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

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Coopering, part2

Most whisky casks are made in the USA, where over a million bourbon casks are produced per annum compared to circa 500 000 (mostly wine) casks in France. Estimated 800 000 used American bourbon casks are sold for Scotch whisky industry per year. It is estimated that about 18 million casks at a time are being used in maturation of whisky in Scotland, suggesting that an average lifespan of one cask is typically 20 years, including the possible refills and rejuvenations.

Cooperage oak is usually harvested at an approximate height of 20-30m, diameter of 35-60cm and age of 60-250 years, according to the species, soil and climate. Fast growing Q.alba and Q.robur are harvested usually at the age of 80-120 years, but Q.petraea grown in dense forests (as in central France) might need up to 250 years. Harvesting time and the age of wood influence the composition of wood. It is believed that wood harvested during winter develops less "green wood"-notes and is easier to cooper due to greater amount of tyloses forming in the spring wood and the astringent tannins diminsh as the tree ages.

The oak used in coopering is cut in quarters to prevent the staves from warping, cracking and leaking. Quartered staves are approximately 25% more waterproof than tangentially sawed staves.

American oaks, especially Q.alba, tend to grow straight and with only a few branches making the coopering easier. High concentration of tyloses and the relative absence of knots allows machinery to be used in cooperages more than in Europe and the staves are often quartered by sawing instead of cleaving in American cooperages. Cleaving is more labour intensive and timeconsuming, but it produces tighter casks and might also affect the permeability of some flavor components. Some Portuguese coopers also use sawing, especially when coopering coarse-grained Q.robur or Q.alba, but usually Q.petraea is cleaved.

After quartering the staves must be dried to ease the coopering and to prevent cracking, shrinking and leaking. The traditional method is to season the staves in columns in oper air for 1-3years, but since the 1800s kiln-drying in 40-60C for about three weeks has been used too, especially in the USA. The French prefer to season their staves much longer, up to 20 years, especially the staves used in maturation of quality cognac or armagnac. The method used to dry the staves does affect the properties of the cask, reducing tannins and adding some flavours from the local microbes.

Seasoned staves are heated and constructed into a cask. A typical whisky barrel consists usually of 31 staves (8 for the heads) and a hogshead of a couple of more. Heating helps to bend the wood and seasons the inside of the cask. American coopers usually use hot steam in bending and a gas burner to char the inside of the cask. Charring is fast; light char is achieved in 15 seconds, medium char in 30s and heavy char in 45s. Further charring results in alligator/grade4 char with blistering burned wood. Europeans commonly use burning wood chips in bending and toasting. Toasting is slower, generally 15 minutes of firing produces light, 30 minutes medium and 45 minutes heavy toasting, although there are variations between cooperages and coopers. Usually the inside of the cask reaches 100-200C in toasting and over 250C in charring. Caramellisation of sugars in hemicellulose and lignin and to some extent in cellulose begins at about 140C.

Charred casks are usually rinsed with water and used to fill bourbon. Toasted casks are usually treated with saltwater, coarse wine, burning sulphur or ammonia and steam to kill potentially harmful bacteria (especially Brettanomyces). Filling the cask with liquid also removes loose wooden chips or sawdust and helps spotting the possible leaks in the cask. In the Jerez region it was customary to ferment the must in new barrels to remove excess resin and tannins from the cask and to produce a furry deposit on the inside of cask, probably consisting of fermenting yeast and bacteria, which was believed to improve the cask. After 1960's most big sherry producers have used stainless steel tanks for fermentations.

Rejuvenation of casks is used in Scotch whisky industry. It usually consists of scraping the inside of the staves about 5-10mm deep, recharring the cask and in some companies seasoning it with sherry or must. This brings new active wood into contact with the spirit and allows the use of cask for refills. Scraping the staves too much increases the risk of leaking or cracking, but on he other hand if the scraping is too thin, the cask may not be active enough. The influence of spirit reaches up to 20mm through the wood. Usually casks are rejuvenated after three fillings, although some companies are rumoured to rechar casks after each filling. There are considerable differences between a fresh cask and a rejuvenated cask as some flavour compounds are extracted faster and deeper from the wood than others in previous fills. Recent innovation in rejuvenation has been a treatment of cask with salt water before recharring or retoasting, which improves the extraction of color and flavour from refill casks.

Various cooperage practices such as wood selection, sawing/cleaving, seasoning/kilning and charring/toasting affect the maturation properties of casks.

REFERENCES:

Chatonnet P, Dubourdieu D. Using Electronic Odor Sensors To Discriminate among Oak Barrel Toasting Levels. J Agric Food Chem 1999; 47; 4319-4322

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, Paterson A, Piggott JR. Changes in wood extractives from oak cask staves through maturation of Scotch malt whisky. J Sci Kilby, K. The cooper and his trade. John Baker Publishers Ltd 1971.

