● BIOCHEMISTRY
Way through the wood
Barrel biochemistry for brewers and distillers
By Ian Hornsey
“Wooden barrels have been arguably the most significant
shipping container in history. They served Romans,
explorers, pilgrims, pirates, pioneers and samurai
through 2,000 years of civilisation.” Diana Twede,
packaging professor and author, 2005.
U
ntil the advent of plastic, wooden
containers, commonly referred to
as ‘barrels’ in the US and called ‘casks’
in the UK (because there the barrel is
a measure), had been a feature of everyday life for nearly two millennia. Capable of holding a variety of liquid and
solid foodstuffs, they were the result of
the skill of coopers who were masters
of woodworking and metalworking.
The UK term ‘cask’ encompasses
a number of container sizes from
4½ gallons (pin) through 54 gallons
(hogshead) to 216 gallons (tun). Those
generally being used for ageing being
towards the mid-range (18 gallons;
kilderkin and 36 gallons; barrel).
Casks/barrels are characterised by
being tapered at each end and consist
of curved wooden slats (staves) along
their long axis; ‘a bulbous cylinder
with flat ends’. In this respect they
have some structural relationship to
the wooden bucket, except that these
have a closure at one end only, and the
component staves are not curved.
The ability to curve wooden slats
would have been learned from early
boat builders, as author Henry Work
related: “The evolution of wooden
barrels interweaves constantly with
that of wooden boats – those built with
bung hole
stave
bilge
head hoop
rivet
cant
head
stave joint
bilge hoop
quarter hoop
chime
croze
Parts of a cask
48 z Brewer and Distiller International April 2016
Ceramic dolia, used to ferment and store wine
www.ibd.org.uk
BIOCHEMISTRY l
Moving wine in Roman times
‘Tubs’ from tomb of Hesy-re; coopered above,
copper below (with lids); ca. 2690 BC
timbers and planks”. But as Twede
says: “Wooden barrels are related to
basketry, which is one of the earliest
packaging technologies,” adding that
“The word ‘cooper’ is related to the
word ‘coop’, which originally referred
to a basket for chickens.” Others have
proposed that ‘cooper’ comes from the
Latin cupa, meaning ‘cask’.
Some early history
The robust wooden cask was an effective wine storage replacement for the
ceramic amphora, which was more
fragile and suffered from an inherent
instability, and the larger, ceramic,
dolium which was used to ferment
and store wine and was often semiburied for that purpose. Apart from
being more robust, casks could be
constructed to greater dimensions and
their very shape allowed close contact
storage; in relation to amphorae,
wooden casks could be made much
larger without adding significantly to
their weight and fragility.
Most authorities would accept that
in the Mediterranean region it was
around AD 500 that wood replaced
clay as the major construction material for wine storage. The main use
for wooden casks today is for ageing
of alcoholic drinks, mainly wines and
spirits although beers now figure more
prominently. For many wines, distilled
beverages, and beers, wood ageing is
one of the most important (and expensive!) quality-defining factors.
The 5th century BC Greek historian
Herodotus of Halicarnassus (ca.484425 BC), who travelled widely, related
that around 800-900 BC, wine was
exported by river from Armenia to
Babylon in casks made of palm wood.
www.ibd.org.uk
Such wood is not ideal and would be
avoided these days because it is difficult to bend and fashion. Because
wooden artefacts do not usually survive and therefore contribute little to
the archaeological record, it is difficult
to ascertain exactly when and where
casks evolved. The ‘favourites’ are the
Celtic tribes of central and western
Europe who were skilled in both woodcraft and metallurgy, but the Vikings
and Romans also have claims. Dates
for the original Celtic involvement with
casks seem to be around 900-800 BC.
Long before this, open wooden
buckets, clearly made by a cooper,
are documented from the end of the
Old Kingdom in ancient Egypt. The
evidence comes from James Quibell’s
drawings from the tomb of the ancient
Egyptian official Hesy-re (ca. 2690 BC),
which show wooden staved buckets as
part of an extensive row of measuring
containers. In Plate XIII he illustrates:
“Four tubs, two of wood two of copper... probably for measuring corn.”
He remarks upon the graining of the
wooden ‘tubs’, “which is best shown
in the largest tub but one, where five
planks are clearly distinguished...it will
be noted that real cooper’s work is intended – barrels with bevelled staves.
The hoops [made of ebony] at top and
bottom are square in section, those in
the middle are rounded.” Later, also
in ancient Egypt, a wall painting in the
tomb of Beni Hassan (1900 BC) shows
raisins (probably) being stored in open
wooden-staved containers.
to casks filled with firewood pitch and
tallow and were using them offensively. Previously, in 238 BC, the emperor
Maximinus, wanting to cross a swollen river in order to secure Aquileia,
a town in north-east Italy, ordered
his engineers to fashion a ‘floating
bridge’ out of redundant wine casks
in order to achieve his goal. So casks
had multiple uses! The celebrated
find of casks found at Silchester, the
Roman city of Calleva Atrabatum,
indicated the extent of trade between
Britain and mainland Europe during
the 3rd century AD. Made from silver
fir (Abies alba), they probably contained Rhenish wine, and had latterly
been used as well linings.
Kenneth Kilby notes “That by the
time of the Crusades (starting 1095)
wooden casks were the standard
means of transporting all manner
of liquids and provisions ... and the
cooper became one of the foremost of
tradesmen.” By medieval times, the
cask was Europe’s primary storage
and transport container, and with the
huge variety of products contained
therein, the nature of a cask’s contents would be known by tradesmen
by its size, style, and markings. By the
16th century, casks were so important
to trade of all kinds that ships were
designed specifically to hold them. In
reality, casks were then a currency of
sorts, their value being a combination
The Silchester casks
From the Roman Empire to the
Crusades
By the latter days of the Roman
Empire, the use of casks was clearly
widespread, for Pliny the Elder’s
Naturalis Historiae, in the section
dealing with ‘Wine Vessels and Wine
Cellars’, we find: “In the vicinity of the
Alps, they put their wines in wooden
vessels hooped around.” He also
describes how Julius Caesar found
himself facing an army of Gauls in
south-west France who had set fire
Cask identification marks (15th, 16th century)
Brewer and Distiller International April 2016 z 49
l BIOCHEMISTRY
Cooperages of different ages
Wooden hoops; Northern Europe, 16th century
of their size and content and because
of this, casks transcended language
and cultural differences. Traders, pirates and customs officials in the 16th
century would have learned the worth
of a cask size in much the same way
people today learn the worth of a £50
or $50 note!
Because of their value, casks containing wine or spirits were susceptible
to tampering. “Sucking the monkey”
is a term that was applied to the act of
drilling two small holes in a cask holding alcohol and the offender would then
blow through one hole which would
induce the liquid to flow from the other.
After the theft, the holes would be
plugged with small pieces of wood.
The wood of choice
The metal hoops used to hold staves
together were not in common use until
the late 18th century, prior to this time
staves were bound by strands of a
flexible wood such as willow, hazel, or
chestnut. With plentiful supplies, oak
became the wood of choice in Europe
and North America for wine storage.
In addition, the porosity of oak wood
permitted evaporation and oxygenation
of the liquid contents. In the days when
beer was fermented and then shipped
in wooden casks, internal pressure
was such that leakage was a major
problem and to counteract this, an inner layer of pitch was added to casks.
Overall, there are two basic types of
cooperage; ‘tight’ (or ‘wet’) and ‘slack’
1st stave cut
3rd stave cut
heartwood
sapwood
2nd
stave cut
quarter sawing
medullary
rays
Stave cuts in timber processing (from Russell, 2003 – by
kind permission)
18th century French cooperage
16th century German cooperage
(or ‘dry’). The former is for storing
and transporting liquids, the latter for
other goods. Some dry cooperage was
for carrying dry cargo that must not
get wet (e.g. gunpowder) and this is
defined as ‘dry-tight’
Wood and drink
Experience tells us that fresh distilled
spirit can be undrinkable because of
its harsh taste and pungency, as well
as the fact that its consumption can
produce harmful side effects. Prolonged storage in oak casks imparts
colour, a complex aroma, and harmony
in the mouth. Not only are complex
wood phenolics extracted into the
spirit, but structural molecules, such
as cellulose and hemicellulose are depolymerised and the products thereof
are also extracted. In all, around 200
substances can be extracted or transformed from oak by ethanol which will
directly participate in the formation of
flavour, aroma, and mouth feel of the
final product.
Since the production of spirits
and wine would have originally been
seasonal activities, it is assumed that
the practice of maturing these drinks
would have evolved from the need to
store the final product. Manufacturers would soon have found that oak
casks were usually the most suitable
containers for doing so. Even so, John
Conner believes that: “Historically the
origins of maturation are obscure, with
the majority of whisky in the 18th and
19th centuries being probably drunk
un-matured.” Bio-geographical criteria would determine the wherewithal
for storage which would vary with tree
species availability. In many parts of
50 z Brewer and Distiller International April 2016
Cooperage at T. & R. Theakston, Masham, ca.
1990
North America or in much of continental Europe, for example, the climax
vegetation could be mixed deciduous
forest and there were plentiful supplies of oak species. This meant that
wines and spirits produced in these areas could be stored in new casks, but
in most of Scotland, where the climax
vegetation is mixed pine/birch forest,
there was a paucity of mature oak and
so a constant supply of new wood was
not forthcoming, meaning that secondhand containers became de rigueur.
These had mostly been used for
the maturation of bourbon or for the
fermentation and shipment of sherry.
In Scotland, ex-bourbon (from US) and
ex-sherry casks (from Spain) are used
repeatedly (‘re-filling’) until they can
no longer fulfil their function (i.e. ‘exhausted’) whence they may be regenerated in the cooperage. Sherry casks
have a longer ‘maturation capability’
than ex-American bourbon casks,
and would normally be preferred but
demand for these exceeds supply.
In the case of Scotch whisky, the
maturation period became so important that it became incorporated
into the legal definition of the drink!
To conform to this, one finds that a
considerable proportion of the production costs of Scotch whisky distillers
are attributable to cooperage oak
investment. Storage in oak casks is an
expensive process in any stored/aged
alcoholic drink.
Tree type and basic wood
chemistry
An oak is a tree or shrub in the genus
Quercus. There are around 600 species of Quercus worldwide, of which
www.ibd.org.uk
BIOCHEMISTRY l
about 20 are economically important.
Just to confuse taxonomy, many
Quercus species readily form hybrids.
Historically, the species most used
in cooperage are Q. alba (American
white oak), and two European species, Q. robur (pedunculate oak) and Q.
petraea (sessile oak). In practice, they
can often serve the same purposes,
but there are differences in wood
extractives; most notably the fact that
American oak wood contains a lower
ellagitannin content than the European
species, and its wood is denser and
more coarse-grained than European
oaks. Other American oak species that
have been used include: Q.bicolor, Q.
macrocarpa, Q. stellata, and Q. lyrata,
while Q.pyrenaica is an alternative
European species. The important
Mediterranean species, Q. suber, is
only used for cork manufacture.
