38-52%), hemicellulose (25-30%), lignin (22-25%) and extractives of the wood (5-10%). Oak cellulose is a linear chain of up to thousands of D-glucose-molecules and quite inactive in terms of flavour extraction during spirit maturation, extracting only some carbohydrates in high temperatures during toasting/charring.

Toffee

Hemicellulose contains several defferent sugars (xylose, mannose, galactose, rhamnose, arabinose, glucose) and forms branched chains of hundreds of molecules. Hemicellulose breaks easily when heated, producing a range of extractable aroma compounds, such as furfural (almond, walnut, grainy), hydroxymethylfurfural (butter, musty, waxy, caramel), maltol (malt, sweet) and cyclotene (maple, caramel, licorice).

Lignins are very complex macromolecules consisting of three monolignol units p-hydroxyphenol (not present in oak), guaiacyl (32% in oak lignin) and syringyl (68% in oak lignin) derived from dehydration and polymerization of cinnamyl alcohols. Usually natural lignin includes various other molecules joined in to the structure, including different sugars, acids and aldehydes. Heating breaks parts of lignin to soluble p-coumaryl-, coniferyl- and sinapyl-alcohols. They can transform into their respective aldehydes, acids and phenols including very aromatic compounds such as guaiacol (smoky), 4-vinylguaiacol (clove), phenyl ethanol (floral, rose), vanillin and vanillic acid. At higher temperatures a range of other volatile phenols are formed. Lignin breakdown continues at a slower rate during maturation by the effect of ethanol. Most of the lignin derivatives and extractibles decribed above are present also in the malted grains, peat and new make spirit.

Lignin synthesis, monolignols. From Nature Reviews

Oak wood extractives include two different natural isomers of oak lactones. The cis-oak lactone gives sweet coconut-vanillin aroma. The trans-oak lactone is spicier (coconut,cloves, celery, incence), but 2,5-20 times weaker if all the synergetic influences of other oak extractives are not taken into account. The trans-lactone is believed to promote the taste of cis-lactone and various other flavour compounds in whisky, producing heavy coconut amd incence aromas at high concentrations. Various other lactones are described as fruity, peach-like and vanillic.

Tannins can be divided into hydrolysable (gallo- and ellagitannins) and the non-hydrolysable condensed tannins, for example proanthocyanidins common in red wines. Oak tannins are more hydrolysable than the more stable wine tannins from grape skins and pips and thus more volatile and active during maturation. Ellagitannins consist of vescalagin, castalagin, their oligomers or their variations such as roburins or grandinin. Tannins impart astringent flavour at least in the early phase of maturation and take part in various oxidative reactions removing sulphury off-notes and promoting color stability, lignin breakdown and alcohol oxidation into acetals producing etheral top-notes.

Dried cloves (Eugenia aromatica)

Other important aromatic oak extractives include different eugenols (clove, cinnamon), β-damascenone (fruity, peach, cooked apple), cyclotene (toasty, caramel), hexanal (grass), trans-2-nonenal (saw dust, greasy), 2-octenal (green leaf, untoasted oak). 

Several extractives from oak wood show significant synergetic effects between each others and lower the odour treshold levels of aromas, for example vanillin and vanillic acid lowers the treshold for lignin-derived aldehydes.

Active carbon layer formed in charring has some filtering potential, removing especially sulphury aromas from the spirit. Some oak derived hydrophobic compounds also suppress the release of volatiles from spirit in room temperatures and mask some aromas especially when nosing whisky.

In conclusion, oak and especially the toasted/charred layer of the cask adds flavour compounds to the spirit and removes some undesired compounds by carbon filtration and oxidation reactions.

Vanilla planifoli

Different oak casks impart different amounts of flavour compounds. The species is the most important factor explaining the differences, but also the origin, seasoning and toasting of staves are significant. Three most common oaks used in whisky cooperage are Quercus alba, Q.robur and Q.petraea. Q.alba grows in northeast America and the latter two in Europe (see previous blogs).

