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Effects of oak wood on the maturation of alcoholic beverages with particular reference to whisky

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Effects of oak wood on the maturation of alcoholic beverages with particular reference to whisky

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Oak casks are used for the maturation of a wide range of alcoholic beverages. Focusing on whisky production, this paper reviews the influence of oak wood properties on the flavour of alcoholic beverages. It examines whether the selection of wood or casks on the basis of their effects on flavour can be justified given our present understanding of the process. The current use of oak casks in whisky manufacture is briefly summarized and the wood properties, both chemical and anatomical, that might influence flavour are described. These characters vary in both the virgin wood and the used casks. The factors influencing this variation are identified. The review also highlights weaknesses in past studies on the subject and proposes research that would allow future work to be more productive and applicable. Despite our incomplete understanding of the role of cask properties in maturation, the selection of wood or casks on the basis of their effects on flavour is feasible.
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Effects of oak wood on the
maturation of alcoholic beverages
with particular reference to whisky
J. R. MOSEDALE
Oxford Forestry Institute, Department of Plant Sciences, University of Oxford, South Parks Road, Oxford,
OX1 3RB, England
Summary
Oak casks are used for the maturation of a wide range of alcoholic beverages. Focusing on
whisky production, this paper reviews the influence of oak wood properties on the flavour of
alcoholic beverages. It examines whether the selection of wood or casks on the basis of their
effects on flavour can be justified given our present understanding of the process. The current
use of oak casks in whisky manufacture is briefly summarized and the wood properties, both
chemical and anatomical, that might influence flavour are described. These characters vary in
both the virgin wood and the used casks. The factors influencing this variation are identified.
The review also highlights weaknesses in past studies on the subject and proposes research that
would allow future work to be more productive and applicable. Despite our incomplete under-
standing of the role of cask properties in maturation, the selection of wood or casks on the basis
of their effects on flavour is feasible.
Role of oak in the maturation of whisky
The use of oak casks for the maturation of
whisky
The whisky industry has traditionally always
used oak casks for the maturation of their prod-
uct and in both the United States and the UK
there are now legal requirements for their use.
In the USA the bourbon industry is required to
store the raw distillate for a year in new,
charred oak casks. In Britain, the law demands
that Scotch whisky be stored in oak casks for a
minimum of 3 years. Legal constraints in both
countries prevent the use of flavour additives
and discourage the adoption of new production
techniques.
The different legal requirements of the UK
and the USA reflect very different traditions in
the use of casks. The bourbon industry, pre-
vented from reusing old casks, is the main pur-
chaser of new American oak casks. In contrast,
the Scotch industry does not generally purchase
new casks, depending instead upon the reuse of
casks already used for the maturation of other
alcoholic beverages. Traditionally, particularly
in the nineteenth century, old sherry casks were
used, but by far the most common source of oak
casks presently used by Scotch producers are
old bourbon casks. It is estimated that between
700 000 and 800 000 used bourbon casks are
sold every year and although some are used for
rum and brandy production, the Scotch whisky
Foronr, VoL 68, No. 3, 1995
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204FORESTRY
industry is a major purchaser. Within the Scotch
whisky industry it is estimated that approxi-
mately 13 million casks are in use at any time.
Oak casks are used for the production of a wide
range of alcoholic beverages, including wine,
brandy, sherry and rum. These industries use a
wider range of oak casks than that used by the
whisky industry.
Process of whisky maturation
The effect of maturation on whisky is quite dis-
tinct, with the unmatured spirit generally hav-
ing few of the desirable properties sought in
whisky taste and aroma. Therefore the impor-
tance of the maturation process should not be
underestimated. The following points are
known about the maturation of whisky
(Nishimura and Matsuyama, 1989):
1 Satisfactory maturation times may vary from
3 to more than 10 years.
2 There is normally a significant flavour differ-
ence between matured and unmatured spirits.
3 Volume and strength are lost due to the evap-
oration of water and alcohol through the
porous wood of the casks.
4 Maturation time and the quality of the
matured spirit may vary according to the type
of whisky, the size, wood type and prior
treatment of the cask and the environment in
which the whisky is matured.
The mechanisms by which maturation in oak
casks occurs are incompletely understood.
Research has been carried out to identify com-
pounds that contribute to the flavour or aroma
of whisky, referred to as flavour congeners.