Deguilloux MF, Pemonge MH, Petit RJ. DNA-based control of oak wood geographic origin in the context of the cooperage industry. Ann For Sci 2004; 61; 97-104

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Jackson, RS. Wine Science. Academic Press 2000

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

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Wanikawa A, Hosoi K, Kato T, Nakagawa K. Identification of green note compounds in malt whisky using multidimensional gas chromatography. Flavour Fragr J 2002; 17; 207-211

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

Coopering



Cooper in the Guinness brewery, late 19th century

Wooden barrels have been used to store wine since ca 500BC. Whisky has probably been stored in barrels from the invention of distilled malt spirits, altough the significance of maturation has been commonly appreciated from the  latter part of 19th century. In 1915 it was made compulsory for Scotch whisky to be warehoused in bond for a minimum of two years and the age limitation was extented to three years one year later. In 1988 the Scotch Whisky Act declared that the maturation had to take place in wooden casks of a capacity not exceeding 700 litres. The wooden casks were specified to be made of oak in Scotch Whisky Order 1990.

Until the 16th century the oak used in cooperages of United Kingdom was mostly English. After that shipbuilding and construction increased the demand for quality oak and imports of oak staves increased. There was even a legistlation from 1543 prohibiting export of casks larger than barrels and making exporters import a corresponding amount of timber for casks. The British imported oak came mostly from the Hansa region (Denmark, Baltic), Russia and America (Virginia, New Orleans). Used wine barrels especially from Spain and Portugal were also common. Spanish and Portuguese coopers preferred American oak, whereas British coopers preferred Memel oak over American, European and English oak.

Memel (now Klaipèda) was an important port in Baltia (Lithuania) providing timber from the Baltic region and Russia. Memel was the golden standard of quality oak for the British until 1930s, but during and after the second world war it became unobtainable in UK due to rise of Soviet Union in Baltia. In addition the Spanish civil war (1936-1939) decreased the amount of imported oak casks from Spain. In the 1930s and 1940s Polish, American and English oak was used. In the 1950s Memel oak was already available, but because of the loss of woods in the wars and difficulties in trade the price was high.

Speyside cooperage

From the war years until 1956 as much as 90% of oak used in UK breweries was Persian oak, probably of species Q.brantii (Persian oak, a white oak), Q.macranthera (Persian oak, caucasian oak, not exactly white oak but similar Q.mesobalanus) or Q.mirbeckii (Q.canariensis, a white oak). The quality of Persian oaks varied greatly, probably due to different species harvested and the long transportations in too hot climates. The amount of Persian oak casks used in whisky industry is not recorded. Wine and spirit shipping casks were used widely and apparently preferred for maturation of whisky, especially sherry casks from Spain and port wine casks from Portugal.

Oak maturation of quality American whiskey became a norm during the late 1800s, whisky from Kentucky being sold with age statements from 1840's. Most American bourbon is supposedly matured in charred new Q.alba, but also other American white oaks and their hybrids are used either on purpose or by mistake, since the species can be quite hard to distinguish even with DNA-testing and practically impossible to differentiate only by inspecting the staves.  Since 1938 bourbon has had to be matured in new oak wood, resulting in great number of reasonably priced used casks available from the USA to Scottish whisky industry. In the 1960's the American cooperages produced up to 2.3 million casks a year, about twice as much as today. Because of the surplus bourbon casks the prices dropped and resulted in diminishing use of sherry casks during the last decades, especially as the amount of sherry shipped in casks dimished. Today over 90% of Scotch whisky casks are ex-bourbon. The barrels are usually dismantled into shooks during transportation and assembled again in Scotland. Usual practice is to convert the barrel sized (150-220l) bourbon casks into bigger hogsheads (about 250l) by adding a couple of extra staves of either new or used wood. Some bourbon producers are now using hogshead-sized casks, too. The casks in Scotch whisky industry are filled usually three times before discarded or rejuvenated by seasoning and/or toasting. Other types of American whiskey (not bourbon) have been allowed to mature in refill oak casks since 1970. Whether the Scotch whisky industry has utilized these American refill whiskey casks is not known. Some American red oak has been used in maturing spirits and beer, most notably Guinness beer from late 1800s to early 1900s, but generally red oak casks are not used anymore.