The best oak wood for casks is
from slow-growing trees grown in
dense groves, which produces an
upright trunk growth with a straight
grain and few knots. Slow-growing
specimens produce wood with tighter
grain. Ideally, trees should be around
100 years old, with a straight unblemished trunk, and around five feet in
circumference. Only the wood from
ground level to the appearance of
the first lateral branches is used and
this should be able to yield up to four
casks, depending on size.
Once staves are cut from the trunk,
they are left to dry for 3-5 years (seasoning), a process that will reduce the
Wood polymers
oak heartwood
major cell-wall components
extractives
lignin
hemicellulose
cellulose
guaiacyl &
syringyl propanes
hexoses,
pentoses
hexose:
D-glucose
phenolics
polyphenols
(ellagitannins)
simple phenols
fatty acids
other extractibles
lactones
alcohols
hydrocarbons
norisoprenoids
inorganic substances
Oak heartwood components
Wood tissue is composed of discreet cells and an intercellular
material that separates the cells, called the ‘middle lamella’,
and three insoluble polymers comprise the main part of (oak)
wood: cellulose, hemicellulose and lignin.
Cellulose, which represents about half of total wood content,
constitutes the wood ‘framework’. It is a linear polymer with a
uniform chain structure, individual glucose-based units being
bound by β(1-4)-glycosidic linkages. Internal H-bonding between
chains yields a cell wall framework onto which other molecules
can adhere.
A matrix for the cellulose superstructure is provided by
hemicelluloses which are branched heteropolymers containing
a variety of sugars, such as hexoses and pentoses. In oak wood,
hemicelluloses comprise some 15-30% cell wall dry weight, and
they are largely xylose based (i.e. xylans).
The third major component, lignin, normally around 1530% in woods, is a highly-branched three-dimensional polymer
structure, and, after, cellulose, is the second most abundant
polymer in the plant kingdom. Lignin confers mechanical
strength to a tree by helping to bond cellulose and hemicellulose
together, as well as providing protection through its antioxidant
capabilities.
HO
A description of lignin
OH coniferyl alcohol
would be: “An
H3CO
amorphous,
OCH3
polyphenolic
material arisHO
ing from an enzyme-mediated
OH sinapyl alcohol
dehydrogenaH3CO
tive polymeriwww.ibd.org.uk
zation of three phenypropanoid monomers; coniferyl, sinapyl and
p-coumaryl alcohols.” The latter is only founds in grasses.
In oak, lignin is formed by the polymerisation of coniferyl alcohol and sinapyl alcohol, yielding ‘guiaiacyl lignin’ and ‘syringyl
alcohol’ respectively. Cell walls contain all three polymers, while
the middle lamella, which cements cells together, consists of a
pectic base mainly impregnated with lignin (around 30% of all
lignin). Oak woods vary in composition, and the heartwood of Q.
alba, for example, is composed of cellulose (49-52%) lignin (3133%), hemicellulose (22%), and a fraction which can be extracted
in hot water or ethyl ether (7-11%). The latter contains volatile
oils, volatile and non-volatile acids, sugars, steroids, tannic compounds, pigments, and inorganic compounds.
In their native form, the three major wood polymers contribute nothing to beverage flavour, but via a series of reactions,
such as hydrolyses, oxidations, esterifications, acetalisations,
and polymerisations, their structures are modified to provide the
required compounds for the visual, taste and flavour characters
for the stored beverage. Pyrolysis of cellulose and hemicellulose during coopering leads to the formation of substituted
furans and pyrans, the most significant entities being furfural
and 5-hydroxymethyl furfural (5-HMF). Phenolics resulting from
the breakdown of lignin and which are often present in seasoned
and toasted wood include hydroxybenzoic acids (e.g. gallic,
syringic, and vanillic) and hydroxycinnamic acids (e.g. p-coumaric and ferulic). Related compounds include hydroxybenzoic
aldehydes, such as vanillin, syringaldehyde, coniferaldehyde,
and sinapaldehyde. There are several other volatile phenols and
most of these compounds can be categorised according to their
biogenic or chemical origin; for example those based upon the
guaiacyl (4-hydroxy-3-methoxyphenyl) or syringyl (4-hydroxy-3,
5-dimethoxyphenyl) nucleus emanate from lignin degradation.
Brewer and Distiller International April 2016 z 51
l BIOCHEMISTRY
Tannins
Classification
Non-hydrolysable tannins were called ‘condensed tannins’, and
consisted of oligomeric and polymeric proanthocyanidins. To complicate matters, some ellagitannins are not totally hydrolysable
because they possess additional bonds (due to the C-C coupling
of their catechin units to glycosidic moieties). Such tannins were
originally called ‘non-classified tannins’, but this changed to ‘complex tannins’. Over the years there have always been nomenclatural problems over the disparity in ‘tannin’ molecule size, with both
large and small molecules exhibiting ‘tannin properties’.
tannins
hydrolysable tannins
complex tannins
ellagitannins have been characterised in oak wood; castalagin;
vescalagin; grandinin, and roburins A-E, and most can be extracted
by aqueous alcoholic solutions (and other organic solvents). The
two most abundant are the stereoisomers vescalagin and castalagin, which comprise 40-60% of all ellagitannins. and the others
are either dimers of these two or differ by presence of a pentose
substituent.
In the European oaks, Quercus robur and Q. petraea, ellagitannins can comprise up to 10% dry weight of heartwood. In addition,
flavanols, including dihydroflavonols, often occur in association with
ellagitannins to give flavano-ellagitannin derivatives such as acutissimin A and acutissimin B. These compounds can occur in whisky,
as can the related gallagyl-glucosides peduncalagin and punicalagin. The level of these latter compounds in a product depends
upon oak species and wood seasoning and toasting.
condensed tannins
gallotannins
ellagitannins
Recent advances in tannin chemistry have seen a reassessment of classification and this has meant that tannins and ‘related
polyphenols’ can now be placed in two categories: Type A, with
“constant structures” and Type B, “of variable composition”. This
distinction can be used alongside the previously mentioned standard classification of ‘hydrolysable’ and ‘condensed’ tannins.
From the point of view of this article, it is hydrolysable tannins
that provide the major compounds in oak heartwood, which contains high concentrations of these phenolic extractives, principally
the ellagitannins. With more than 500 natural products identified
so far from the plant kingdom, these are by far the largest group of
known tannins. Gallotannins are the simplest hydrolysable tannins,
containing a polyphenolic and a polyol residue, mostly derived from
D-glucose. The hydroxy functions of the polyol residues may be
partly, or fully, substituted with galloyl units. Gallotannins are easily degraded by many microbes, while ellagitannins are relatively
resistant because of the more complex structure conferred by the
further C-C coupling. The presence of gallotannins in oak heartwood has not been unequivocally determined.
Ellagitannins are derived from
HO
biosynthetic stepwise oxidation
of gallotannins and subsequent
O
HO
oligomerisation processes, and
OH
can be classified according to their
OH
HO
biogenic oxidation stages. They are
HO
esters of hexahydroxydiphenic acid
OH
(HHDP) and a polyol, usually gluO
cose or quinic acid. HHDP is the
product of the first-stage biogenic
OH
oxidation of galloyl groups. Ellagitannins have enormous structural Hexahydroxydiphenic acid (HHDP)
variability because of the different linkages of HHDP residues with
the glucose moiety and their strong tendency to form dimeric and
oligomeric derivatives.
Ellagitannins in oak wood
When exposed to acids or bases, ester bonds are hydrolysed and
HHDP spontaneously rearranges into water insoluble ellagic acid
(EA). Ellagitannins are thought to derive from a common gallotannin biosynthetic precursor, penta-O-galloyl-β-D-glucose. Eight
52 z Brewer and Distiller International April 2016
The eight oak heartwood ellagitannins
The amount of vescalagin and castalagin that is extractable by
organic solvents declines as the heartwood ages, due to a decrease
in solubility caused by tannin polymerisation. Hydrolysis of castalagin gives ellagic acid and castalin and that of vescalagin gives
ellagic acid and vescalin. Despite the high solubility of hydrolysable ellagitannins in ethanol-water solutions, they are not found in
spirits or wines that have been matured in oak casks. Admittedly,
ellagitannins are rapidly denatured when wood is heated during
toasting but since they are not present in spirits aged in nontoasted casks, this cannot totally explain ellagitannin absence in the
final product. It has long been known that the relationship between
known wood-derived compounds and distilled alcohol is not always
straightforward, for some, such as vanillin, are often found in newmake spirit, albeit at low levels. In malt whisky production, this is
likely to be attributable to barley lignins emanating from the wash.
Using the ratio of vescalagin to castalagin, it has been shown
that the composition of ellagitannins varies with heartwood and that
this is independent of the decline in the concentration of extractable ellagitannins with increasing heartwood age. Wood density
is another important factor, as is flexibility which is governed by
way in which the long wood (‘medullary’) cells are arranged. The
conversion of sapwood into heartwood is critical since it determines
the number of blockages (‘tyloses’) formed in medullary cells and
these reduce the likelihood of leakage.
www.ibd.org.uk
BIOCHEMISTRY l
Brown-Forman.com
likelihood of cask leakage and extract
more tannin from the wood. Cutting
the trunk for stave production is a real
skill and follows a set pattern. Seasoning results in wood dehydration,
until it becomes aligned with ambient
humidity. During the process, there is
a reduction of hydrolysable polyphenols, such as ellagitannins, which are
polymerised and precipitated – thus
decreasing astringency. Most of this
occurs on the wood surface, but is also
known in inner wood. A combination
of the effects of rainfall, temperature
variation, and UV radiation contribute
to these chemical changes.
Toasting the wood
Later, during coopering, it is toasting
(or charring, at higher temperatures)
where the most significant changes
take place, the most important being
the decrease in ellagitannins and the
concomitant increase in ellagic acid.
In addition, the three main polymers
pyrolyse to yield volatile extractables,
such as guaiacol, 4-methylguaiacol,
trans-oak lactone, cis-oak lactone, and
vanillin which are formed mainly from
the degradation of lignin. Over 100
volatile phenolics have been identified
from oak wood, with the oak lactones
being most important. Heat will also
destroy undesirable resinous compounds in wood (e.g. ‘sawdust’ aroma,
mainly due to trans-2-nonenal).
When considering seasoning and
toasting oak, it is important to realise
that many of the numerous flavour
compounds are produced at different
temperatures, and that oak species
toast differently. Seasoning and toasting ensure the structural integrity of
the finished cask. The former prevents
shrinkage, while toasting is aimed at
stabilising the curve of the wood. It is
the wood ultra-structure of certain oak
species that marks them out as ideal
for tight cooperage. For details of cask
manufacture see Conner et al., (2003)
and Work, (2014).
From sapwood to heartwood
Large quantities of tannins and related
polyphenols are laid down in oak during the transformation of sapwood
into heartwood. These compounds
provide protection for the plant against
invasive micro-organisms; such resistance being conferred by the ability of
tannins to complex with proteins and
polysaccharides. In addition to species
differences, the content of oak extractives varies with factors such as geographical origin and forestry practice.
www.ibd.org.uk
Tannins are polyphenolic secondary
metabolites of higher plants, sometimes called ‘plant polyphenols’ and,
because of their enormous structural
diversity, have enjoyed several disparate ‘definitions’ over the years. For
example, one (not so ancient!) organic
chemistry text defines tannins as: “Cand O-glycosidic derivatives of gallic
acid (3,4,5-trihydroxybenzoic acid)”,
which is far too specific because not all
tannins necessarily contain a galloyl
unit or derivative (i.e. condensed tannins which are built up from flavanoid
precursors. The name ‘tannin’ was
originally given to any plant extract
that exhibited astringency, without any
regard for its chemical structure.