Correlations between descriptive flavours and
chemical analyses of mature whiskies, have
identified over 400 flavour congeners (Philp,
1989a). The principle ones are esters, carbonyls,
sulphur compounds, lactones, phenols, and
nitrogenous bases, including both desirable and
undesirable components. In some cases the ori-
gin and method of synthesis have been further
studied and the involvement of the maturation
stage confirmed. Changes in taste or aroma will
be due to changes in these flavour congeners.
The methods by which this may occur, during
the maturation of whisky in oak casks, arc
listed below (Nishimura and Matsuyama 1989):
1 Direct extraction of wood chemicals.
2 Decomposition of wood macromolecules and
extraction of these into the distillate.
3 Reactions between wood components and
constituents of the raw distillate.
4 Reactions involving only the wood extrac-
tives.
5 Reactions involving only the distillate com-
ponents.
6 Evaporation of volatile compounds through
the cask.
However, as emphasized by Piggott et al.
(1992) it is the concentration in the 'headspace'
(the air space in the cask or container) rather
than in the mature spirit that determines the
influence on flavour of many volatiles. The con-
centration of volatiles in the headspace will be
influenced by any factors affecting their solubil-
ity in the distilate, including the concentrations
of involatile compounds.
Canaway (1983) described how the variation
of samples from different casks could equal the
variation between samples of differing age. Such
variation between casks could be due to differ-
ences in the raw distillate, the conditions of
maturation or the cask wood.
Although both the raw distillate and particu-
larly the conditions of maturation may play an
important role in determining the result of mat-
uration, the oak cask in which maturation takes
place appears to be of prime importance to the
final flavour of whisky (Williams, 1983a). The
type of cask can affect both the taste, colour
and aroma of whisky. The desired effect of mat-
uration will depend upon the nature of the
immature whisky. It may sometimes be desired
that the oak wood contribute significantly to the
flavour, while for other whiskies, perhaps with
an already characteristic taste, the desired effect
of maturation may be less. The time taken to
reach a satisfactory condition is of financial and
practical concern for the manufacturer and
varies according to the type of cask used.
Cask and oak types
The source of oak wood used for the construc-
tion of casks will normally be one of two gen-
eral types. Most commonly used is American
oak, which is predominantly Quercus alba, but
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MATURATION OF WHISKY IN OAK WOOD205
may include wood from 10 or more other
species of American white oak (Singleton,
1974).
The majority of casks used by both the
bourbon and Scotch whisky industries will be of
this type. Less often used is European oak, con-
sisting of wood from either Quercus robur or
Quercus petraea. Spanish sherry casks may be
manufactured from either American or Euro-
pean oak, and it is possible that a single cask
may include both types of wood, particularly
after repair or reconstruction work.
The division of wood into American and
European oak will encapsulate more than sim-
ply the botanical species. The two types are
associated with different environments and are
generally used by very different cooperage
industries. It is also important to note that these
two simple classifications of cask wood do not
form homogeneous groups. There is a long tra-
dition of using different types of French oak for
different purposes, as the effect on flavour is
thought to vary according to the forest where
the oak is grown. However, there is great uncer-
tainty over what determines the different geo-
graphic types. Wood from different locations,
even if of the same species, may have grown in
different climatic or silvicultural regimes. Fur-
thermore the felling and later selection and
treatment of wood may vary, with many wood
types associated with particular cooperage
methods as well as geographic origins. There-
fore it is often difficult to discriminate between
wood and cask origins. Although most of the
oak used derives from the USA or Europe, many
other sources of oak have been used for the pro-
duction of whisky and other alcoholic bever-
ages.
Table 1 lists and describes some past and
present sources of oak cask wood. Wood other
than oak is occasionally used to store alcoholic
beverages, although the cask is normally coated
on the inside by paraffin or silicone to prevent
leakage and the release of unpleasant odours
(Knox personal communication). Robinia
pseudoacacia has been reported as being used
for wine casks in Hungary (Lamfalussy, 1953;
Molnar et al., 1985), apparently without any
coating. Trials in India on the suitability of 12
native timbers to mature whisky found Termi-
nalia tomentosa and Shorea robusta to be the
best substitutes for imported oak. Other woods
Table 1: Sources of oak wood used for maturation of alcoholic beverages
Wood originsSpecies reported as being
used for cooperageMain cask usesComments
America
Western Europe
(mostly France)
Eastern Europe
Japan and Asia
Near East
South America
Q. alba and related white
oak species (see Singleton
1974).
Q. robur, Q. petraea.
Q. robur, Q. petraea,
Q. cerris.
Q. dentata, Q. crispula,
Q. mongolica.
Q. mirbeckii and possibly
others.
