Russell I. (ed.) Whisky. Technology, Production and Marketing

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From a log quarter, the first saw cut is removed from one of the flat surfaces

parallel to the radius of the tree. The quarter is then turned 908 and the second

cut is removed from the other flat surface. This is repeated until the amount of

timber left in the quarter is too small to be of any use. The sapwood and any

dead heartwoo d are then removed from the sawn timber to produce the fin-

ished stave and heading blanks.

The next stage of the process is the drying or seasoning of the timber. In the

USA, all of the oak for the production of bourbon barrels is dried in a drying

kiln over a period of approximately one month. This reduces the moisture

content of the wood to a wo rkable level of approximately 12 per cent.

Throughout the drying process the timber is held under specific temperature

and humidity conditions to ensure that the drying procedure is efficient and

that wood damage, such as the appearance of splits or cracks in the stave ends,

is minimized.

In Spain the seasoning of timber for the production of sherry casks is quite

different. The stave and heading blanks are initially air dried in the growing

regions of northern Spain for a period of approximately nine months, which

reduces the moisture content of the wood to approximately 20 per cent. The

timber is then shipped to the warmer sherry-producing regions in the south of

Spain, where it is further air seasoned for a period of approximately six to nine

months, or until its moisture content is redu ced to a workable level of approxi-

mately 14–16 per cent.

214 Whisky: Technology, Production and Marketing

Figure 7.1

Stave cuts in timber processing.

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Bourbon cask construction

In the manufacture of bourbon barrels, the dried, rough-cut stave blanks are

initially jointed to produce staves that have smooth angled edges and are

slightly wider in the middle compared to the ends. This is essential for

forming the desired barrel shape. The assembly of the barrel begins by

arranging the straight staves into a circular structure that is closed at one

end. This is then steamed for a period of between ten and twenty minutes at

a temperature of approximately 958C to soften the fibres of the wood and

enable the staves to be bent. The staves are then drawn into the conventional

barrel shape using a windlass, and a temporary iron hoop is used to hold

them in place.

The next part of the process is to heat treat the inside of the cask shell. In

terms of whisky maturation this is probably the mos t important stage of

barrel production, as it defines the cask’s ability to matur e spirit. The cask

shell, which is still wet from the steaming process, is initially heated to

between 230 and 2608C for a period of approximately fifteen minutes.

This drives off the surface water and sets the staves in the shape of a barrel

(Hankerson, 1947). The shell is then charred, which involves setting the

inside on fire and allowing it to burn until the required degree of char

has been obtained. The role of the formed char layer during maturation,

and the toasted layer that lies beneath, is described in detail in the follow-

ing section.

Barrel ends are produced by pinning together pieces of oak wood of appro-

priate thickness using wooden dowels. This is then cut into the circular head

and is given a bevelled edge. The ends, which are also charred, are then

inserted into the cask shell. Their bevelled edges locate into grooves, known

as the croze, cut into each end of the shell. Finally, the hoops are driven on to

produce the finished bourbon barrel.

Sherry cask construction

The staves for a 500-l sherry butt are tapered in the same way as in the

production of a bourbon barrel, but are longer and thicker. Where the 180-l

barrel requires around 30 staves for its construction, the larger butt will con-

tain approximately 50 staves. These are initially raised into a circular structure,

as in bourbon barrel production, which is held in shape using temporary

hoops. This is then placed over an open fire at approximately 2008C, which

both heat treats the wood, turning it brown in colour, and makes it more

pliable, allowing it to be slowly pulled into the conventional butt shape

using a windlass. No steaming is involved in the construction of a sherry

cask, but water is applied to the outside of the staves during heating to prevent

cracking. As in the production of a bourbon barrel, the ends are initially pre-

pared by pinning together heading timber using wire dowels. These are then

cut into the circular head and inserted into the cask shell to produce the

finished sherry butt.