Main differences between species are most likely in the concentrations of oak lactones, eugenol, tannins and other polyphenols. The variation inside the species is most distinct in Q.petraea, as Q.alba and especially Q.robur tend to be more predictable in terms of whisky maturation.

Coconut (Cocos nucifera)

Oak lactones are important flavour extractives in oak wood. The cis-isomer is usually dominant and imparts sweet vanillin and coconut aroma. The trans-oaklactone is more spicy and herbal in low concentrations, but in high concentrations produces heavy coconut and incence, part of this phenomenon is probably due to synergistic nature of trans-isomer with other lactones and polyphenols. The ratio of cis/trans-isomers differs between species: It is highest in Q.alba and almost non-existent in Q.robur. The ratio in Q.petraea varies, but it is usually less than in Q.alba. The only oak with greater proportion of trans-isomer is Japanese oak (reported as Q.mongolica, but it is more likely Q.crispula). Japanese Q.dentata is similar to American Q.alba and Japanese Q.serrata is similar to European Q.petraea in both oaklactone-concentrations and cis/trans-ratios. Total amount of oak lactones is highest in Q.alba and Q.crispula, usually less in Q.petraea (although some very high concentrations have been measured) and very low in Q.robur.

Grapes (Vitis vinifera)

Another significant difference between oak species is the amount of tannins and other polyphenols. Q.robur has most tannins, especially the more water-soluble vescalagin, castalagin and roburins. Q.petraea has about third of the tannins content of Q.robur, but the tannin contents in Q.alba and presumably in Q.crispula are very low. These differences are partly due to the growth speed of the species as older trees have less tannins than younger oaks, but the growth speed alone does not explain the differences between the concentrations.

The amount of lignin is quite similar in all the oaks, although it seems that Q.robur might be richer in lignin than Q.alba. Again there probably is more variation between different Q.petraea trees. It seems that Q.alba has little less soluble monolignols than the European species. Lignin- and hemicellulose- derived vanillin and furfural contents are usually highest in Q.robur. The differences in coopering and especially in toasting/charring practices influence the monolignol and vanillin contents apparently more than the variation between the species.

It was earlier believed in wine industry, that tight-grained (slow growth) oaks produced less tannins and more sweet notes, but the grain width is not significant if the species are taken into account. The observation is true in the sense that Q.petraea is usually tight-grained oak (about 1mm) and has less tannins than coarser Q.robur. Q.robur and Q.alba have usually coarser grains of about 3mm.

The origin of oak has some effects, too. East European oaks (not depended on species) have usually more lactones, eugenols and vanillin but less tannins than French or Spanish oaks.

It should be noted that although the sherry casks used in Scotch whisky industry are called Spanish oak casks, they most likely often are Spanish coopered American oak casks. Although made of the same Q.alba oak, these casks produce very different Scotch whiskies from the ex-bourbon casks due to the differences in cooperage practices, seasoning effects the sherry wine and the different sizes of casks.

References and further reading:

Boudet AM et al. Biochemistry and molecular biology of lignification. New Phytol 1995; 129; 203-236

Bryce JH et al (ed). Distilled spirits, production, technology and innovation. Nottingham Univ Press, 2008

Clyne J, Conner JM, Paterson A, Piggott JR. The effect of cask charring on Scotch whisky maturation. Int J Food Sci Technol 1993; 28; 69-81

Conner JM et al. Release of distillate flavour compounds in scotch malt whisky. J Sci Food Agric 1999; 79; 1015-1020

Kilby, K. The cooper and his trade. John Baker Publishers Ltd 1971

Garde-Cerdàn T et al. Effects of composition, storage time, geographic origin and oak type on the accumulation of some volatile oak compounds and ethylphenols in wines. Food Chem 2010; 122; 1076-1082

Günter Berger R. Flavours and Fragrances: Chemistry, Bioprocessing and Sustainability. Springer 2010
Jounela-Eriksson P. The aroma composition of distilled beverages and preceived aroma of whisky. Academic Press 1978