Manuel González Gordon

Sherry producers use mostly new American oak, usually toasted and not charred, but at least some Spanish Q.robur from Galicia is used. In the 1960's and the 1970's Costa Rican red oak varieties Q.costaricensis, Q.eugeniifolia and white oak Q.copeyensis were used (and esteemed) in the Sherry region, but I have no information of the scale of this nor whether they are still used. The sherry producers have been using at least some American oak from the 1600s and nowadays use it almost exclusively. A note from 1807 by Spanish botanist Esteban Boutelou (1776-1813): "The great wine shippers cooper themselves the casks they require for their trade...They use oak almost exclusively; and they esteem more than any that which comes from the United States of America, next the Northern oak, then the Italian and lastly the Spanish... wines ferment better in butts and wooden vats than in earthenware, which belief is shared by the Sanlucar growers.". Hardman (1878) reports that American oak is used exclusively for sherry and marsala production. The great sherry authority Manuel González Gordon writes in 1948 (and in later editions 1972 and 1990) that "In Jerez no other wood has been used for many years, as American oak has given the best results for the fermentation, maturing, ageing and shipping of Sherry." and "In recent years some Spanish oak has been used [for shipping sherry], due principally to the difficulties of importing American timber". Of the big sherry importers Gonzalez-Byazz, Sandeman, Williams-Humbert and Lustau declare they use mostly American oak and I have not yet found a firm using only European oak. It is safe to assume that the whole Sherry region has used mostly American oak for at least the last two centuries and the majority of the Spanish oak casks used in Scotch whisky industry are in fact Spanish-coopered American oak casks. Some European oak (Q.robur, Q.pyrenaica) sherry casks have sold to Scotch whisky producers either by chance or by special order from for example Edrington Group (owner of Macallan, Glenrothes, Highland Park,Tamdhu, Glenturret), Glengoyne and Gordon&MacPhail. Sherry shipments by the cask have practically ended since Spain became member of EEC in 1986 and bulk shipments of wine fortified over 15.5% ABV became prohibited inside EEC. Usually only some fino sherries are below 15.5% ABV and the olorosos preferred in whisky maturation are often over 18% ABV. In addition, if the lower strenght sherry wines with the status of Denominación de Origen (DO) are to be shipped in bulk, the containers must be approved and sealed by the Consejo Regulador de Jerez and the wine must be accompanied with detailed certificates of analysis. Only the biggest sherry houses (Bodegas de Crianza y Expedicion) are allowed to ship sherry in bulk (64 different houses at the moment). Understandably practically all sherry casks are shipped empty or dismantled to shooks.

Apparently there is no official definition for "sherry cask" in Scotch whisky industry, so some of the so called sherry casks might be ex-bourbon casks treated with sherry or similar products such as paxarete wine (a heavy sweet wine from the Sherry region). The size of sherry cask is by law under 1000 litres, usually a butt contains 30 arrobas (about 500 litres), although a variety of different size casks has been used (octaves, hogsheads, gordas, bocoys etc).

In Portugal "imported oak is preferred". American, Polish, French and even Brazilian (no idea of the species) oak has been used, although considerable amounts of Q.pyrenaica and even Castanea sativa (chestnut) has been used as well, at least in red wine maturation and port wine shipping casks. Since 1986 the same EEC shipping rules apply to fortified port as to sherry (see above). The size of port cask (pipes) is usually slightly bigger than the cask size for sherry or other wine casks.

French oak has been used extensively in wine cooperages and lot of research has been made concerning diffrent oak species and local variance of wood as well as its influence on wine maturation. Traditionally local oak has been used to mature local wines, but recently the knowledge on oak qualities has lead to various experiments with different oaks and combinations of oak species. There are lots of variation between the species and the forests, but generally Q.petraea is considered to be of better quality for wines, especially for the whites. Wine casks are usually barrique sized 225l.

Q.robur (Limousin, Gascony, Cîteaux, Galicia) is used to mature spirits, most notably cognac and armagnac. Armagnac producers do use different species of oaks, but the most appreciated is "black oak" of Monlezun forest in Gascony. Black oak is probably a variation of Q.robur. Cognac producers prefer Q.robur from Limousin; new casks are used to mature inferior cognacs or for just a short period (months) in the beginning of maturation before reracking the spirit into old (usually bigger) casks for longer maturation. Cognac casks are usually sized 270-450l, the most popular size being 350l.

Q.crispula, Q.dentata, Q.serrata and probably Q.mongolica are used in Japan. The Japanese mizunara casks are commonly 500l casks and often seasoned with sherry.

General rule of the thumb seems to be that casks made of European Q.robur or American Q.alba are preferred in spirit maturation over Q.petraea in various spirit producing areas and for longer maturation periods refill casks are preferred, except for bourbon.

http://whiskyscience.blogspot.fi/2011/02/coopering-part2.html

References:

Calabrese S. Cognac, liquid history. Cassell&Co 2001
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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
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Lepais O et al. Species relative abundance and direction of introgression in oaks. Molecular Ecology 2009; 18; 2228-2242

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

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