Features that distinguish tannins
from other types of plant polyphenol
are the ability of the former to bind
to proteins, basic compounds, large
molecular compounds, pigments, and
metallic ions. Many tannins also have
important biological characteristics,
including anti-oxidant, antimicrobial,
and anti-tumour activity. The properties of tannins, which are moderately
size molecules, are based on the fact
that they have two or three phenolic
hydroxyl groups on a phenyl ring. According to the polyphenol groups in
their molecules, tannins were once
classified into two groups: pyrogalloltype and catechol (catechin)-type.
Subsequently, these two groups were
renamed to ‘hydrolysable tannins’ and
‘condensed tannins’ respectively.
Whisky
Regarding the chemical nature of
oak woods, most attention has been
paid to trees grown in France and
North America, where there are clear
species differences as well as those
due to geography, etc. American oak
has a lower ellagitannin content than
its French (European) counterparts,
and higher levels of β-methyl-γoctalactone particularly the cis isomer.
These cis- and trans-lactones are of
great importance in Scotch whisky
ageing and are known as the ‘whisky
lactones’. They are formed during
charring of bourbon casks or toasting
of wine casks alongside coloured compounds, which give the finished whisky
a ‘mature colour’.
Certain types of molecule, exhibit
great variation between these two oak
types and can be used as markers for
oak wood origin. Most notable in this
respect are norisoprenoids, several of
which have been found in significant
amounts in American oak, but are
absent/present in trace amounts in
European oak. Conversely, 4-oxo-7,8dihydro-β-ionol was a major norisoprenoid in European oak, but absent
in Q. alba. Over thirty norisoprenoids
have been identified from oak, the first
being β-ionone.
In bourbon manufacture, casks
tend to be re-used many times, resulting in a diminution of the overall
quality of substances leached from
the wood and the depth at which
leaching occurs increases. In some
experiments with cask staves prepared from Q. alba, it was found that
repeated exposure to whisky led to
less leaching. New charred (unused)
bourbon staves had more extractable
lignin hydrolysis products, whisky
lactones and coloured substances
than either used Bourbon or first fill
Scotch staves, and ‘exhausted’ Scotch
staves had the least. It has been
Brewer and Distiller International April 2016 z 53
l BIOCHEMISTRY
Wood-ageing beer at Epic Brewing, Denver, CO
suggested that the concentrations of
lignin-derived guaiacyl and syringyl
compounds could be used as an indicator of cask exhaustion.
Beer
Most research into wood ageing
relates to spirits and wines, with
relatively little published on beer.
With the never-ending search for new
products, especially by craft brewers, beer wood-ageing is now a fertile
field of study and there is an increased
consumer interest in such beer.
Superficially, one might propose that,
using the same wood and conditions,
a different rate and type of
uptake in wood extractives might occur
in spirits,
wines and
Oak wood
beers because of the decreasing level
of ethanol contained in these drinks.
For convenience, most experiments on beer seem to have been
carried out by using oak chips. During
brewing, malt and hops are the primarily responsible for imparting aromatics to beer. Phenolic compounds
such as ferulic acid, gallic acid, vanillic
acid, quercetin and vanillin are known
beer phenolics and contribute to flavour, colour and body as well as haze
and astringency.
In a Brazilian study, a young
Pilsner-style beer was subjected to a
3-month maturation period (at 0°C)
in the presence of French oak cubes
with different toasting regimes.
Oak barrels were also used. Beer
sample strength varied between
4.34 - 5.19% v/v and samples were
analysed monthly. Results indicated
that there were some interactions
between wood and beer but they
were “smaller than expected,” and
this was attributed to the low level of
alcohol in the beer, low maturation
temperature and short maturation
period. The lager stored with the
most heavily toasted cubes had the
highest concentration of low-molecular weight phenolics, followed by
beer matured in an oak barrel. It was
agreed that neither oak cubes nor
wooden casks affected the quality of
the beer.
In experiments carried out at KU
Leuven over a period of 60 days with
an 8.3% ABV ‘neutral blonde beer’
suffused with oak chips (identical
dimensions) of Q. robur and Q. alba,
toasted to different degrees (medium
54 z Brewer and Distiller International April 2016
and heavy), an increase in concentration of nine monophenols (vanillin,
acetovallinone, syringaldehyde, acetosyringone, guaiacol, 4-ethylguaiacol,
eugenol, thymol, and salicylaldehyde) was observed. Wood origin and
degree of toasting influenced both
monophenol concentrations and their
sensory effects. Medium toasted
chips promoted eugenol, thymol, and
salicylaldehyde levels in beer, while
the other six monophenols were most
associated with heavy wood toasting.
Beer flavour was greatly influenced
by wood, with notes such as ‘vanilla’, ‘woody’, ‘smoky’, ‘creamy’, and
‘burned’ being to the fore.
Further work with model solutions showed that process parameters
influenced the availability of the nine
monophenols mentioned above. They
all increased quasilinearly in intensity
with increasing amounts of oak chips
and their extraction was enhanced
by low pH (3.5) and higher ethanol
content (→8-10%v/v). Elevated storage
temperature (20°C) also enhanced
extractability, but, conversely, presence of yeast and oxygen decreased
extraction of some phenolics. Vanillin,
acetovanillone, syringaldehyde, and
acetosyringone are particularly unstable when oxygen is present.
Other wood for cooperage
In a comparison of East European (Romania, Ukraine, Moldova) Q. robur and
Q. petraea wood with that from Frenchgrown samples, and with American
Q. alba, discrete chemical differences
in their extractives were revealed. As
judged by their ellagitannin and whisky
lactone levels, the greatest disparity
was between American- and Frenchgrown wood, whereas East European
samples were somewhat intermediate between these extremes. Eastern
European wood could be characterised
www.ibd.org.uk
BIOCHEMISTRY l
by high levels of eugenol, 2-phenylethanol and aromatic aldehydes.
Oak wood is still the main material used in cooperage, certainly
for products such as vinegar,
cider, and some spirits such as
brandy, although only oak and
chestnut (Castanea sativa)
are at present approved by
the International Organisation of Vine and Wine (OIV) for
winemaking. Some producers prefer their drink to be
non-oak matured, and this is
not always on financial grounds.
A compromise would be to use oak
casks with non-oak staves incorporated.
When assessing other woods for
ageing suitability, it is the phenolic
fraction that is the most important single parameter. Among woods that have
been studied extensively for possible
cooperage use are chestnut (Castanea
sativa), , cherry (Prunus avium) acacia
(Robinia pseudoacacia), ash (Fraxinus
excelsior; F. americana), and to a lesser
extent, mulberry (Morus alba; M. nigra).
Before toasting, each wood exhibits
a different and specific polyphenolic
profile, with both qualitative and quantitative differences. Toasting changes
these profiles according to the level
of heat applied and wood thus treated
will assume its own set of phenolic
markers.
In toasted cherry wood, for example, methyl syringate, benzoic acid,
methyl vanillate, p-hydroxybenzoic
acid, 3,4,5-trimethylphenol, and pcoumaric acid, the flavonoids naringenin, aromadendrin, isosakuranetin
and taxifolin can be used as markers.
Cherry wood also contains many condensed tannins of the procyanidin type
which can be useful markers. In acacia
heartwood, where there are relatively
low levels of condensed tannins and
no hydrolysable tannins, one finds high
levels of the flavonoids robinetin and
dihydrorobinetin, which are found in no
other cooperage woods. The latter is a
marker for ‘upmarket’ vinegars aged
in acacia.
Chestnut wood
Chestnut wood, which is especially
rich in gallic acid and ellagitannins,
has been used enologically around
the Mediterranean for years. It is now
known that chestnut heartwood has a
polyphenolic profile very similar to that
of oak. A difference, however, revolves
around 1-O-galloyl castalagin, which
seems to originate from the esterificawww.ibd.org.uk
acacia, cherry and mulberry (in
that order). Over 50 different
volatiles were identified from
these woods, with each species showing a characteristic
profile. Oak proved to be
the richest in volatiles, with
chestnut running second;
mulberry had the lowest
levels of volatiles.
Accelerated ageing
Chestnut
tion of castalagin or vesacalagin with a
gallic acid residue.
Chestnut and oak are both in the
same flowering plant family, Fagaceae, as is beech (Fagus),and both are
characterised by having high levels of
ellagitannins after seasoning, especially vescalagin and castalagin. After
light toasting of seasoned chestnut,
concentrations decreased by 70%,
and this was increased to a 95% loss
after an intense toasting. Levels of
ellagitannins in seasoned and lightly
toasted oak followed the same pattern, but detected levels were lower
than chestnut; even an elimination of
ellagitannins from heartwood surface
layers being observed after heavy
toasting. Since it is only stave wood
that has to be toasted, higher levels
of ellagitannins will be available for
extraction from cask head wood.
Gallotannins are present in chestnut
wood and were found at their highest
levels in seasoned wood. They also
showed a 70% decrease after a light
heat treatment and a 95% reduction
at higher temperature. Such facts
allow us to distinguish chemically
between these two woods.
When oak, chestnut, acacia,
cherry and mulberry woods were
exposed to a 50:50 ethanol-water
solution (‘model spirit’) and a 12%
ABV ‘model wine’ solution, there was
considerable variation in polyphenol extractability. The model spirit
extracts of chestnut and mulberry exhibited the highest total polyphenols,
followed by cherry, acacia and, finally,
oak, whereas the model wine solution
extracted most polyphenolic material from oak – followed by chestnut
For drinks that must be matured for extended periods, such
as brandy, where some 200 phenolics may be extracted from wood,
any means of hastening the process
has to be considered. Whatever the
method, there should be no deleterious effect on those phenolics necessary for taste, aroma, etc. Several
physical methods have been used to
accelerate wine ageing processes,
including γ-irradiation, ultrasonics,
UV visible light, as well as electric
fields (EFs), including pulsed AC
electric field (PEF) and static electric
field (SEF).
In general, these methods are
efficient, non-thermal, and inexpensive. For example, it has been shown
that treatment with 20 kHz ultrasonic
waves aged rice wine much more
quickly than standard methods, and
that γ-irradiation rapidly produced
very high quality rice wine. Treatment
of a young red wine with PEF gave an
increase in the concentration of most
phenolics.
When EF treatments were applied to brandy ageing, promising
results were obtained. Most beneficial
substances, such as tannins, total
phenols, volatile phenols, and, esters
were increased, while some harmful
ones (acetaldehyde, acetal, and higher
alcohols) were decreased. Most of this
sort of work is carried out on small
casks (i.e.2L or 5L), but scientists
believe that similar effects should
exist when EF treatments are applied
to production size casks (i.e. 225L or
above).
References
Kilby, K. The Cooper and His Trade. J. Baker,
London. 1971.
Russell, I. (ed.) Whisky, Technology, Production and Marketing, Academic Press,
London. 2003.