Probably Q. copeyensis
(see Singleton, 1974).
Bourbon and subsequently
Scotch whisky. Wine and
sherry.
Wine and brandy.
Wine, brandy, beer.
Whisky and brandy.
Sherry and whisky casks.
Low tannic content but
high levels of volatiles.
Varies depending upon
precise origins.
Present state of oak
forestry uncertain—but
potentially a major source
of cask wood.
Q. crispula reported to
release a sweet taste
(Kanazhashi personal
communication).
Oak staves imported from
Iran and Turkey during
1940-50s (Williams,
1983b).
Costa Rican oak reported
to have been exported to
Spain.
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206FORESTRY
that gave tolerable results included Quercus
dilata and W. semecarpifolia (Anon., 1950).
The cooperage industry
The methods and regulations of cooperage dif-
fer between America and Europe, with tighter
control generally being found in the supply of
American oak. In America the cooperage indus-
try is more automated and operates on a much
larger scale than most European cooperages. It
accounts for approximately 3 per cent of all the
American white oak harvested each year. In the
American mid-west 950 000 to 1,200 000 casks
per year are made: the majority destined for the
bourbon industry (Knox personal communica-
tion).
In comparison the French cooperage
industry, which is by far the largest in Europe,
produced around 160 000 casks per year in the
early 1990s. However, while new American
casks may cost between £50 and £100, new
French casks will normally be over £250 each.
A variety of construction methods and practices
are used, but some of the main steps in cask
construction are listed in Table 2.
Table
2:
Stages of cask construction in chronological
order
1 Selection and felling of suitable timber.
2 Sawing of staves—initial cleaving of wood carried
out by some European coopers.
3 Seasoning—air or kiln drying.
4 Precise cutting of staves, bevelling and manufac-
ture of cask heads.
5 Raising of cask—toasting or steaming used to
bend staves.
6 Further heat treatments of cask—various heating
treatments including the intense charring of bour-
bon casks.
7 Testing of cask integrity and strength.
8 Use, repair and rejuvenation treatments of cask.
Oak forestry and distribution
Oaks form a major part of the forest flora in
both Europe and America. Oak forests fre-
quently exceed over 25 per cent of total forest
area in many European countries (France,
Greece, UK, Hungary and others) and are often
of significant economic value. Their growth, sil-
viculture and exploitation have a long history,
particularly in Europe.
Kleinschmit (1993) describes 24 oak species
and different hybrid forms existing in Europe.
Eight of these are of economic importance, with
only the two most important species commonly
recognized as suitable for cooperage: Quercus
robur and Q. petraea. These two species are
found across most of Europe up to an elevation
of 1600 m for Q. petraea in the French Alps,
with a high degree of range overlap. The two
species hybridize, but there is still much contro-
versy over the frequency and importance of
hybrids. A similar situation is found among the
species of white oaks of north America, where
Burger (1975) and others have described the
problems of ascribing strict biological species to
the various types of oak.
Selection of cask and wood types
The most important factors governing the
choice and use of oak casks are practical con-
cerns,
such as the ease of supply and economics
of use. Therefore, although their previous use is
often claimed to contribute to the taste of the
mature whisky, the purchase of used casks is
due primarily to their low cost. Economic con-
straints demand the reuse of casks within the
industry, despite casks decreasing in viability
with each use.
Singleton (1974) claimed that whisky produc-
ers have nearly always displayed some prefer-
ence for the source of oak casks used, but the
preferred wood has often been identified simply
by the port of importation. Despite widely
made claims and traditions that different types
of oak will influence maturation in varying
ways,
at present the flavour effect of oak does
not play a major role in the selection of casks.
However, the producers of other alcoholic
products, particularly wine and brandy, show
more discrimination between different types of
cask. This is reflected in the much higher cost
that French coopers are able to demand for
their products, and it is interesting to observe
that despite their own sizeable cooperage indus-
try, the USA purchases around half of all French
cask exports (Knox personal communication).
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MATURATION OF WHISKY IN OAK WOOD207
Wood properties affecting the maturation
of whisky
'Wood chemistry
The effects that wood-derived compounds may
have on whisky maturation have already been
outlined. Flavourful whisky is thought generally
to contain high levels of wood-derived com-
pounds. Therefore particular focus has been
placed on the role of oak extractives as either
flavour congeners in themselves, or involved in
their formation or breakdown.