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Heat treatment chemistry

Heat treatment has always played an important role in the manufacture of

casks for maturing distilled spirits. Two distinct methods of heating are used:

toasting is a milder but more prolonged form of heat treatment, while charring

is more rapid and involves heating the inner face of the cask with a gas burner

until the inner face catches fire and becomes carbonized. Despite these differ-

ences, the objectives of the treatments are the same:

. Degradation of wood polymers to yield flavour compounds

. Destruction of resinous or unpleasant aroma compounds present in the wood

. Production of a layer of ‘active’ carbo n on the inner surface of casks (charring

only).

Degradation of wood polymers to yield flavour compounds

The main effect of heat treatment is the degradation of wood polymers to form

colour and flavour compounds. Most studies in the past have focused on the

degradation of lignin to produce aromatic aldehydes and acids, such as vanil-

lin and vanillic acid. However, recent research has highlighted the importance

of polysaccharide brea kdown in the formation of a number of flavour com-

pounds. Thermal degradation of polysaccharides (cellulose and hemicellulose)

produces large quantities of furaldehydes, of which furfural predominates

because of the preferential deterioration of the hemicellulose. Furaldehydes

themselves have little sensory impact, but their formation is accompanied by

that of many other mo lecules with sweet, caramel and toasted aromas. Maltol

and 2-hydroxy-3-methyl-2-cyclopentenone were identified in toasted oak after

heating (Nishimura et al., 1983), but comparison of the amounts present with

their odour thresholds suggests that their sensory impact may be limited

(Cutzach et al., 1997). More recently, furylhydroxymethyl ketone and 2,5-fur-

andicarbaldehyde have been identified in extracts of toasted oak, being

formed by the direct pyrolysis of sugars (Cutzach et al., 1999). Extracts also

contained 2,3-dihydromaltol, furaneol, and 2,3-dihydro-3,5-dihydroxy-6-

methyl-4H-pyran- 4-one, which are the products of the Maillard reaction

between amino acids and sugars (Cutzach et al., 1997).

The products of lignin degr adation, such as vanillin, syringaldehyde, con-

iferaldehyde and sinapaldehyde (Nishimura et al., 1983; Reazin, 1983) undergo

further oxidation, either in the wood or in the maturing spirit, to give vanillic

and syringic acids. Of these, vanillin is of greatest sensory importance on

account of its low odour threshold. Analysis of whisky matured in new

toasted casks shows that vanillin exceeds its odour threshold within the first

six months of maturation (Con ner et al., 2001). Synergism has been reported to

decrease the odour threshold of mixtures of lignin breakdown products

(Maga, 1985). The intensity of heat treatment can affect the levels of aromatic

aldehydes and acids generated. Studies using oak chips have shown that

toasting temperatures up to approximately 2008C increase levels of aromatic

aldehydes and acids (Nishimura et al., 1983). Higher temperatures and char-

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ring decrease levels, due to the formation of volatile phenols such as guaiacol

and syringol, or carbonization of the aromatics. In casks however, charring

increases the levels of lignin breakdown products extracted by the spirit.

Although the char layer contains few aromatics, heat penetration to sub-sur-

face layers promotes thermal degradation reactions, increasing aromatic alde-

hydes and acids, over a depth of 6 mm (Perry et al., 1990). Consequently, with

increasing charring time there is increased thermal degradation of wood con-

stituents behind the char layer. Although deeper in the stave, behind a thicker

char layer, this does not hinde r the extraction of these comp onents because the

disruption of the wood structure during charring increases the penetration of

the maturing spirit.

Other constituents of wood are affected by heat treatment. Complex ellagi-

tannins, such as vescalagin and castalagin, are greatly reduced by heating and

charring. Levels are generally low in distilled spirits (Mosedale, 1995), possi-

bly as a result of conversion to ellagic acid (Hale et al., 1999). Two aroma

compounds present in untreated oak are eugenol (clove-like aroma) and oak

lactone (coconut). While some studies show that toasting and charring

increase levels, others suggest there is little effect (Mosedale, 1995).

Although heat treatment may increase the formation of these compounds,

their low boiling points result in volatilization from the wood surface.