Mangas J et al. Volatiles in distillates of cider aged in american oak wood. J Agric Chem 1996; 44; 268-273

Monica Lee KY, Paterson A, Piggott JR. Origins of flavour in whiskies and a revised flavour wheel. Review. J instit brew 2001; 107; 5; 287-313

Mosedale JR, Puech JL. Wood maturation of distilled beverages. Trends in Food Sci Technol 1998; 9; 95-101

Mosedale, JR. Effects of oak wood on the maturation of alcoholic beverages with particular reference to whisky. Forestry 1995; 68; 3; 203-230
Mosedale, JR. Variation of the flavour and extractives of european oak wood from two french forests. J Sci Food Agric 1996; 70; 273-287

Mosedale JR, Charrier B, Crouch N, Janin G, Savill PS. Variation in the composition and content of ellagitannins in the heartwood of european oaks. Ann Sci For 1996; 53; 1005-1018

Nykänen P, Suomalainen H (ed). Aroma of beer, wine and distilled alcoholic beverages. Akademie-Verlag, Berlin 1983.

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

Poisson L, Schieberle P. Characterization of key aroma compounds in an american bourbon whisky. J Agric Chem 2008; 56;5820-5826

Prida A, Ducousso A, Petit RJ, Nepveu G, Puech JL. Variation in wood volatile compounds in a mixed oak stand: strong species and spatial differentiation in whisky-lactone content. Ann For Sci 2007; 64; 313-320

Prida A, Puech JL. Influence of geographical origin and botanical species on the content of extractives in american, french and east european oak woods. J Agric Chem 2006; 54; 8115-8126

R. Rowell (ed.) The Chemistry of Solid Wood’ . Am. Chem. Soc.,Washington D.C. (1984)

Reale S et al. Mass spectrometry in the biosynthetic and structural investigation of lignins. Mass Spect Rev 2004; 23; 87-126

Ronde I (ed. Malt whisky yearbook 2011. MagDig Media 2010
Russell I (ed). Whisky, technology, production and marketing. Academic Press 2003.

Spillman PJ, Sefton MA, Gawel R. The effect of oak wood source, location of seasoning and coopering on the composition of volatile compounds in oak-matured wines. Aust J Grape Wine Res 2004; 10; 216-226

Tanaka H et al. Determination of total charge content of whiskey by polyelectrolyte titration: alteration of polyphenols. J Food Sci 2002; 67; 8; 2881-2884

Viriot C et al. Ellagitannins and lignins in aging spirits in oak barrels. J Agric Food Chem 1993; 41; 1872-1879

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

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

Peat

Pagoda-roof of a kiln

Heat is needed in kilning to dry the malts. Peat has been traditionally used as fuel for kilning in Scotland, especially in the areas where coal was not easily available, such as the islands, Campbeltown and nothern highlands. During the 20th century more affordable coal, gas and oil became more available and gradually the use of peat in kilning dimished. In 1940 it was common to use 25-50% peat for Lowland malts, 50-75% peat for Highland malts and usually 100% peat for Islay and Campbeltown malts. The rest of the fuel was usually coal or anthracite. Nowadays the Islay, Orkney and Campbeltown distilleries are famous for their peated malts, but also some mainland distilleries have experimented with peated malts.

Sphagnum bog

According to Encyclopedia Britannica peat is spongy material formed by the partial decomposition of organic matter in wetlands. Usually peat consist of decomposed Sphagnum and low growing plants such as heather and different sedges (cotton grass, rushes, grasses). Wetlands can be formed by heavy rainfall or by a water basin filled by ground water. Scottish peatlands are usually formed by heavy rainfall and therefore contain more sphagnum moss and heather and less any woody vegetation. The surface layer of a peat bog is thin and aeriated moss and the deeper layers below 50cm are usually waterlogged. As the moss grows the deeper layers decompose. Because of the waterlogging there is less oxygen available and thus the decomposition of organic matter is slow and incomplete. The growing moss on top creates pressure on the deeper layers, producing thicker peat especially below the waterlevel.