Extractives are compounds found in oak
wood that are soluble in either water or organic
solvents. There is no precise definition, with the
composition dependent upon the solvent used,
method and conditions of extraction. The syn-
thesis of most extractive material is associated
with the formation of heartwood, and is
thought to be controlled by both genetic and
environmental factors (Sjostrom, 1981). Those
compounds believed to be of importance in the
maturation of whisky include the following
groups.
Tannins and their derivatives The most impor-
tant group of phenolic compounds are the tan-
nins,
being a loosely defined group of water
soluble plant polyphenols (Haslam, 1981). The
main characteristic responsible for their biolog-
ical activity, including their astringency (Hager-
man and Butler, 1991), is their ability to bind
proteins.
The tannins may be further divided into two
separate groups. The principal group in oak is
the hydrolysable tannins, including gallotannins
and ellagitannins, which are described by
Hagerman and Butler (1991) as having a polyol
(normally D-glucose) as the basic structural unit,
of which the hydroxyl groups have been
esterified by gallic acid or hexahydroxydiphenic
acid (HHDP acid). These tannins are easily
hydrolysed either enzymically or in acid or base
conditions, to form free gallic or HHDP acid,
the latter spontaneously lactonizing to give
ellagic acid. Their biosynthesis probably occurs
at the transition zone boundary, during the
transformation of sapwood to heartwood
(Hillis, 1987), with their precursors thought to
derive from the shikimic acid pathway (Haslam,
1981;
Gross, 1992; Haslam, 1992). The ellagi-
tannins have been found to make up to 10 per
cent of heartwood dry weight (Scalbert et al.,
1988a). The most common ellagitannins in oak
have been identified as vescalagin and castalagin
(Mayer et al., 1967), with eight water soluble
ellagitannins being characterized by Herve du
Penhoat et al. (1991a and b). The second cate-
gory are the non-hydrolysable tannins (con-
densed tannins or proanthocyanidins). These
are oligomeric or polymers of flavonoid units,
linked by carbon—carbon bonds, that are not
susceptible to hydrolysis (Hagerman and Butler,
1991).
In oak heartwood they are found in much
lower concentrations than hydrolysable tannins
(Scalbert et al. 1988a and 1988b).
The solubility of tannins may vary, according
to their type, size and various binding reactions
with other compounds. Solubility will generally
decline with increasing molecular weight.
Therefore polymerization is likely to lead to a
decrease in the level of soluble tannins. This
effect of polymerization has been widely
reported for condensed tannins (Hagerman and
Butler, 1991), but fewer studies have examined
hydrolysable tannins. However, Peng et al.
(1991) studying tannins in the wood of Castanea
sativa and Quercus petraea concluded that insol-
ubilization of tannins in heartwood probably
results from their slow oxidation, leading to
polymerization or copolymerization of both
condensed and hydrolysable tannins with cell
wall components. Such oxidation reactions
could be enzymic (involving peroxidases), but in
heartwood are more likely to be non-enzymatic.
The degree of polymerization would be depen-
dent upon how readily tannins oxidize, with
condensed tannins considered more vulnerable
to such reactions. Oxidation and polymeriza-
tion of tannins will considerably modify their
astringency and toxicity (Peng et al., 1991). The
lower solubility of tannins, due to polymeriza-
tion, is thought to cause the loss of astringency
as fruit ripens (Hagerman and Butler, 1991) and
also to explain the reduction in central heart-
wood durability and resistance to fungal attack
(Han and Hillis, 1972; Peng etal. 1991; Scalbert
1992b). Partially oxidized and polymerized
ellagitannins may also be responsible for a large
part of the heartwood colour (Haluk et al.,
1991;
Charrier, 1992).
Numerous studies have described tannins in
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208FORESTRY
whisky as arising through direct extraction
from the wood, the concentration in maturing
whisky increasing rapidly in the first 6 months,
after which the rate of increase declines (Bald-
win etal., 1967; Reazin etal., 1976; Baldwin and
Andreasen, 1974). However, the role of tannins
in the flavour of whisky and other alcoholic
beverages, has not been well established despite
the taste often being described in terms of a tan-
nin character. Studies have been further con-
fused by imprecise measurement and use of the
term tannins. Most early studies used solely the
Folin Denis or Folin Ciocalteau methods of esti-
mating 'total tannins', or more accurately 'total
phenolics', which measured both tannins and
non-tannin phenolics. Puech et al. (1990)
showed that the Folin Denis measure of total
phenolics gave a good correlation with the tan-
nin content of wood extractive but not for the
amounts found in maturing spirits, due to the
higher levels of ellagic acid and lignin derived
phenolics. Many recent studies report very low
levels or no ellagitannins occurring in many
spirits (Puech etal., 1990; Ford and Done, 1991;
Puech and Moutounet, 1992). Viriot etal. (1993)
describe the level of ellagitannins in maturing
brandies as increasing over the first 5 years, but
then subsequently decreasing with further age-
ing, probably due to chemical degradation
through hydrolysis. In a comparison between
wine and an alcohol—water solution, both of
which had been stored in oak casks for 12
months, Chattonet et al. (1989) found that the
wine contained lower levels of ellagitannins.