Twede, D. The cask age: the technology and
history of wooden barrels. Packaging and
Technology Science. 18(5): 253-264. 2005.
Work, H.H., Wood, Whiskey and Wine: A
History of Barrels, Reaktion Books, London.
2014.
Brewer and Distiller International April 2016 z 55
● DISTILLING
The Cask
An amazing element in the
wonderful process of making whisky
©istock.com/ksmith0808
By Billy Mitchell
Much has been researched
and written to explain in
great technical and scientific detail the various
processes used in the production of whisky – many
articles have been written
on raw materials, malting,
mashing, yeast, fermentation and distillation of many
new make whiskies.
ith this article I hope to provide
some generic insight to another
key element in the process of making whisky – the cask. This is not a
detailed scientific explanation of cask
construction, wood treatment or even
the maturation process itself. I would
say this is an introduction into yet another amazing element of the wonderful world of whisky.
Cask – the history
The use of watertight, barrel shaped
wooden containers that could be rolled
and stacked has been recorded in history dating back to around 300 – 400
years BC. These containers proved to
be very strong and could withstand
the stresses of the basic methods of
transport at that time. It is likely the
techniques of bending wood into shape
©istock.com/EdStock
W
A cooper works on a cask at the Speyside Cooperage
by heating were borrowed from the
boat builders of yester year who used
this technique to bend planks of wood
for the hulls of ships.
Historically the barrel was used
to hold water, wine and other liquids
such as vinegar, oil and water, but also
became the method of choice for storage and transportation for solids such
as metals, nails and gun powder. It
could also be used for the preservation
of foods such as fish and other sweet,
slated or pickled food. Historic records
22 ❚ Brewer and Distiller International May 2015
show that ancient coopers’ tools dating
back to 100 years BC have been found
and recorded.
The barrel or cask has proven to be
the most convenient method of storage
and shipping for almost 2000 years
– apart from storage and transport
of many of the materials previously
noted they were ideal for carrying all
types of bulk goods. The wooden cask
proved much easier to manhandle
and was significantly sturdier than the
cheaper bags and crates – on a weight
www.ibd.org.uk
DISTILLING ●
The various sizes and shapes of the most commonly used oak casks
for weight basis they could be readily moved. However, much the same
as the whisky and other industries
introduced more modern production
processes, the wooden cask lost its
place as method of choice for storage
and transportation and was eventually
replaced by pallet based and containerisation systems introduced during
the 20th century.
With the movement of casks across
the globe to support all types of trading activity it should be of no surprise
to learn that the early distillers, either
illicit or eventually legal, came across
the opportunity to store their product
in these wooden containers. During
the years of illicit whisky production in
Scotland the transport of the distilled
liquid was limited as concealment of
the distillate was required. Transportation was mostly carried out using
disused wine, sherry and port casks
originally from Southern Europe. The
transport of whisky in wine, port and
sherry casks brought about differences
in the whisky which is still recognised
in the maturation processes of today.
I suspect very little was known or
understood about the importance of
wood at that time other than it had an
effect on the liquid. Eventually as the
Whisky Industry grew it became very
aware of the importance of maturation
in wood to soften otherwise a very immature product. The role of wood was
finally recognised!
There has been much research
into this very important facet of whisky
production and I recommend and direct
your attention to the respective chapters
by John Conner, Ken Reid and Frances
Jack in the first edition and by John
Conner in the second edition of Whisky,
Technology, Production and Marketing edited by Inge Russell and Graham
Stewart, for more detailed reading.
The cask – a maturation container
for Scotch whisky
There are several types of cask generally differentiated by the volume of
liquid they hold and their shape.
www.ibd.org.uk
By definition Scotch whisky, “has
been matured only in oak casks of a
capacity not exceeding 700 litres”. All
of the casks in Table 1 have been used
to mature Scotch whisky. They range in
capacity from the American Standard
Barrel holding ~ 200 litres to the largest, the Puncheon holding around 560
litres. The American Standard Barrel
has the greatest internal surface area
to liquid volume ratio and is often recognised as the industry standard for
maturing both malt and grain whisky.
The oak tree is the starting point
for all casks used to mature Scotch
whisky. There is yet another very large
industry supporting the sustainability
of the European and American oak
forests – I will not get into any real
discussion in this article but needless to say production volumes can
and do fluctuate. Once the mature oak
trees have been cut down, logged and
processed through the saw mill into
useable materials the wood can then
be turned into casks. The fluctuation in
new wood supply can have an impact
on the availability of wood entering the
coopering businesses although this
can be mitigated to some extent by
the requirement to hold large stocks
The wooden cask and its parts
A section through a head
A schematic showing all the elements which combine to
form a cask
of immature wood to dry and season
prior to raising into casks. This storage
acts as a buffering capacity between
the availability of new wood from the
saw mills and seasoned wood entering
cask production.
Oak is used to mature Scotch
whisky due to its physical and chemical properties. Oak wood has physical
strength due to its natural development during growth which allows the
wood to be bent by heating without
splitting. This gives strength when
formed into a cask. Oak has a tight
grain which prevents leakage of
product - it is also porous and allows
Size and Shape
Type
BUTT
HOGSHEAD
American
Standard
Barrel (A.S.B)
PUNCHEON
DUMPY
PUNCHEON
Approx. Capacity
(litres)
500
254
191
558
463
Height (m)
1.27
0.86
0.88
1.13
0.99
Max. Diameter (m)
0.90
0.72
0.63
0.96
0.95
Internal surface
area (m2)
3.55
2.25
1.90
3.78
3.34
Surface/Volume
Ratio (cm2/l)
71
88.5
100
67.5
72
Table 1 – Cask specifications
Brewer and Distiller International May 2015 ❚ 23
● DISTILLING
An example of the automated equipment for the mechanical
tightening of cask hoops – cask hardening.
permeation of air (oxygen) in and out
of the cask. The oak is also different to
other wood species like pine or rubber
trees which contain resin materials that could eventually pass strong
flavours into the maturing liquids.
Chemical transformations can be produced on the internal surfaces of the
oak casks by the use of various heat
treatments after the cask has been
raised. These treatments can include
various degrees of toasting and charring of the internal surfaces – each different heat treatment has the potential
to deliver different flavour profiles in
the maturing whisky.
There are two main sources of
cask for the Scotch whisky businesses.
The majority of casks are sourced as
ex-bourbon wood from the bourbon
industry in the United States. They are
all produced from American White Oak
(Quercus Alba) and these are the most
commonly used cask in the Scotch
whisky industry. The other source of
casks is from the European sherry,
port and wine businesses – these
casks can also be produced from
American White Oak but are more
commonly made from Quercus Robus
and Quercus Petraea, both European
species of oak.
By definition all bourbon must
be aged in new, charred oak barrels known collectively as American
Standard Barrels (ASB’s). The scale of
this industry, either through expansion
or contraction, has a corresponding
knock on effect on their sourcing for
Scotch whisky maturation. American
Standard Barrels are a fairly new addition to the Scotch whisky industry
becoming readily available from the
end of the Second World War. A new
law was formulated at that time by the
Cooper’s Union in the United States
that stipulated that bourbon must be
matured in new oak barrels – this was
done to boost the coopering industry
after its collapse during Prohibition.
This resulted in a massive increase in
numbers for Scotch whisky matura-
tion. Initially these casks were broken
down in “shook” bundles for transport
across the Atlantic before being rebuilt
in cooperages in Scotland – although
these practises still remain in some
cases, ASB’s are now more commonly
transported as casks by container to
Scotland.
Similarly in Europe, sherry, port
and wine casks are generally only
used once in the maturation cycle for
these products and again they then
become available for Scotch whisky
maturation. Changes in scale of these
European businesses also impacts on
the availability of this wood. Importation of sherry from Spain to the United
Kingdom started in the mid-1800s –
the casks used to transport the sherry
were made from Spanish oak – these
are still used by many distilleries even
although they are substantially more
expensive than the American bourbon
cask.
Some distilleries in Scotland also
use specialist oak casks for finishing product – these are generally
sourced from specific oak species
to deliver alternative flavours to the
final product. American oak is seen
as perfect for whisky cask construction – the trees are fast growing with
tall straight trunks, giving quality wood
with high levels of flavour compounds.
It is said that approximately 90% of
all the world’s whisk(e)y is matured in
American oak ex-bourbon casks. The
Three casks sit in the firing chamber at various stages of the charring and heat treatment process as part of cask rejuvenation.
24 ❚ Brewer and Distiller International May 2015
www.ibd.org.uk
DISTILLING ●
Ideal maturation
Taste
Cask too strong
Additive Activity
Cask too weak
Distillery character
Time
Un-matured whisky (subtractive activity)
Cast maturation (additive activity)
Figure 1 shows an overview of the maturation process
key flavours associated with American
oak include vanilla, honey, nuts, fudge
and ginger spice – this is not a fully
inclusive list.
Spanish or French are the two
predominant sources of European oak,
although there is now wood available
from the Eastern European countries –
these were either made for the sherry
or wine industries in their respective
countries. They are generally much
slower growing species when compared with American white oak. The
key flavours associated with theses
oaks include sherry, dried fruits, wood,
caramel, orange and various spices –
again this is not a fully inclusive list.
Maturation – a brief overview.
Many of the flavours and characteristics of whisky are picked up from the
oak casks in which it is matured – by
law this must be for a minimum of
three years in Scotland.
Research has shown that changes
in raw materials, pre-distillation and
distillation processes can alter the
quality and character of the new make
whisky distillate – this is also applicable to the maturation process where
wood type, wood treatment by toasting
or charring and length of time in cask
can have a significant impact on the
flavour and character of the matured
liquid.
There are three main contributors
to the total maturation phase – these
are the actual distillery character of
the new make whisky, the subtractive
activity of maturation and the added
activity.
All casks used in the Scotch whisky
industry have either been toasted or
charred prior to use in their original roles as maturation vessels for
bourbon, sherry, port or wine or have
www.ibd.org.uk
ing maturation, this well known filter
aid is capable of removing odours,
particularly sulphur compounds, from
the maturing liquid which were formed
during the fermentation or distillation
processes.
undergone some form of rejuvenation
prior to their ongoing use in whisky
maturation. Charring an oak cask involves a number of complex reactions
which take place over a very short
timeline. Whisky casks are charred
to create flavour, colour and different
aromas - a char layer acts as a filter
and to break down the oak cell walls
so the spirit can extract flavour from
the oak.
Charring a cask requires the total
interior of the cask shell to catch fire
for a pre-determined length of time.
This will either take place when the
cask is first raised or during a rejuvenation process where an exhausted
cask has had the original charred
layer removed leaving a new fresh
oak surface area available for a new
charring process. Ideally the charring
process should take place with both
ends removed to ensure consistent
and efficient firing. The inside of each
cask catches fire and in that short
period of time a char layer is created
on the internal shell of the cask. As the
heat from this process gradually penetrates into the oak the chemistry is
significantly modified in these interior
wood layers – sugars are caramelised
giving both flavour and colour and
vanillin and other flavour characters
are created. Each whisky producer
will have their own standard of char
which supports the quality required for
their own product portfolio – once the
desired level of char has been reached
the casks are normally sprayed with
water to halt the charring process.