 

Heather (calluna vulgaris)

Peatland plants consist mainly of cellulose, hemicellulose and lignin. The lignin in grasses contains all the monolignols; coniferyl, sinapyl and p-coumaryl-alcohols, as lignin in heather is mostly coniferyl-sinapyl-type (see previous blog). Sphagnum is very different in structure consisting of a sort of polyphenolic network. Sphagnum moss is therefore richer in p-hydroxyl-phenols instead of the usual lignin-derived compounds. Therefore burnt sphagnum releases more simple phenols and burnt wood more syringol and guaiacol-derivatives with slightly different smoky aromas. Wooden stemmed plants with more cellulose and hemicellulose decompose into simpler carbohydrates. Surface layer has proportionally more carbohydrates and less phenols and deeper layers have increased levels of potentially harmful nitrogen compounds and hydrogen sulphide (aroma of rotten eggs). Nitrogen compounds are probably produced by a range of fungi. Hydrogen sulphide is generated by bacteria in anaerobic conditions, usually below the waterlevel, from other sulphur compounds.

Anthracite coal

The smoky flavour of a peat reek is supposedly coming from simple phenols, such as phenol, its alcohol-derivatives and creosols, and to some extent from guaiacols, furans and pyrans. Syringyl-compounds are not thought to be of major significance in producing smoky aromas. Different carbonyl-compounds seem to soften the phenolic aromas. Without the carbonyls and guaiacols the phenols can taste ashy, sharp and hard, whereas together they produce aromas of smoked meat, savory "maggi" and burnt sugar. Large amounts of nitrogenated compounds give higher levels of pyridines and result in astringent, green and rubbery flavours. In addition the nitrosamines produced by the nitrogen oxides in malt are carcinogenic. The formation of nitrosamines can be blocked by sulphur oxides, which can be produced by burning sulphur-containing coal or rock sulphur with peat or by adding gaseous suphur dioxide to non-sulphurous gas. Paradoxally the rubbery, unpleasant "sulphury" odor (from nitrogenated compounds) in a whisky can result from not using enough sulphurous fuel in kilning. Also the right temperature in firing is important as more smoke and lignin-derived aromas are extracted and less nitrogen released if the peat burns without flames in relatively low temperatures.

Peat layers

There are considerable differences between peats from different origins. Islay peat is usually richer in phenols, guaiacol, vanillic compounds and nitrogens but poorer in carbohydrates than the peat from the mainland . This is probably because of the greater amount of Sphagnum and lesser amount of wooden stemmed plants in Islay bogs. Wooden plants, especially decideous plants contain grater amounts of syringol-based aromas compared to phenol-rich Sphagnum and relatively guaiacol-rich bog plants. Orkney peats are of an intermediate type as they contain more carbohydrates than Islay peats and more phenols than mainland peats from Tomintoul. The extraction depth is also important, as especially in Orkney there are great differences in peat composition as surface peat is closer to the mainland peat and deeper layers resemble Islay peat. The best extraction depth seems to be just above the water level near the surface. This is probably because of greater amount of carbohydrates and lesser amount of nitrosamines and hydrogen sulphide in the surface layer. The drying of peats and the controlling of burning temperatures are also easier if the peat is not too thick.

The middle cut (from bruichladdich.com)