This is probably due to reactions with wine
constituents, involving binding with proteins or
oxidation reactions, whether they be direct,
coupled or after hydrolysis. Tannins or their
products may be important to the maturation
process as oxidative catalysts, or in the removal
of sulphides (Chatonnet et al, 1991). The
hydrolysable products of tannins, such as gallic
and ellagic acids, have been found in many spir-
its (Jindra and Gallander, 1987; Wilker and
Gallander, 1988; Puech and Moutounet), sug-
gesting the breakdown of the hydrolysable tan-
nins.
Sefton (1991), describing results of Somers
(1990),
claimed that in regards to wine matura-
tion the sensory role of oak was not related to
total phenolics, while the role of the involatile
tannins was unknown. Viriot et al. (unpub-
lished) were of the opinion that ellagitannins do
not play an important role in the maturation of
cognac or other spirits, while Herve du Penhoat
et al. (199b) thought that tannins contribute
indirectly to the taste of brandies, through their
complexing or reducing properties. Therefore,
despite their abundance in the extract of oak
wood, the role of tannins in the flavour of
whisky remains uncertain.
A variety of other phenolic compounds are
also found in the extract of oak wood. The
fluorescent cumarin compound scopoletin is
used as an indicator of spirits having been
matured in wooden casks (Puech and
Moutounet, 1988). A wide range of volatile phe-
nolic compounds, particularly aromatic aldehy-
des and acids, derived from lignin are also
thought important and these are discussed
below.
Lignin degradation products A number of
compounds found in mature whisky derive from
oak lignin. Mechanisms for their formation
have been proposed by many authors (Baldwin
et al., 1967; Puech et al., 1977; Reazin, 1981;
Nishimura et al., 1983; Conner et al., 1989;
Nishimura and Matsuyama, 1989; Sarni et al.,
1990) and the following pathways have been
verified (Nishimura and Matsuyama, 1989) for
the origin of lignin degradation products in
matured distillate.
1 Degradation of lignin to aromatics due to
toasting or charring of casks.
2 Extraction of monomeric compounds and of
lignin from the wood.
3 Formation of aromatics by ethanolysis of
lignin.
4 Further conversion of compounds in the
spirit.
Ethanolysis involves the reaction of the distil-
late ethanol with lignin, to produce an alcohol-
soluble form of lignin. As the solubulization
involves the splitting of alkyl aryl ether covalent
linkages it is a slow process likely to occur
throughout the ageing process (Viriot et al.,
1993).
Puech and Sarni (1990) outlined three
stages in the delignification process:
1 Degradation of cell walls, with lignin poly-
saccharide bonds breaking and lignin depoly-
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MATURATION OF WHISKY IN OAK WOOD209
merization to dissolvable smaller molecules.
2 Inactivation or repolymerization of small
mass molecules, possibly with recondensation
on fibres.
3 Subsequent hydrolysis of smaller molecules.
They described an easily extracted lignin
complex, that amounted to approximately 4 per
cent of the total lignin, and observed that higher
levels of tannins appeared to increase the rate of
delignification. A variety of phenolic com-
pounds may be produced, which may readily
oxidize to give various aromatic aldehydes such
as vanillin and syringaldehyde, as well as their
respective acids (Baldwin et al., 1967). Puech
(1981) described how lignin underwent intense
oxidation when in contact with spirits and oxy-
gen, forming aromatic aldehydes. Such reac-
tions may also occur by means of heating or
charring the wood.
The role of lignin-derived products on flavour
is uncertain, with none of the many studies of
them providing conclusive evidence of an effect
on flavour. Indeed, their levels in matured dis-
tillate are often lower than their individual
flavour thresholds (the concentration at which
they produce a detectable flavour). However,
Maga (1985) found that compounds were syn-
ergistic, with a mixture of seven congeners giv-
ing a very low mutual flavour threshold of
about 2 p.p.m. in 40 per cent alcohol. There-
fore,
despite individually low concentrations,
they may none the less influence flavour. Lignin
degradation products also highlight the
difficulty in defining the extractive content of
wood, with many compounds being formed
indirectly through subsequent reactions.