Subtractive Activity
The process of charring produces
carbon on the internal surface of
the oak staves. While this does not
directly contribute to the flavour dur-
Lighter char levels tend to show fruit
esters and spice notes while the heavier char gives rise to extraction of oak
flavours into the spirit more quickly.
During the process of caramelisation,
wood sugars are transformed into
many new aromatic compounds – the
“toasty” compounds in the wood will
be extracted into the liquid during
maturation contributing to the colour
and flavour of the product.
Figure 1 gives a very simple
overview of the maturation process
over time. It can be seen the distillery
character changes very little over
time and this is driven by raw material selection, mashing and fermentation conditions and the total distillation process. The subtractive activity
is driven by the condition of the char
layer on the internal surfaces of the
cask and the quality of the original
new make whisky. The additive activity
is driven by the many chemical reactions within the wood / liquid interface
and can be variable depending on the
condition of the cask.
If the maturing liquid is left too
long in an active cask it may eventually
be described as woody while conversely if matured in an unreactive cask
for a short period of time it may be
described as immature. What seems a
very simple process from an external,
non-blender perspective is in fact a
very complicated algorithm with many
complicated, interactive and interdependent reactions. This is where the
experiences, skills and understanding
of each of the Master Blenders from
the Scotch producers comes into play
– they will fully understand the many
variables in the production processes
through maturation and will ensure
that stocks are laid down to ensure
final product quality is achieved on an
annual basis.
Key to the whole process will be
the provision of a wood policy, most
likely to be specific for each whisky
business, which, when applied, will
ensure that all the variables previously
noted are addressed to ensure that
product quality is not compromised.
These Master Blenders will also use
their skills and expertise in both wood
and liquid quality to continually create
Brewer and Distiller International May 2015 ❚ 25
● DISTILLING
A selection of coopers tools including plucker (or downright), croze board, topper, heading knife, driver, cooper’s
hammer, crumb knife, punch and adze.
new products for the ever changing
demands of the market place.
whisky maturation process. Generally
coopers working in these larger types
of cooperages were known as PBR
(paid by results) coopers and their salary was directly related to the output
and performance of their own work.
There were many payment schemes
where coopers were paid by either
minute values (time allotted for each
repair type) or by the scale of the tasks
required on each cask and even by the
number of staves replace or repaired.
Needless to say these roles, either as a
service cooper or PBR cooper required
great skill, energy, stamina and overall
physical strength in what was and
remains a very demanding job.
Much has changed over the years
where centralised cask filling and
maturation warehouse operations have
more or less replaced the small tra-
ditional filling stores and warehouses
at distillery sites and the coopering
industry has followed a similar model
– centralised coopering operations
are the norm with little or no coopering presence on the majority of the
distillery sites. The physical demand
on the service or PBR coopers led
the industry to seek out mechanical
solutions and aids to minimise many
of the physical, labour intensive tasks
required to breakdown, repair and
re-build casks. There now remains a
much smaller coopering footprint in
Scotland where mechanical, semimechanical, and traditional manual
operations are undertaken to repair,
rejuvenate and re-build casks for the
whisky industry.
In the early 1970s there were
some 1151 coopers and apprentices
The cooper – a master craftsman.
“Cooper” is the name given to the
tradesman who makes or repairs
casks or wooden barrels – the cooper
would generally work in a cooperage.
The whisky industry has changed
markedly over the years with industry
consolidation, distillery closures and
distillery and central warehouse developments, varying levels of production
and modern technology replacing some
of the more traditional processes – a
similar picture can be painted of the
coopering industry. When I started my
career in the whisky industry in 1970
almost every distillery would have at
least one cooper working on site – in
fact most distilleries had a cooperage,
the size being dependent on the scale of
distillation, cask filling and warehouse
activities – Her Majesty’s Revenue &
Customs officers also had a significant
presence on the distillery and maturation warehouse sites at that time. The
primary role of the distillery coopers at
that time was to repair leaking casks
during maturation and also to ensure
that casks were fit for filling prior to
maturation to ensure any loss of revenue was minimised – generally their
role was that of a service cooper.
At that time there were also a significant number of larger, more centralised cooperages where casks were
raised from shook bundles purchased
in the US or Europe and where American Standard barrels were enlarged
into hogsheads by the inclusion of
additional staves and enlarged heading wood. They would also be used to
repair and rejuvenate both old and new
stocks of wood prior to entering the
These photographs give a view of the differences in scale between a large centralised cooperage and filling operation (top) and a small distillery operation
26 ❚ Brewer and Distiller International May 2015
www.ibd.org.uk
DISTILLING ●
employed in Scotland compared to the
211 employed at the end of 2013. This
represents a huge change in coopering
operations in Scotland and could have
impacted on the provision of casks for
the whisky industry. Thankfully this did
not happen and the apprentice population has now risen to around 17.0 %
of the total cooper population in 2013
compared to some 13.0 % in the 1970s.
Historically an apprentice cooper
was required to serve some nine years
learning his trade before becoming a
fully qualified cooper – this has subsequently been reduced to seven then five
and now finally four years over time. An
apprentice will now likely spend three
three years in a traditional apprentice
workshop with a final year four in the
mechanical processes where these are
employed. It should be noted it takes
considerable skill to raise a cask from
anywhere between 30 and 35 oak staves
of different width, mild steel hoop iron
and end wood produced from other oak
staves - this is even more so when you
consider that there are neither nails or
glue, nor any other fixings or fastenings
used in this process.
The coopering industry has always
been and will always be dependent on
the scale and volatility of whisky operations and will expand and contract
in a similar fashion – this is inevitable
but the cooper will always play a huge
role in the quality of the final bottled
product.
Casks – the logistics piece
Alan Gray reported in his 2014 annual
review of the whisky industry there
were some 3.7 billion litres of alcohol
in maturation warehouses in Scotland
– this would equate to about 5.8 billion
litres of bulk whisky at 63.5 % alcohol
by volume.
If all this volume was matured
in American Standard Barrels there
would be some 29 million casks in
bond – conversely if this volume was
matured in large wood (butts), there
would be some 11.6 million casks in
bond. Obviously these figures are at
the extremes and the reality is the
actual number will fall somewhere
between the two, with significantly
more of the smaller casks in bond so
the actual number will trend towards
the higher figure.
If we assume the average age of
bottled Scotch is, say, six years then
a sixth of the maturing stock will be
disgorged on an annual basis and the
same volume of new make will be
filled to maintain a stock balance - in
www.ibd.org.uk
An apprentice hard at work in the workshop
learning the arts, crafts and skills of the cooper.
actual fact the volume of new make
filled currently is higher than that
disgorged due to the growth in the
industry.
If these assumptions mirrored
reality there will be anywhere between 1.9 million and 4.8 million casks
disgorged in a given year with a higher
number being filled. If we assume
there are, on average around 80 casks
per lorry load there will be anywhere
between 23,750 and 60,000 lorry movements per year with a similar number
associated with new make fillings. The
truth is actually somewhat different to
the above assumptions as the range of
product in market ranges from product
at three years of age to many single
malts anywhere between 16 and 28
years of age.
I have used these assumptions to
demonstrate the huge logistics operation employed to support the industry
in Scotland. Using these extremes
and assumptions gives those outside
the industry an overview of the scale
of operations behind the disgorge of
mature product and the new make
whisky supply chains. In reality much
of both new make and mature product
is tankered between production, filling
stores, maturation warehouses and
bottling and blending operations to
simplify logistics and minimise haulage costs. Obviously consolidation and
centralisation of whisky filling, coopering activities, maturation warehouses
and mature whisky disgorge operations also goes some way to mitigate
A palletised cask warehouse
A racked cask warehouse
the overall impact of transport and
haulage costs.
This simplified overview of the
cask, the cooper, maturation and
the logistics piece gives yet another
great insight into the many varied and
skilled roles across yet another part of
the whisky production process – this
adds to the other skills and roles in
the malting and malt and grain whisky
production processes and should
demonstrate the real opportunities
available to those still looking for
challenging roles in what is currently a
vibrant industry.
Brewer and Distiller International May 2015 ❚ 27
● DISTILLATION
Is the oak industry under threat
from rapid maturation techniques?
iStock.com/liveslow
Moving things along
A look at rapid whisky maturation methods
By Roger Putman
It is an oft-quoted old saying, particularly amongst the
brandy fraternity, that: “You make brown spirits for your
sons and daughters and drink the ones your father made.”
This would suggest that it takes time to mature a spirit into
something of which a father might be proud. According to
California-based whisky writer, Bozkurt Karasu, Americans
lack one important virtue – and that is patience.
H
ence there has been a lot of interest lately, particularly in the USA
with its 1000 craft distillers, in producing an acceptable spirit in weeks or
months rather than waiting for years
and suffering concurrent losses of
volume and alcohol, high stock valuations, on-going warehouse storage
costs and having the Excise breathing
down your neck all the time.
On the more cautious side, Rosemary Gallagher at the Scotch Whisky
Association observed that “Traditional
production practices have built up the
reputation of whisky as a category. Consumers expect whisky to be matured in
wood and if they don’t get that, they are
being cheated. All whisky sold in the EU
must be matured in wooden casks for
at least three years, with the definition
of Scotch whisky requiring maturation
in oak. One can experiment with different techniques, but whether the final
product complies with the definition of
whisky in the market concerned (and
can therefore be described as whisky) is
a different matter. Such products would
have to be described as something else
– they should not benefit from being
described as whisky.”
Before we take a look at the various
methods supposed to save distillers a
36 z Brewer and Distiller International November 2016
fortune, perhaps we should consider
what occurs during maturation and
judge whether the science behind these
accelerations is likely to have any commercial effect. During the preparation of
this article, I was told of magic and black
arts by a number of Scots. Many text
books gloss over ten long years in a dark
cellar by the Hebridean Sea by saying
that the raw spirit fresh off the still is
diluted to around 64%ABV in casks previously used in the US bourbon industry,
the angels take their share and hey
presto, a few years later the magic happens after the black arts have weaved
their spells and the blender is presented
with something to play with!
Wood and brown spirits
Brown spirits are matured in wood and
include whiskey, brandies (cognac,
armagnac, pisco, aguardiente and
other grape products) as well as some
rums, cachaças and tequilas. I note
that the innovative souls at Watershed
in Columbus, Ohio, first produced a
barrel-matured gin back in 2012. They
were surprised when the sale of a
few bottles was announced and 750
folk turned up to buy some. Hernö in
www.ibd.org.uk
DISTILLATION l
Char 1
Brown spirits of course start off
from the still entirely colourless. The
brown comes from the oak cask, its
charring and toasting and the colour of any previous incumbent be it
bourbon or sherry. Although still a
niche category, brand owners see an
angle in acquiring finishing casks that
earlier held port, Madeira or wines
like Sauternes, Tokaji or even Grand
Cru champagne or Chateau Margaux.
France’s Brenne farm distillery near
Cognac makes a single malt and does
things the other way around, starting
in new Limousin oak then finishing in
newly disgorged cognac casks.
Char 2
Char 3Char 4 (Alligator char)
Charring the casks at the Independent Stave Company and the various levels of char available,
from Char 1 to Char 4 which is the classic Alligator
Sweden has gone further and used a
juniper cask for its spell of maturation.