Heat treatment also increases the level of coloured compounds extracted

from the wood by the spirit. However, the exact chemical nature of cask colour

has not been elucidated. Heat treatment of model compounds suggests that

colour formation is mostly the result of hemicellulose and lignin degradation,

with little or no contribution from cellulose. The level of extractable colour

generally increases with the intensity of toasting and charring. However, as for

the aromatic aldehydes and acids, the char layer contributes little colour.

Destruction of resinous or unpleasant aroma compounds present in the

wood

Maturation of wines and spirits in new barrels can result in a rancid, sawdust

aroma. The major compound responsible for this aroma has been identified as

trans-2-nonenal, though other unsaturated aldehydes and ketones such as

trans-2-octenal and 1-octen-3-one may enhance this character (Chatonnet and

Dubourdieu, 1998). The levels present varied greatly between samples of

wood and their most likely route of formation is by chemical auto-oxidation

of linoleic acid during seasoning. The amount of trans-2-nonenal could be

markedly decreased, and the off-flavour completely eliminated, by increasing

toasting intensity during cask manufacture.

Production of a layer of ‘active’ carbon on the inner surface of casks

The formation of the ‘active’ carbon layer on the inner surface of the cask is the

result of carbonization of the polymeric constituents. This layer contributes

little in the way of colour or extractives to the maturing whisky. It does,

however, play an important role in the removal of immature character.

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Experiments have shown that it promotes the oxidation of dimethyl sulphide

(Fujii et al., 1992) and may reduce the concentration of other sulphur com-

pounds by a combination of adsorption and oxidation (Philp, 1986). Also, the

break up of the wood structure near the surface may allow easier penetration

by the spirit and increase the extraction of degradation components from sub-

surface layers (Mosedale, 1995).

Control of heat treatment

The control of toasting and charring of casks has a major effect on the sensory

properties of the matured spirits. The intensity of toasting is generally con-

trolled by time, with light, medium and heavy toasts the result of five to ten,

ten to fifteen, and fifteen to twenty minutes’ heating respectively (Mosedale

and Puech, 1998). Toasting normally uses oak-chip fires maintained by indi-

vidual coopers, and consequently there are large variations between different

cooperages and from one cooper to another. New methods are being investi-

gated to provide a better classification of toasted casks, and these are based on

either chromatographic analysis of the volatiles produced during toas ting or

their assessment using metal oxide-based odour sensors (Chatonnet, 1999).

The degree of char ring is also controlled by the burn time, with 15 seconds

used for a light char, 30 seconds for medium char and 45 seconds for a heavy

char (Mosedale and Puech, 1998).

Cask regeneration

Casks that fail to produce a satisfactory maturation, because continued re-use

has depleted the level of available extractives, can undergo regeneration.

These casks are first de-charred, using a rotating brush or flail system, and

the de-charred casks are then re-charred using a gas burner. When casks are

re-charred thermal degradation of lignin and polysaccharide occurs, yielding

similar flavour compounds to those produced in a new charred cask.

However, other constituents of oak are not regenerated, such as oak lactones

and hydrolysable tannins. Consequently the balan ce of wood extra ctives in

regenerated casks is very different from that in a new charred cask.

Re-charring is controlled either manually or by a timer. Reliance sole ly on

time can produce variable results, because casks from different sources have

variable moisture and spirit contents. This affects the drying time of the wood

before ignition, and as a result the degree of charring and the levels of colour

and extractives produced are not controlled. This problem can be overcome by

control measu res based on the surface temperature of the cask wood, or by the

colour of the flame. Initially the cask burns with a blue flame partly fuelled by

the spirit volatilizing from the cask wood; when dry the wood ignites and

burns with a strong yellow flame (Perry et al., 1990). Ch arring time s of 30 to 40

seconds are generally employed, although longer burn times can generate

higher levels of cask extractives and colour.

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Scotch whisky maturation

Maturation can be viewed as the specific combination of one type of distillate

with any one type of cask leading to the development of a flavour profile

relative to time (Philp, 1986). Modern analytical techniques have been used

to identify an increasing number of reactions that take place during whisky

maturation. Most of these reactions are identified by chemical changes, and

their influence on the sensory properties of a mature spirit has not been clar-

ified. This lack of sensory understanding is due to the complex nature of

whisky flavour. Overall flavour is the result of an interaction of a large number

of different aromas, some of which have yet to be identified. Frequently wood

or spirit treatments change more than one compound (or group of com-

pounds), and this can compromise the modelling, and therefore the prediction,

of the sensory impact of a single aroma compound.