Oak lactones Masuda and Nishimura (1971)
identified the two "y-lactone isomers (cis and
trans) as being major components of the volatile
fraction of oak wood extractives. These lac-
tones,
the so called oak or whisky lactones,
derive solely from oak and may be formed from
the oxidation of lipids (Maga, 1989b). Tsukasa
(1988) reviewed the chemistry of oak lactones,
examining a number of different synthesis path-
ways.
These lactones are known to increase in
concentration in the distillate during maturation
in oak casks, reaching concentrations of up to
10 p.p.m. (Otsuka et al., 1974). Otsuka et al.
(1974) found a direct correlation between oak
lactone concentration and assessed quality
scores of different whiskies. Studies on their
concentration in red wine (Chatonnet, 1991)
suggest they are beneficial to flavour in low con-
centrations but detrimental in excess, having an
aroma of new oak and coconut. Reazin (1981)
claimed that their flavour was modified by the
presence of furfural. The precise role of oak lac-
tones in whisky flavour is unknown, although
relatively high levels in mature whisky are con-
sidered desirable.
Acids Measurements of volatile and fixed
acids have found that both may increase during
maturation (Reazin, 1983). The amount of
acetic acid has been found to increase dramati-
cally during maturation (Franco and Singleton,
1984),
with studies by Reazin etal. (1976) show-
ing that most derives from wood extractives
rather than the distillate ethanol. Nykanen
(1984) identified dicarboxylic acids as generat-
ing aroma compounds and catalysing reactions
forming lactones, esters and other compounds.
Fatty acids and other apolar extractives The
apolar fraction of oak extract has received
relatively little study, despite it being thought
that many compounds arc important flavour
components. The group includes steroids and
triglycerides, as well as palmic, steric and oleic
acids.
Carbohydrates The levels of many sugars are
found to increase in maturing spirits, normally
displaying a hyperbolic increase over matura-
tion time (Reazin, 1983). Many of the reducing
sugars probably derive from the breakdown of
hemicellulose and hydrolysable tannins (Wilker
and Gallandcr, 1988). Nykanen (1984) found
that the most abundant sugars in the maturing
spirit were glucose, arabinose, and proto-
quercitol, while other studies (Charrier, 1992)
have found fructose and glucose as the most
common sugars in oak extract. The thermal
degradation products of cellulose and hemicel-
lulose, such as furan and pyran volatiles, have
also been reported frequently in whisky, most
recently by Clyne et al. (1993). However they
are not thought to have a major influence on
flavour (Chatonct etal., 1991).
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210FORESTRY
Nitrogenous compounds Both polyphenoloxi-
dase and peroxidase activity have been found in
heartwood of oak (Ebermann and Stich, 1992).
Amino acids and other nitrogenous compounds,
such as pyrazines and pyridines (Maga, 1985)
have been detected in charred oak extract, with
concentrations of 17 and 2 mg respectively per
100 g dry weight of wood. Although it has not
certain that they influence flavour, such com-
pounds are known to have very low flavour
thresholds.
Terpenes This group of volatile compounds
has received relatively little attention, despite
being important flavour and colour compounds
in spices, perfumes and other aromatic prod-
ucts.
Studies by Sefton et al. (1990a), Nabeta et
al. (1986) and Nishimura et al. (1983) have
identified monoterpenes, sesquiterpenes and
various norisoprenoids among the constituents
of oak wood. Sefton (1991) describes the noriso-
prenoid group as the most diverse and identifies,
in oak extract, precursors of compounds
patented as flavour additives.
Other compounds A range of additional com-
pounds found in the extractive content of oak
may be of relevance to whisky maturation, but
have yet to receive attention. Those of impor-
tance include both volatile and non-volatile
compounds. Over 200 volatile components of
cask wood have been identified (Sefton et al.,
1990a), and many others may be present. Non-
volatiles, such as tannins, may influence flavour
by direct or indirect effects, such as affecting the
solubility of volatile compounds (Piggott et al.,
1992).
As already indicated, while whisky may
obtain some flavour congeners by direct extrac-
tion, many others, particularly aromatic aldehy-
des and their related acids, will derive from
both extraction and further reaction of com-
pounds from the cask.