In the UK, Chris Hayman reports that
for his wood-aged 1850 Reserve he
had to alter the gin recipe to take account of the woody flavours interacting
with the botanicals.
I do hope we are not about to see
a botanical-infused whiskey but the
whole market is highly competitive and
innovation to get that edge in the market
is intense. Developing a USP has never
been more critical. Once upon a time the
sector was driven only by %ABV, package design, age or cask finish. Scottish
products no longer feature as regularly
in Jim Murry’s Whisky Bible top five as
they used to. While at the classic end of
the market, a 12yo whisky might now be
more common than a declared 8yo but
there is more and more Scotch on sale
with no age statement. In Ireland, Cooley
is selling a poteen straight off the still;
I trust that it is more potable than the
product which surreptitiously appeared
at times in Bass’ Belfast Brewery!
Hybrids are everywhere; PernodRicard has Zaconey, a blend of Irish and
US whiskies with berry extracts; England’s Lakes Distillery produced a blend
of whisky from all parts of the United
Kingdom called ‘The One’ to sell before
its own whisky comes of age. Mosswood finished a 7yo Tennessee whiskey
in a cask which had been seasoned with
espresso coffee. High West Whiskey’s
Campfire combines American rye with
peated imported Scotch. Similarly Jim
Beam Kentucky Dram blends Kentucky
Straight Bourbon with an Ardmore
peated malt to ‘marry smoke and vanilla’. Diageo’s Whiskey Union recently
launched Huxley Rare Genus Whiskey,
comprising Scotch, Canadian and
American casks while its Boxing Hares
infuses hop flavours.
Adelphi has mixed single malts
www.ibd.org.uk
from Longmorn and Glen Garioch with
whisky from the Hanyu distillery in
Japan to produce an offering called
Glover. Perhaps on the more bizarre
but still eye-catching end of innovation
is from independent bottler Cadenhead’s Stupid Cask range with Fishky
which supposedly put Bruichladdich
in a herring cask; pundits Whisky Cast
were not complimentary, suggesting
that: “The nose hides the secrets of
the whisky well and the finish is salty,
greasy and nasty with no redeeming
qualities.” Westland Distillery also
put out a press release on April Fools
Day with Inferno, finished in a Tabasco
cask. Roundstone Rye probably had
more luck with a maple syrup cask.
Vibrant sector
So the sector is indeed vibrant and as
Bill Owens at the American Distilling
Institute points out, if you listen carefully to the supply chain for American
whiskey, there is a distinct sucking
sound as demand threatens to outstrip
the ability of the industry to supply.
The market is huge, with Bill’s 1000 or
so US craft distillers representing just
1.5% of American spirits sales (over
there craft beer is 12.2% of the market).
We have already seen %ABV reductions and a shortening of the period
in cask so small wonder that brand
owners are interested in any permitted
accelerations as long as they do not
damage brand credentials. No doubt
much work is going on behind closed
doors to avoid accusations of pennypinching corner-cutting but enquiries to
PR departments are greeted with sharp
intakes of breath and quiet tutting
sounds for your even suggesting that
such shenanigans are going on! Heriot
Watt University in Edinburgh has also
conducted some research but under a
client non-disclosure agreement.
Unaged spirits in the past
During the nineteenth century, much
whisky was bottled soon after distillation. But like brewing, whisky-making
tended to be seasonal in nature and
the oak barrel would have been the
most readily available container for
storage. Discerning producers and
customers must have picked up on
the changed character of the spirit.
The current minimum storage period
embedded in the Scotch Whisky Regulations is a little known result of Lloyd
George’s efforts to bolster munitions
production during the Great War when
drink was apparently causing more
problems than the whole fleet of German submarines.
Total prohibition was not an option
but a number of measures to curb
immoderate consumption were introduced. UK brewers are familiar with
the restriction of pub opening hours,
making the buying of rounds illegal
– and the nationalisation of pubs and
breweries around the munitions plants
at Enfield in London and Gretna on the
Scottish border – but the Immature
Spirits (Restriction) Act of 1915 banned
the sale of any spirit less than two
years old. This was increased to three
a year later.
The Act certainly reduced spirit
availability and took immature products off the market believing that they
caused more drunkenness than older
and obviously more expensive brands.
Today the arms factories are long
gone, but the three-year rule still applies for Scotch whisky along with the
restrictions on raw materials, upper
distillation strength and a minimum
alcoholic strength of 40%ABV in bottle.
Immature spirits disappeared from the
market for a century.
The benefits of aging in oak have
long been appreciated, rather than
understood. Even today we have only
Brewer and Distiller International November 2016 z 37
l DISTILLATION
Ian Palmer from Scotland’s newest whisky distillery Inchdairnie says he has looked at lots
of rapid maturation options, not that he wanted to use them but just to understand them.
He tried adding wood into the barrels but this just tasted of sawn wood where the young
spirit flavours are masked as opposed to maturing them out. He added “I am looking for
complexity, mouth feel, long finish and sweetness. Many of these ideas, for a sophisticated
palette, can be just one dimensional which leads to disappointment. Time is only a problem
when you do not have enough of it.”
elucidated parts of it and the discipline
of wood management has emerged
utilising our current knowledge of a
highly complex and lengthy process.
Oak wood is a potent source of vanilla
and coconut like flavours.
The continuing popularity of vanilla
ice cream is testament to how this spice
is inculcated into our flavour preferences
from an early age. Yet it is not all about
adding flavours, there is subtraction, in-
teraction and masking of low flavour
threshold, less desirable characters.
The tight grain of American oak slows
chemical interactions so the internal
surface of the staves (not usually the
ends) are charred or toasted to create
both a carbon layer and to damage the
wood so that lignin derivatives can be
leached out by the spirit. The staves of
uncharred casks have also been damaged by heat applied during the curving
of the staves to make the barrel shape.
For charring, a gas flame is
deemed more controllable than the
traditional fire of oak chippings, with
the intensity of flame and duration
dictating different levels of charring.
Heavy charring can leave a smokiness
and acrid taste which is not always desirable. Toasting in front of electrically
Oak still rules
In Scotland at and Arran and the currently closed Bladnoch distillery
38 z Brewer and Distiller International November 2016
heated elements provides a similar if
less pyrotechnic effect.
Sherry casks remain popular
although they are no longer procured
from Bristol bottlers as since 1981,
sherry must now be packaged in
Spain. Users should note that only
about 90% of sherry casks are derived
from European oak with lighter sherries demanding wood from America –
buyers need to ask the right questions.
Making the casks last longer
The level of charring, initially carried out
to help mature the bourbon, is still active
when the cask is used for Scotch. It will
remain active for up to five fillings over
perhaps 40 or 50 years but each successive use exhibits a drop in activity, so
immature flavours are slower to go and
mature flavours take longer to develop.
This is best tracked by looking at the
colour which is extracted. By carefully
choosing the US cooperage supplying
the used bourbon casks, it is possible to
get a level of activity to suit the product
you are trying to mature. After all, if you
have to hold stock for a statutory three
years there is little point in procuring
casks which will have moderated the
new-make flavours in two!
In the old days exhausted casks
would be destined for the garden centre
but now they can go through a dechar/
char process where the old layer is
scoured off down to bare wood and the
surface then charred again with sufficient heat to damage the upper wood
layer to allow a further few fillings. Conditioning with sherry used to be popular
www.ibd.org.uk
DISTILLATION l
but today if you have both grain and malt
whiskies, the newly-charred cask is just
refilled with grain spirit. Otherwise they
might be used for a ‘finishing’ period
with matured whisky to condition the
wood. There is obviously a limit to successive dechars before the structural
integrity of the stave is compromised.
Balance is essential. A heavily peated Islay may still dominate the finished
product even after long maturation
in a highly active cask. Conversely, a
delicate Lowland would be swamped by
the use of first-fill sherry casks leaving
a cask flavour masking any contribution
of the stills themselves. Having said
that the heavy sweetness from sherry
is popular with some customers but it
should not be overbearing.
Over 500 components
Industry guru Alan Rutherford points
out that mature whisky contains over
500 identifiable molecular components, although it is likely that fewer
than a hundred of these have a primary
influence on organoleptic character.
These compounds, or congeners,
include higher alcohols, aldehydes, esters, nitrogen and sulphur compounds,
carbonyls, phenols and lactones. He
groups the long slow chemical reactions occurring during maturation into
three general categories.
Firstly, the removal of undesirable
or immature components such as heavy
sulphur compounds by absorption on
to the oak/char surface and to a lesser
extent by evaporation (light carbonyls)
through the porous wood. Then desirable components such as lactones migrate from the oak to the spirit. Finally
there is chemical interaction within the
spirit between organic compounds and
with oxygen which diffuses through the
oak to produce desirable flavour congeners such as acetals.
Furthermore, the concentration
of ethanol, increased by loss of water
to the atmosphere or decreased by
the loss of ethanol if the humidity is
high, will complicate the reaction even
more. For a more detailed review of
the reactions which take place, Ian
Hornsey published a survey in last
April’s Brewer and Distiller International
and John Conner’s chapter in Inge
Russell’s Whisky Technology: Production
and Marketing is invaluable.
New make is typically cut to
63-64%ABV into cask, there may be
temptation to increase this to save on
barrels and warehouse space but such
spirit matures more slowly so it will
have to be in cask for longer. Casks may
be held on their bilges (bellies to the
brewer) two high in traditional warehouses or else on steel racks as high
as your building costs and fork lift truck
reach will allow. More and more will
be conveniently stored on their ends
(heads to the brewer) on pallets perhaps five high or six high. Temperature
seems to be important and warehouses
should be as full as possible to reduce
fluctuations but casks on the topmost rows will get a lot hotter even in
Scotland unless the ceiling is specially
insulated. Even in Scotland a steady
2% of volume is lost to the angels every
year meaning that after ten years only
85% of the volume will remain but the
loss of water being relatively greater
will usually slightly elevate the %ABV.
Experimental warehouse
Sazerac opened its Buffalo Trace
Experimental Warehouse X in late 2013
Three casks sit in the firing chamber at various stages of the charring and heat treatment as
part of cask rejuvenation
www.ibd.org.uk
Learning more about maturation conditions.
The front and rear elevations of Buffalo
Trace’s experimental 150 cask Warehouse X at
Frankfort in Kentucky
to explore some of the storage issues.
Warehouse X is comprised of four independently operating chambers that
allow specific variables like natural
light, temperature, humidity and airflow to be tested. There is only space
for 150 casks but it uses $1m worth of
monitoring equipment.
After a couple of years, results are
said to be encouraging, yielding ‘interesting experimental insights’. Sazerac
CEO Mark Brown does not believe
rapid aging is the solution to current
shortages but he has launched Whiskey Thief – a bourbon with added oak
inserted into the barrel. Such contact
is said to double the age profile of the
product. Meanwhile Buffalo Trace is to
spend $200million on capacity expansion at its Frankfort base in Kentucky.
Reducing the raw new-make
flavours
A large company like Diageo with 28
malt distilleries has to know the character of the various new-make products in formulating a wood management policy. These flavours may range
from perfumed, appley, grassy or fruity
to cereal, sulphury, feinty or smokey.