Another drawback is that the research has been fragmented, with different

researchers looking at different products in different parts of the world. As the

number of reactions increases, maturation can no longer be thought of as a

homogeneous process with the same reactions occurring irrespective of the

cask type. Most probably maturation is a mixture of different reactions, and

the nature and extent of each is determined by the type of cask used.

Consequently, this description of maturation is in two sections; the first part

outlines the different reactions that have been identified, and the sec ond

relates these reactions to different cask types.

Maturation reactions

The reactions that occur during maturati on can be separated into additive and

subtractive activities:

. Additive activity includes reactions that introduce or form new aroma com-

pounds

. Subtractive activity includes reactions that remove or alter constituents of

new-make spirit.

The main example of additive activity during maturation is extraction of cask-

derived congeners. These can originate from the unprocessed heartwood, the

thermal degradation of wood polymers during cask manufacture, and carry-

over from previous use of the cask. These extractives can also be supplemen-

ted by the hydrolysis of wood constituents during maturation, and interac-

tions between wood and distillate components that form new aroma

compounds. Subtractive reactions may involve the removal of constituents

by physical processes such as evaporation, adsorption/degradation by the

charred surface of the cask, and chemical degradation reactions such as oxida-

tion and the masking of distillate aromas either directly or through changes in

the whisky matrix.

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Additive activity

Cask charring generates high levels of colour and extractives in the cask

wood. A large proportion of these are extracted during the first use of the

casks, though the actual amount will depend on variables such as the

length of the matur ation period and the warehousing conditions. In sub-

sequent use further extraction of these thermal degradation products occurs,

and this is supplemented by the breakdown of lignin and hemicellulose

polymers in the wood by the combined actions of oxidation and hydrolysis.

Two mechanisms have been proposed for the degradation of lignin during

maturation (Reazin, 1981; Puech, 1984). The first involves extraction of an

ethanol–lignin complex by the spirit, which breaks down to form coniferyl

and sinapyl alcohols. These alcoho ls are then oxidized into conifer- and

sinapaldehydes, with further oxidation to vanillin and syringaldehyde

respectively. The second mechanism involves similar reactions, which take

place in the wood to produce aromatic aldehydes that are later extracted by

the spirit.

Degradation of hemicellulose under these condi tions has not been fully

investigated, but may give rise to sugars such as xylose and glucose rath er

than the furan compounds produced during toasting and charring. The level

of sugars obtained, however, does not reach the level required to impart

sweetness to the maturing spirit.

There are also a number of aroma co mpounds present in oak wood that

are not formed by the degradation of structural polymers. Of these, both oak

lactone (coconut aroma) and eugenol (spicy, clove aroma) have been identi-

fied in mature spirits. The sensory importance of oak lactone is complicated

by the fact that two diastereoisomers are present in oak, each with a different

sensory impact. The relative amounts of these depend on oak origin and its

pre-treatment. The highest concentrations are present in American oak, in

which the cis isomer predominates. Grain whisky matured for four years in

new, toasted American oak casks has been found to contain cis oak lactone at

a concentration twenty times greater than its odour threshold (Conner et al.,

2001). On its own, this level of cis oak lactone has a pronounced coconut

aroma, which was not detected in the grain whisky matured in the new

charred cask. Although it is undoubtedly an important aroma compound,

other aroma compounds present in the cask wood must mo dify its sensory

impact. With cask re-use the level of this compound extracted into the spirit

decreases, with a corresponding reduction in its sensory impact. Re-charring

casks produces only a small increase in oak lactone levels. The level of

eugenol in grain whisky matured in new, toasted American oak casks is

around its odour threshold level after four years. Consequently its sensory

impact is a lot lower, but cannot be discounted as it may play some role

in modifying the sensory impact of other aroma compounds such as oak

lactone.