Wood anatomy
Effects of wood anatomy The use of oak wood
for the manufacture of casks is primarily due to
its ability to contain liquids with little leakage.
The wooden cask must also meet other cooper-
age criteria, such as suitable strength and flexi-
bility. Anatomical features could influence the
maturation process through two possible mech-
anisms. Firstly the location of extractive
deposits and any factors that influence the per-
meability of wood, are likely to affect the avail-
ability of wood extractives to the maturing
distillate. Secondly, the cask wood will influence
the maturation conditions and environment.
For example, any features that influence the
movement of gases through the wood will affect
both the rate of evaporation of the distillate and
the availability of oxygen. As well as influenc-
ing whisky maturation directly, anatomical fea-
tures may correlate with other properties that
affect maturation. If chemical requirements can
be shown to correlate with physical properties
the selection of wood for flavour effects could
be easier.
Anatomy and properties of oak wood Oak
wood is ring porous. The early wood is laid
down at the start of the growing season and
consists mostly of large vessels. The late wood
has a greater proportion of fibres and only small
vessels are present. The rate of growth deter-
mines the size of annual rings and the propor-
tion of late to early wood increases with ring
width. Sapwood, containing living parenchyma
cells and frequently starch, eventually transform
into heartwood. This transformation involves
cell death, the removal of starch and laying
down of extractives. Structurally the woods are
very similar, despite the lower permeability,
greater durability and darker colour of heart-
wood. The heartwood periphery may be undu-
latory, and does not normally correspond with
a specific growth ring.
Tyloses are occluding structures found in ves-
sels which develop through pit apertures from
adjacent parenchyma cells. The formation of
tyloses is generally associated with the conver-
sion of sapwood into heartwood. However they
also form in the sapwood of felled timber dur-
ing storage to an extent depending upon the
conditions and duration of storage (Alexander,
1972;
Bolton personal communication). Tyloses
may also form as a response to fungal infection
or injury. Wheeler and Thomas (1981) report
the findings of Williams (1942) who found that
in white oak the late wood vessels rarely have
tyloses, in contrast to the larger early wood ves-
sels.
The precise stimulus for their formation
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MATURATION OF WHISKY IN OAK WOOD211
remains uncertain, with exposure to air and
changes in the concentration of ethylene among
the proposed causes (Hillis, 1987). Tyloses are
not found in the wood of all species of oak,
with Q. rubra being a notable case, making their
wood unsuitable for leak-proof cooperage.
The number and structure of wood rays are
thought to influence the radial permeability of
wood, possibly to both gases and liquids. Rays
may consist of either a single (uniserate) or 5-30
(multiserate) rows of cells across a transverse
section and be hundreds of cells in height. The
size of rays was found by Feuillat (1991) to be
inversely correlated with their number.
Permeability Low wood permeability is neces-
sary in order to ensure a tight cask with little
leakage. The impermeable nature of oak wood,
together with its wide availability, were proba-
bly the main reasons for its initial use. Tyloses
are generally considered to be the primary cause
for heartwood impermeability to both gases and
liquids. Lehmann (1988) found that air perme-
ability of Fagus syvatica heartwood was closely
correlated to tylose formation. Likewise Kuroda
et al. (1988), in studying twenty hardwood
species, found that the presence of tyloses in the
wood could be predicted from measuring per-
meability. The ring porous nature of oak may
also be important in restricting its permeability.
The most relevant measure of permeability is
the tangential permeability, which corresponds
to movement through a stave, from the inside to
outside of a cask (see Figure 1). The size, abun-
dance and distribution of vessels, degree of clo-
sure by tyloses, the number of wood rays and
other anatomical factors are all likely to
influence tangential permeability. Due to the
ring porous nature of oak, the size of annual
rings may also be important, as this will deter-
mine the relative abundance of large vessels,
which are found only in the early wood. The
anatomy of oak wood is very variable, even
when the ring size is similar.
However, studies examining the permeability
of wood in relation to specific anatomical fea-
tures have rarely found any strong correlations.
Kuroda et al. (1988) found that neither vessel
radius, nor percentage of vessels by volume nor
total number of vessels per cross sectional area,
gave good agreement with permeability in ring
Quartm-Mwri ttove
Amuti rings
Sipwood,
TWo raw o* «tavs« out (ran hMrtwood
Direction of ponotnllon of wood by dtoflsts
Figure
1.