Malt adsorbs the smoky flavours best when hand dry (15-30% moisture), therefore peat must be burned in the early stages of kilning. Also the grinding and composition of the malts affects the aroma, as the husks are more prone to absorb the phenols. Usually the malt is specified by measuring the phenol-content with high performance liquid chromatography (HPLC); lightly peated malt has <5 ppm phenols, medium peated 5-15 ppm, heavily peated 15-50 ppm and some experimental peated malts have well over 100 ppm phenols. The taste tresholds for different phenolic compounds vary greatly, for example 3 µg/l for guaiacol, 10-68 µg/l for creosols and 7100 µg/l for phenol. The phenol content and the strength of the smoky aroma in the spirit is usually much lower than in the malts and little lower than in the wort, but it depends heavily on the distillation practice. The foreshots contain barely any smoky flavour, the middle cut is also quite subtle, but the last part of the cut is very smoky, about six-fold compared to the middle cut and about the same intensity as the wort. The tails (feints) has about third of the phenols in the last cut and twice of that in the middle cut. To produce heavily smoked malt it seems important to continue the middle cut as long as possible bearing in mind that too much feints produces unwanted off-flavours. None of the distillers have (yet) used only the last cut to produce very smoky spirit. In the table below are some phenol contents and middle cut alcohol contents. Possibly the more intense smoky flavour of the southern Islay whiskies (Ardbeg, Laphroaig, Lagavulin) is partly due to their longer middle cuts.

Lagavulin 1969 (from Whisky Exchange blog)

Phenols decrease during maturation, but the exact mechanism has not been described. It is estimated that 25 ppm phenol content in a new make becomes 10 ppm after 10 years, 8 ppm after 15 years and 6 ppm after 30years of cask maturation. Altough the synergetic nature of oak derived compounds (guaiacol, vanillin etc) can enhance the peaty flavours, it usually softens and diminishes during aging.

Phenol-levels of malts and new-makes in different distilleries and the ABV of the middle cut. (modified from Misako Udo: The Scottish Whisky Distilleries)

DISTILLERY	MALT PHENOLS (ppm)	NEW MAKE PHENOLS (ppm)	MIDDLE CUT ABV
Ardbeg	54 (42-70)	24-26	73-62.5
Bowmore	20-25	8-10	74-61.5
Bruichladdich	3-4		76-64
Port Charlotte	40	20-25
Octomore	129 (in 2003)	46 (in 2003)
Brora	7-40
Bunnahabhain	1-2 (peated malt 38)		72-64
Caol Ila	30-35	12-13	75-65
Highland Park	35-40 (and unpeated malt used together)	2	70 and then 2h40min
Lagavulin	35-40	16-18	72-59
Laphroaig	40-45	25	72-60.5
Springbank		7-8 (formerly 15-20)	68-63
Longrow	55
Old Ballantruan (Tomintoul)	55

REFERENCES, FURTHER READING:

Bozkurt  S et al. Peat as a potential analogue for the long-term evolution in landfills. Earth Sci Rev 2001; 53; 95-147
Bryce JH et al (ed). Distilled spirits, production, technology and innovation. Nottingham Univ Press, 2008
Da Porto C et al. A study on the composition of distillates obtained from smoked marc. Anal Chim Acta 2006; 563; 396-400

Guillén M et al. Carbohydrate and nitrogenated compounds in liquid smoke flavorings. J Agric Food Chem 2001; 49; 2395-2403 

Guillén M, Manzanos M. Study of the components of a solid smoke flavouring preparation. Food Chem 1996; 55; 3; 251-257

Guillén M, Manzanos M. Characterization of the components of a salty smoke falvouring preparation. Food Chem 1996; 58; 1-2; 97-102

Harrison B, Priest F. Composition of peats in the preparation of malt for scotch whisky production. J Agric Food Chem 2009; 57; 2385-2391

Harrison B et al. Differentation of peats used in the preparation of malt for scotch whisky production. J Inst Brew 2006; 112; 4 ; 333-339

Jefford A. Peat, smoke and spirit. Headline 2004 

Kostyra E, Barylko-Pikielna N. Volatiles composition and flavour profile identity of smoke flavourings. Food Qual Pref 2006; 17; 85-95
Lehtonen M. Phenols in whisky.  Chromatographia 1982; 16; 201-203

Russell I (ed). Whisky, technology, production and marketing. Academic Press 2003
Smith GD, Wallace G. Ardbeg, a peaty provenance. GW Publishing 2008  

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

Valaer P. Scotch whisky. Industr Engineer Chem 1940; 32; 7; 935-943

Voigt J et al. New highly aromatic products and distillates from smoked malt. In Distilled Spirits, Nottingham Univ Press 2010

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