This immaturity is attenuated to yield
a desirable product over time before
the whisky is deemed saleable and
can be passed on for blending. Newer
producers should look at the materials
and processes which might lead to the
new-make product being more pungent, sulphury and feinty thus needing
a longer maturation period.
One Speyside distiller I spoke to was
most affronted when I suggested his
new-make was pungent, raw or feinty.
Clearly he has a finely-tuned process,
he thought he would get the sack if
Brewer and Distiller International November 2016 z 39
l DISTILLATION
Maturation in action
Spirit
Character
Maturation effects
Interactive
Distillery Character
Immaturity
Maturity
Subtractive
Additive
Time
This diagram from Diageo illustrates how the maturation process attenuates the raw spirit
immaturity over time, the distillery character interacts with the wood which also develops
maturity traits – all over time
Distillery Character A
Immaturity
Distillery Character B
Maturation effects low level of initial maturity
Spirit
Character
Spirit
Character
Maturation effects initial high level of immaturity
Distillery Character A
Distillery Character B
Immaturity
Time
Time
These slides show higher levels of immature character take longer to attenuate
Maturation effects highly active cask
Distillery Character A
Immaturity
Distillery Character B
Spirit
Character
Spirit
Character
Maturation effects exhausted cask
Distillery Character A
Distillery Character B
Immaturity
Time
Time
An exhausted cask will remove immaturity far more slowly than a reactive one
Maturation effects highly active cask
Distillery Character A
Distillery Character B
Spirit
Character
Spirit
Character
Maturation effects exhausted cask
Distillery Character A
Distillery Character B
Maturity
Maturity
Time
Time
Similarly, the maturity level in an exhausted cask will build more slowly than in a highly
reactive one
Maturation effects highly active cask
Distillery Character A
Immaturity
Distillery Character B
Maturity
Spirit
Character
Spirit
Character
Maturation effects exhausted cask
Distillery Character A
Maturity
Immaturity
Time
Time
Putting the two effects together, you can see the slow loss of immaturity and the slow gain of
mature character in an exhausted cask compared to the more rapid reduction in raw flavour and
increase in mature flavour to a higher level if the cask is more active
40 z Brewer and Distiller International November 2016
it wasn’t and the spirit was not ready
on the 1096th day after filling. Others
might choose to alter the wash conditions considering the wort clarity and
concentration, totally enclosed or open,
stainless or pitch pine fermenters, pitch
rate, type of yeast used, pitch temperature, wort oxygenation and period
in fermenter. All will alter the volatile
compounds present in the wash.
Within the still, the ‘copper conversation’ is critical as active sites on the surface will remove sulphury compounds.
These active sites will get used up like a
spent Brillo pad and need rejuvenation
by resting the still, opening the hatch to
allow air inside and also having a gap between one distillation and another. There
must be some mileage in the nano particle copper surfaces espoused by Italy’s
Green Engineering (Brewer and Distiller
International, June 2016) as surface area
for reaction is increased dramatically.
Still design with respect to the angle of
the lyne arm, the reflux geometry in the
neck and the temperature of operating
the condensers so that if they run hotter
with the vapour condensing further down
the worm so that there is more spirit
vapour ‘conversation’.
Then there is the generosity of the
cuts from foreshots to spirits and then
to feints. Whether there are separate
tanks for foreshots and feints and
the way these recoveries at different
ethanol concentrations react with the
still all have an effect. It has always
amazed a humble brewer how these
unwanted compounds go round and
round the process yet somehow the
levels remain in balance and you do
not get the accumulated methanol
from operations since stills were
legalised in 1823!
Scotland is a relatively small area
compared to the United States or Canada
where temperatures can easily range
from 30oC in summer to -30oC in winter.
Eight thousand feet up in the Rockies in
Colorado will see a 20% evaporative loss;
there you will see casks covered in large
plastic sheets and misters trying to keep
the atmospheric moisture up. There
are over a hundred Coloradoan craft
distillers, situated over 5000 feet above
sea level, tackling challenges unknown
at sea level but with many Americans
tracing their ancestry back to Scottish
immigrants, the draw to produce a traditional Scotch is high indeed.
Oak is big business
The Scotch Whisky Association reckons
there are 20 million casks maturing spirit in Scotland. I am told that a
www.ibd.org.uk
DISTILLATION l
commercial rental for a cask is around
£15 per year. Applying that figure
across the industry shows how storage
alone amounts to a not inconsiderable
sum. The whisky market worldwide is
around 11.5% of the total spirits market
(3127m 9L cases), so add the brandy
and rum casks, all the wine casks and
even Tabasco, Worcestershire Sauce
and maple syrup barrels, you have an
awful lot of wooden containers.
Today the brewing contribution is
negligible with traditional producers like
St Austell, Wadworth and Sam Smiths
as well as kolsch makers in Cologne
still using a few casks. It is thus hardly
surprising that one of the earlier papers
exploring rapid maturation appeared in
a journal called Forestry back in 1985.
Oak is a large industry. A tree might take
80-120 years growing tightly together to
ensure a tall straight trunk. The timber
below the first branch is taken and that
might only yield enough wood to make
two or three casks. English oak is said
to be too knotty although that did not
seem to bother Admiral Nelson and the
shipbuilders at Chatham Dockyard.
The vast majority of casks used for
whiskey are sourced from the USA.
Stateside regulations demand that
whiskey be matured in a new cask,
which leads to the availability of a
large number of second-hand casks.
The Scotch industry has based its
strategy around this continuous supply.
Another April 1st post in 2014 suggested that Diageo and Brown Forman
were tussling with US regulators (The
Alcohol and Tobacco Tax and Trade
Bureau – TTB) over the definition of
Tennessee whiskey with B-F wanting
to allow the second use of a cask.
This would solve any US cask
shortages but the supply of cheap
casks for Diageo’s 30 Scottish plants
would dry up – putting pressure on using casks more economically. A scary
spoof perhaps, but could it happen?
Speeding things up
We have had a thorough look at the
challenges of more traditional cask
maturation, so how can this be accelerated? Imitating maturation is rather
challenging in view of complex multi
stage and subtle processes. A quick
recap might be helpful.
During maturation, additive, subtractive and productive mechanisms
operate which change the colour and
the flavour of the new-make spirit. The
main groups of compounds extracted
by spirit from oak are hemicelluloses,
lignins, tannins, and lactones. Lignin
www.ibd.org.uk
degradation products (such as vanillin,
syringaldehyde and coniferaldehyde)
provide floral, spicy, smooth, mellow
attributes to flavour and aroma. Tannin
contributes to bitterness and astringency. Subtractive reactions include
evaporation of volatile compounds
such as ethanol, acetaldehyde and
dimethylsulfide.
Oxidations involve ethanol to
acetaldehyde and dimethylsulfide to
dimethyl sulfoxide and ethanolysis
of acrolein to 1,1,3-triethoxypropane.
Productive or interactive reactions
include oxidation of acetaldehyde to
acetic acid, esterification of acetic acid
to ethyl acetate. Acetal formation from
acetaldehyde and ethanol, oxidation
and esterification of lignin degradation
products and water-ethanol interactions through hydrogen bonding leading to the formation of stable clusters
of water and ethanol and an increase
in viscosity. Stable clusters of water
and ethanol are thought to contribute
to the ‘mellowness’ of the spirit.
Maturation enriches the sensorial
characteristics of the product through
reduction of DMDS and DMTS from
grain and ‘nutty, cereal, oily or feinty’
characters of new-make spirit. All this
complexity suggests that any attempts
at acceleration are not likely to deliver
an absolute match to any traditionally
matured product.
John Conner, the Scotch Whisky
Research Institute’s maturation guru
divides the approaches into three.
First is what amounts to the addition
of wood extracts and this can take a
multitude of forms depending on the
required character: American v French
oak, the degree of toast, staves, cubes
or chips and whether soaked in wine. The use of ultrasonics is simply a
way of increasing the rate of extraction. The second camp concentrates on
the removal of immature characters
and a variety of techniques appear to
be used – activated charcoal, light and
metal catalysts.
Lastly there are one or two patents
espousing the ammonium ion but Dr
Conner is at a loss to think of any reason this would promote a naturally occurring reaction. The patent describes
the background as ‘alkaline distillation
causing protein hydrolysis and amino
acid deamination, the resulting NH3+
ions react with the naturally present
aldehydes to form ammonium aldehydes. These in turn enter into aldol
condensations with the esters and/
or the congener molecules to give the
complex aldehyde mixture character-
istic of aged product’. A 1976 patent
from Belarus proposes that staves are
placed in a vessel under vacuum for
half an hour to remove most of the air
from within; gaseous NH3 is introduced
under a pressure of 1bar and the
staves are held for 0.5-5 h at 20-35°C
in an alkaline medium, in which oxidation with O2 is carried out at 20-35°C
for 0.5-3 days. Excess NH3 is removed
under vacuum.
John continues: “From a Scotch
whisky perspective, there are two problems with the first method. A big sticking point is that most Scotch whisky
is matured in used casks and this is
virtually impossible to mimic using oak
sticks. The chips provide a rapid and
almost complete extraction whereas
the physical structure of the stave plays
an important role in regulating the
extractives that dissolve in the spirit.
“Also, we have very limited information on how wood extractives change
over the course of maturation. Without
this knowledge it is impossible to say
whether the various methods in use
accelerate these reactions, so what you
have added is extract and not mature
character.
I have similar reservations about
the methods used to reduce immature
characteristics. “Again we do not know
enough about these reactions to say
whether they are accelerated by the
various methods. Increasing the energy
input in whatever form or the use of
catalysts may not just accelerate reactions that would naturally have taken
place, but the chemist in me can’t help
thinking it could help others over
their activation energy barrier
and put in motion reactions that
wouldn’t normally occur.”
IBD whisky tutor Brian Eaton
adds: “There are other maturation effects that cannot easily be
mimicked – the loss of volatiles
and oxidation. This was shown
Tuthilltown Distillery in New York State were early
exponents of maturation in smaller casks using 3, 5, 10 and
14 gallon barrels. Even the bottle for its Baby Hudson is
small at 375mL
Brewer and Distiller International November 2016 z 41
l DISTILLATION
ture identical or artificial and referred
to as GRAS (Generally Recognised
as Safe). Several of the components
imitate the composition of congeners
formed during fermentation which
would be released into the spirit.
These would be esters notably ethyl
acetate ethyl caproate, ethyl caprylate,
ethyl caprate, ethyl laurate, isoamyl
acetate and phenylethyl acetate,
which impart diverse fruity and flowery notes. Small amounts of other
chemical compounds such as higher
alcohols/fusel oils, fatty/organic acids
and aldehydes are also incorporated to
provide a balanced product character
and palate appeal.
Flavour houses can supply treatments containing some of the critical
components developed during the
maturation process such as furfural,
vanillin, lactone, eugenol, guaiacol etc,
but incorporating tannin and phenolic
aldehydes such as coniferaldehyde,
sinapaldehyde and syringaldehyde
in appropriate proportions is often a
limiting factor.