Another grou p of constituents extracted from oak during maturation are the

hydrolysable tannins. These include gallic and ellagic acids and the various

complex combinations of these acids with sugars, which are known as gallo-

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and ellagitannins (e.g. vescalagin and castalagin). These compounds are

generally non- volatile and have no aroma, but may play a role in modifying

the mouth-feel and taste of mature whisky. They may also be important as

oxidative catalysts in the removal of sulphides from the spirit, and so contri-

bute indirectly to flavour changes duri ng maturation (See Chemical degrada-

tion, below). It should be noted that in many older studies Folin-Denis and

Folin Ciocalteau reagents were used to determine the level of tannins in

whisky. This method more accurately measures total phenolics, which

would also include any lignin breakdown products extracted from the cask

wood. Hydrolysable tannins gradually deplete with repeated cask use.

Although re-charring casks may give increased levels of ellagic acid, it has

little effect on the levels of gallic acid.

Another potential sour ce of aroma compounds in refill casks is from the

spirit or wine matured in a previous fill. In-drink by the cask dur ing matura-

tion results in the retention of some components of the distillate by the cask

wood, and these are then released during subsequent maturations. This effect

is most obvious in casks that are used to mature malt and then grain distillates,

with the grain spirit acquiring some of the characteristics (such as peatiness) of

the malt spirit. It is also possible to identify congeners from sherry and bour-

bon in ex-sherry and ex-bourbon casks, but it is not known if these constitu-

ents make a significant contribution to the distinctive flavo ur profile obtained

from such casks.

After a number of fills, heartwood constituents and thermal degradation

products have all but been depleted. The extraction of wood components

then relies on a combination of hydrolysis and oxidation reactions.

However, the proportion of lignin in the wood that is degraded by these

reactions is relatively small (Conner et al., 1993). With repeated use, suscep-

tible lignin in the inner surface of the wood becomes depleted and the locus of

degradation retreats further into the stave (Conner et al., 1995). This combina-

tion of depletion and migration from deeper in the stave wood slows the rate

of extraction and consequently maturation. Eventually a point is reached

where the cask fails to produce any sensory improvement, and it is then

termed ‘exhausted’ (Philp, 1989).

Subtractive activity

Changes in distillate character during maturation may be the result of the loss

or suppression of aroma compounds. This may involve:

. Evaporation of low boiling point compounds through the cask

. Adsorption/degradation by the charred surface of the cask

. Chemical reactions resulting in a less volatile product or one with different

sensory characteristics

. Masking of im mature characters either by sensory interaction or by physical

changes in the whisky matrix.

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Evaporation

Evaporation of volatile compounds through the cask surface occurs during the

course of maturation. For a model whisky, the rate of evaporation ranged from

32 per cent of the total present in the spirit for acetaldehyde to 5 per cent for

iso-amyl alcohols and 1 per cent for ethyl hexanoate and acetic acid (Hasuo

and Yoshizawa, 1986). Evaporation is thought to be the main route for the loss

of dimethylsulphide (Fujii et al., 1992) and dihydro-2-methyl-3(2H)-thiophene

(Nishimura and Matsuyama, 1989). Although evaporation occurs from all

casks, it is not known how factors such as porosity and stave thickness affect

its rate, and in turn spirit quality. Different warehouse conditions (tempera -

ture, humidity and the airflow round the cask) will also affect the evaporation

rate and again mature quality. As evaporation progresses the level of spirit in

the cask decreases, creating an air space. The increase d headspace may pro-

vide a larger pool of air to replenish the dissolved oxygen in the spirit that is

used up in oxidation reactions during maturation.

Adsorption/degradation by char

The ‘active’ carbon layer on the inner surface of the cask plays an important

role in the removal of immature character from the maturing spirit.