Alignment of stave wood.
porous woods such as oak, which have high
variation in vessel size. Sato etal. (1990) studied
the penetration depth of cask staves by malt
whisky, which correlated with neither ring
widths nor the alignment of the wood relative
to the radial direction. The dominant role of
tyloses in determining the permeability through
wood may explain the failure to detect such cor-
relations. In contrast Maga (1989b) stated that
it was the preponderance and spacing of large
rays that made it difficult for liquid to pass
through oak wood. The importance of rays in
liquid or gas movement in wood is uncertain,
although the estimated 25 per cent lower tan-
gential permeability compared with radial per-
meability is thought to be due to rays allowing
greater radial flow (Kumar and Kohli, 1988).
Wheeler and Thomas (1981) suggested that the
permeability of rays depends on their width,
with single cell uniserate rays being the most
permeable. Other features that are thought to
influence permeability include the deposits of
extractable material, which may block vessels
and cell pits (Wheeler and Thomas, 1981;
Kumar and Kohli, 1988).
The permeability also varies between differ-
ent liquids. Kiseleva and Zoldners (1986) found
that pure water diffused through birch wood
four times faster than pure alcohol, when
restricted to passing through cell walls, as is
likely to be the case when movement through
vessels is blocked by tyloses.
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212FORESTRY
Wood density Zhang et al. (1993) describe the
close correlation of oak wood density with
both ring width and cambial age (the number
of rings away from the pith). Ring width
influences density predominantly by the ratio of
early to late wood, with the early wood having
a lower density than the late wood. Quickly
grown oak, with wide rings, tends to produce
wood with a higher density than slowly grown
oak. Keller (1987) described the best cask wood
as being less dense and therefore more perme-
able than wood used for most other applica-
tions.
Wood colour Klumpers et al. (1993) found a
tentative correlation between wood colour and
extraaive content, with heartwood colour being
loosely correlated with the tannin content. The
darker colour of heartwood is frequently
claimed to be due to the laying down of pheno-
lic extractives. The colour of the wood almost
certainly correlates with some measure of
extractives, as the wood becomes visibly paler
after extraction with ethanol or acetone solu-
tions.
The majority of studies addressing the
colour of oak wood have focused on the prob-
lem of discolouration during seasoning of the
wood (for example Haluk et
al.
1988 and 1991).
Charrier (1992) found that brown discoloura-
tion was associated with a decrease in the levels
of ellagitannins, with a corresponding increase
in the concentration of ellagic acid and possible
coloured degradation products.
Wood grain This empirical term is much used
among wood traders and coopers to describe
cask wood. The grain of the wood is deter-
mined by the visual impression produced by the
size of wood elements, particularly vessels. Dif-
ferent types of grain may include fine, coarse,
tight and loose. Feuillet et al. (1992) describe
how the term has varied in its use over history
and how it presently most often refers to the
ring width and wood texture. This in turn
relates to the porosity of the wood and the
abundance and distribution of large vessels.
Although the term lacks objective precision,
none the less its continued use among profes-
sionals, particularly those working in French
forests, makes it important to understand the
properties to which the term relates.
Summary of the role of wood in the maturation
process
The properties of the cask wood clearly have a
major effect on the maturation process. Of par-
ticular importance, and having been the subject
of numerous studies, is the amount and compo-
sition of the extractive content. Various flavour
congeners are thought to derive directly or indi-
rectly from extractive compounds and a number
have been identified as being of likely impor-
tance. Anatomical characteristics of the wood
may also influence the maturation process, par-
ticularly the rates of evaporation, oxidation and
extraction. Any factors that cause variation of
these properties will thereby influence the mat-
uration of whisky.
One cannot define a set of properties, be they
chemical or physical, or the cask wood that may
be used as criteria in the selection of wood suit-
able for whisky maturation. However, a num-
ber of characteristics are accepted as influencing
maturation. One can examine the variation in
these characteristics to determine the feasibility
of selecting wood for its effects on maturation.
The next section examines evidence for varia-
tion in relevant properties and likely causes and
patterns in this variation.
Factors determining cask properties
Numerous studies have compared the effect of
using different types of cask and many have
found apparent differences. However it is often
found that numerous factors such as the method
of cooperage, age and drying of the wood, in
addition to the source of oak used, vary
between cask types. Therefore although many
studies have shown that there is variation in
cask properties and effects, the source of this
variation has not often been identified. An
attempt will be made to discriminate between
variation in the properties of the virgin wood
and that among used casks.
Variation in oak wood properties
Variation between species Many studies exam-
ining variation of the properties influencing
maturation fail to identify the species of oak
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