Staves, cubes and chips
Innerstave products are typical of oak inserts:
staves installed in a vessels, a fan of slats for
suspension in a cask, a nylon bag of ‘blocks’, a
chain of toasted laths fits into the barrel and
oak cubes
a few years back with shrink-wrapping
of casks in Scotland with the aim of
minimising alcohol loss – the whisky
did not mature well. It has also been
found recently that stacking casks
on pallets instead of on their bellies increases the time required for
maturation by about a month per year,
presumably due to poorer air flow
around the cask. So it is not just about
adding wood character.”
What has been tried?
There are lots of cautions out there but
perhaps you would expect those connected with an industry where there is
tremendous heritage and a strict legal
time limit to be apprehensive but let’s
see what has been tried. Bozzy Karasu
takes up the story: “To start with, using
any cask size smaller than standard
53-gallon barrel is for one goal only:
accelerated maturation. It is a method
perfected over the years in the wine
industry and they have been pretty
successful with it.
When craft distilleries started to
pop-up all over the country they also
adapted this method. Ralph and Gable
Erenzo at Tuthilltown Distillery started to
use three- and five-gallon casks allowing
them to bring their Hudson Baby Bourbon whiskey to a colour and taste profile
at a decent level in under six months. A
lot of distillers are using the micro-oxygenation method – again inspired again
from the wine industry. Those microoxygenators are not that expensive to
buy. They pump medical oxygen into the
barrels in intervals to oxidise the spirit
like it would over the years.
Another method is the pressure
cooker invented by Tom Lix at Cleveland Whiskey. He puts some wood
chips and staves in his spirit in a big
pot and alternates the pressure low
and high over several days.”
Smaller casks alter the surface to
volume ratio. Buffalo Trace reported
back in 2012 that trials with small
casks (5, 10 and 15 gallons) over six
years were not successful citing a lack
of depth of flavour.
At the budget end of the market in
India, commercial flavours are added
to molasses-based neutral spirit to
make cheaper whiskies. These added
flavours are classified as natural, na-
42 z Brewer and Distiller International November 2016
Most accelerators will use wood in the
form of staves, cubes or chips. There
is a choice of oak and toasting. Andre
Alcarde at Brazil’s Sao Paulo University
tested chips with cachaça and found it
made no difference whether the oak
was American or European but toasting levels did. Wood to make a matrix
of staves within a tank or just cubes or
chips are available from Independent
Stave, Oak Wise and Innerstave.
This latter company, based in California, is typical, with a choice of oaks
and toast levels – toasted staves can
be mounted inside tanks where 1.7 m2
will mimic a 225L cask, they can be in
the form of a fan of 22 laths measuring
28x2x5/16” which can be dangled in on
a chain. Shorter laths can be drilled
and linked so that a chain of 17 can
be used and each mimics a third of a
barrel. Half-inch cubes suspended in a
nylon mesh bag with a drawstring and
a sack with 6.3m2 surface will treat 600
gallons within the year. Chips the size
of broken matchsticks are used at 10
to 20 pounds per 600 gallons, they too
can be contained in a nylon bag.
Infusion Spirals, part of the Barrel
Mill Cooperage, increase the surface
area by milling a spiral configuration
into 48-inch long poles around 30mm in
diameter. Five in a mesh bag will treat
1000 gallons. Nine-inch lengths can
form a chain inside a long nylon bag
and six of them are said to mature 70
www.ibd.org.uk
DISTILLATION l
Joe Paglioni’s Oak Bottle claims to ‘infuse’
spirits with the same amount of flavour as
lengthy barrel ageing
gallons. Different oak sources and toastings can be used together. The company
also makes smaller diameter spirals
which can be inserted into bottles.
Time and Oak sell small toasted
sticks branded Whiskey Elements
which are cut to increase surface area
and apparently need only to spend a
day inside a bottle of neutral spirit to
make a passable whisky. You can buy a
750mL oak bottle from Amazon to keep
your vodka in and surprise granddad
at Christmas! Thousand Oaks Barrels in Virginia will make you a cask
from one litre size upwards and also
supply a range of essences if you really
want to cheat! Independent Stave can
make casks with up to five different
types of oak wood to give you the best
of all woody worlds and Makers Mark
offers private customers the chance
to formulate their own barrel in a new
program called Private Barrel.
A number of distillers use chips
within a conventional cask. Copper Fox
in Virginia which incidentally floor-malts
its own barley, pops a mesh bag of oak
and apple chips into the bourbon barrel
with the whiskey. After the year, the chip
bag is removed and reused, then the
whiskey is transferred into another barrel which is heated and then rolled, still
hot, to ensure that the alcohol achieves
maximum penetration into the charred
internal layers of the barrel. After
cooling, the process is repeated several
times over the next few months until the
spirit is ready for bottling
The effect of long journeys
Agitation could be a factor. There have
been various publicity stunts like Pete
Brown trying to take a cask of Burton
IPA to Calcutta until it disgorged its
contents all over his hotel room in
Brazil. The idea was to take the beer
on a six-month journey and compare it
Time and Oak claims its Whiskey Elements
sticks add the effects of three years of ageing
to any whisky in just 24 hours
Oak spirals from Infusion Spirals…
A bottle spiral kit you can use in spirits, beers
or cocktails
www.ibd.org.uk
or oak chips from Add Oak
with a cask of the
same brew flown
out from Burton.
In the whiskey
world, Jefferson
Bourbon recreated
the route used 150
years ago from
Louisville down the
Mississippi across
to Florida and north
to the East Coast –
and went on to take
a cask between five
continents crossing the equator
four times to see
whether ten months
on water would enhance seven years
in a warehouse on
C&C Shine’s Californian moonshine
dry land.
(from a Kentucky recipe) came with
French whiskey a toasted oak rod. This is placed in
maker Black Moun- the bottle which can be regularly
tain strapped a 280L tasted to assess how long it takes
cask to the deck of a to obtain the wood flavour you like
sailing vessel which took tourists around
the Bay of Biscay. It was on board for six
months. A 300L cask of Jack Daniel’s
cocktail as formulated by the Savoy in
London was on the open deck of the
Queen Mary II for a 41,000mile trip lasting
four months. Last year Suntory put some
whisky (not in barrels) on the International Space Station in a maturation
experiment echoing the Ardbeg exercise
which kept vials of Islay congeners gravity free for three years. The continuing
saga allowed the release of several top
end offerings celebrating the 90-minute circumnavigation of the weightless
containers – we await the results.
Going not quite as high we have
seen Montanya Rum matured at 8,900
feet up in the Rockies inspired by some
Columbian rum which was kept at
17,000 feet. A day at Crested Butte in
Colorado can see temperature fluctuations of 40oC! Meanwhile, Cayman
Spirits Company’s Seven Fathoms
Rum matures at a depth of 42 feet
under the Caribbean Sea. The motion
of the tides gently rocks the casks
and the company keeps the location a
closely guarded secret.
We know the wood ‘breathes’
allowing oxygen and other molecules
out, so is the micro movement allowing the layer of liquid close to the stave
to mix with the body of the cask and
avoid concentrations which might slow
reactions. How many distillers roll
their casks before sampling? Is there
a concentration gradient in the liquid
towards the wood surface?
Brewer and Distiller International November 2016 z 43
l DISTILLATION
Agitation in action.
Jefferson took two casks from Louisville to New York by sea and then took another cask
on a voyage visiting all continents crossing the equator four times. Seven Fathoms from
the Cayman Islands is matured 42 feet down in the Caribbean
Listen to the music
The boys at Tuthilltown
also installed sub-bass
speakers into their
warehouses to give the
barrels some rumble
so that the magic at
the air, wood and spirit
interfaces can be enhanced. Copper and Kings,
a brandy distillery in Kentucky says its
casks prefer the works of Lenny Kravitz, The Doors and Bruce Springsteen
to stretch their pores. New Zealand gin
maker Rogue Society has also experimented using a local band and you get
a CD of the ‘music’ when you buy the
Electric Wire Hustle X Rogue Society
Gin Limited Edition.
Colonial American Inspired Rum at 62%ABV from the Lost
Spirits Distillery in California – oak matured in the Lost
Spirits reactor (inset)
Thea One
There are a number of patents for
ultrasonic treatments to hurry the
molecules along a bit but apart from
Terrapure, tests do not seem to have
got beyond the laboratory bench. There
is one high-energy input system which
is causing a bit of a stir despite the
inventor getting a pretty rough ride at
an ADI symposium.
Bryan Davis has spent years investigating the aging of spirits. He first
founded an absinthe operation in Spain
and moved back the States to make
whiskey at a plant called Lost Spirits
beside the Pacific at Monterey. He had
a wooden still, obviously steam heated,
and the worm passed through his
swimming pool so that he could have a
relaxing dip after the run!
With the help of crowd funding he
launched Lost Spirits Technology and
built a portable reactor which claims
to make a good quality spirit in just six
days. He has impressive GC-MS charts
to show that the peaks are all in the
right places. His board boasts some
retired industry heavyweights, there
are five prototype installations christened Thea One in operation and he
is in talks with another 65 distilleries
looking at forming a partnership. Thea
One will produce over 500L of ‘20yo’
each week.
New-make spirit and oak chips
are put in the reactor. The process
first involves a heating spell under
just 6psi top pressure at temperatures between 60-75oC. The spirit is
cooled and subjected to light radiation from 400-1000nm passing
over 2m lux hours. Putting a carboy
in a Californian greenhouse for a
week would pass just over 4m lux
hours. The light treated spirit is then
44 z Brewer and Distiller International November 2016
reheated to 60-75oC to finish off the
operation.
Terrapure from Terrassentia based
in South Carolina makes claims for a
similar system which uses ultrasonics
and pumps new-make through an oxygenated chamber. It offers own-label
spirits to some 50 hotel and restaurant
chains but details of the actual process
are currently rather vague.
The Allegro process from Kairos
Global also offers a reaction vessel under controlled temperature and oxidation conditions passing spirit through
a comminuted wood matrix. Kairos
offers a service to match the ethyl
acetate in a matured product which
might take four years by incorporating
an addition into the reaction.
The wine industry has tried passing
electric currents in the presence of
oxygen through young acidic red wine
and reported more mellow and aromatic results. Three minutes produced
the best wine while anything over eight
yielded a product worse than the control. Pulsed electric currents have also
shown encouraging results. Rice wine
has been reportedly enhanced by both
γ-radiation and ultrasonic treatments.
Any application to spirit maturation
has yet to be published.
Bozzy gets the last word: “I have
tasted hundreds of spirits that went
through accelerated aging one way
or other; they have a different taste
profile you can get right away. But it is
not necessarily bad. Anyway, they do
taste different – exactly like small barrel wines do. Distillers should not try
to match the flavour profile of longer
maturation periods of Scotch distilleries. It is a fight they cannot win. Nothing can replace longer maturation in
bigger barrels in milder climates. They
use a different method which leads to
a different taste.
“Be OK with it, craft distillers have
a six-month old whiskey matured in
a different way and tastes different.
It is a new product, maybe a new category – and they should work on how
to make that unique product better. I
think we have all learned that in this
industry, we cannot replace time with
smart ideas.”
Acknowledgements
The author would like to thank Bill
Owens, Margarett Waterbury, Graham Stewart, James MacTaggart,
Ian Palmer, Alan Winchester, Gary
Spedding, Binod Maitin and Douglas
Murray for helpful direction during the
preparation of this article.
www.ibd.org.uk