Experiments have shown that it promotes the oxidation of dimethyl sulphide

(Fujii et al., 1992) and may reduce the concentration of other sulphur com-

pounds by a combination of adsorption and oxidation (Philp, 1986). Two

mechanisms are possible for these reductions. It may be that char preferen-

tially adsorbs these compounds, or chemical degradation may play the major

role. Fujii et al. (1992) showed that approximately half the dimethyl sulphide in

model solutions was oxidized in the presence of char, but was unable to

identify the route by which the remainder was lost. Another potential role

of char is in retaining both cask extractives and wine/spirit congeners from

previous fills. Although fresh char contains few components, after maturation

it contains higher concentrations of wood and wine/spirit components than

the sub-surface layers. The char may therefore act as an important reservoir for

congeners that are released during the subsequent use of the cask.

Chemical degradation

Chemical reactions that alter distillate components include oxidation and

acetal formation. Examples of oxidation include the formation of acetaldehyde

and acetic acid from ethanol (Reazin, 1981), and the formation of dimethyl

sulfoxide from dimethylsulfide (Fujii et al., 1992). The breakdown of sulphur

compounds may also be enhanced by cask extractives, particularly hydroly-

sable tannins such as gallic and ellagic acids. Interaction between these tan-

nins, dissolved oxygen and copper ions produces active oxygen and peroxide,

which degrade sulphur compounds (Philp, 1986). The dynamics of this reac-

tion change during matur ation. Copper in the spirit is an important promoter

of this reaction, and is adsorbed by the cask during maturation (Muller and

McEwan, 1998). The availability of oxygen may also depend on cask factors

such as porosity, evaporation rate and the air space within the cask. The

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situation is further complicated by a recent study (McPhail et al., 1999) that

identified these cask extractives as important antioxidants in mature spirits

with the ability to adsorb free radicals. Therefore, the role of these compo unds

and the mechanisms of oxidation in maturation need further clarification. If

oxidation reactions are initiated by active oxygen and peroxide, then hydro-

lysable tannins will act as promoters. If, however, it is initiated by free radi-

cals, then these compounds will act as inhibitors!

Acetal/aldehyde equilibria are established for most aldehydes, and are

important in whisky aroma. Aldehydes frequently have sour and pungent

odours, whereas acetals are pleasant and fruity (Perry, 1986). The equilibrium

between free aldehyde, hemi- acetal and acetal is affected by spirit pH (Perry,

1986), and hence is partly influenced by cask type. During maturation the

concentration of esters generally increases due to the esterification of free

acids by ethanol. A large part of this is due to the formation of ethyl acetate

from acetic acid, either extracted from the cask wood or the product of ethanol

oxidation (Reazin, 1981). Trans-esterification reactions are also thought to

occur, which in the presence of the large excess of ethanol favours the forma-

tion of ethyl esters.

Masking

The immature character of a spirit may be masked in a number of ways. The

first is a direct sensory interaction, where the presence of strong wood aromas

lessens the impact of sulphury or feinty characters. Less dominant wood aro-

mas may also interact by enhancing the perception of positive distillate char-

acters. However, the nature and extent of this type of interaction has not been

studied owing to the difficulties in creating realistic models of whisky aroma.

Masking may also occur through changes in the whisky matrix that reduce

the volatility of distillate components. The reduction in pH during maturation,

which may be cask-dependent, affects the ionization state of weak bases and,

consequently, their volatility (Delahunty et al., 1993). Decreases in pH had the

greatest effect on pyridines, due to their pKa values, greatly reducing their

perceived aroma in whisky.

It has also been known for some years that wood maturation of spirits

produces physicochemical changes in the liquid that are detectable by differ-

ential scanning calorimetry (Nishimura et al., 1983), small angle light scatter-

ing (Aishima et al., 1992) and mass spectrometric analysis of liquid clusters

(Furusawa et al., 1990). Recent research has shown that ethanol and water do

not form a homogeneous mixture over the whole compositional range

(D’Angelo et al., 1994). At spirit concen trations, ethanol exists as a micro-

emulsion in water. Whiskies consist mostly of ethanol and water, with

flavour-active components maintained in the aqueous emulsion by ethanol

(Conner et al., 1998). The aggregation of ethanol is affected by the presence of

wood extractives, which increases the solubility of aroma compounds and

consequently reduces their release into the headspace (Conner et al., 1999).

This effect, and the physicochemical changes, is consistent with an increase

in large ethanol polymer hydrates in wood-matured spirits, which have a

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