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

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ever, the ethanol concentration at any specific point in the syst em should be

constant over time, but varies with the position in the still, rising from about 10

per cent at the base of the rectifier to at least 94 per cent at the top.

Figure 6.14 shows the distillation of typical compounds of types A, B and C

in the batch process. Line A shows collection of type A compounds principally

in the foreshots, and line B (i.e . type B) shows collection throughout the entire

distillation process in a similar profile to ethanol, but of course in much smal-

ler amount. Line C1 of Figure 6.14 shows a type C compound distilling at

maximum amount about the end of spirit collection, but a different compound

(also of type C) could appear late in the spirit cut, increasing through out the

distillation of feints (line C2). These effects can be explained in terms of their

relative volatility to ethanol as shown in Figure 6.13. In the continuous process,

however, compounds of type C are unlikely to make any contribution to the

rectifier column at all. A small amount may be vapori zed from the top plate of

the analyser and carried over to the rectifier, only to be condensed in the lower

202 Whisky: Technology, Production and Marketing

Figure 6.14

Separation of flavour congeners according to volatility during batch distillation (from

data provided by United Distillers Ltd). A, congeners that are more volatile than

ethanol; B, congene rs of similar volatility to ethanol; C, volatiles (C1 and C2) that

are less volatile than ethanol.

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levels and returned to the analyser in the hot fe ints. A similar effect occurs

throughout the analyser, and the lower volatile material (type C) will gradu-

ally cascade down the column to leave eventually in the spent wash.

So the hot spirit vapour flowing into the base of the rectifier column will be

rich in type A and B compounds but contain only a small amount of type C,

which will soon be returned to the analyser. Type A congeners, being more

volatile at all concentrations of ethanol, migrate to the top of the column.

According to the operating conditions of the top condenser, individual com-

ponents of that mixture will be vented to atmosphere, recycled in the reflux to

the top plate of the rectifier, or drawn off in the cold feints for recycling to the

top of the analyser. The situation with type B congeners is more complex, since

the ethanol concentration varies throughout the height of the column. These

compounds will stabilize at the level in the column where their volatility and

that of ethanol are equal.

Figure 6.10 shows the distribution of the principal alcohols, ethanol, n-pro-

panol, butanols and iso-a myl alcohol, within the rectifier. Note that the spirit

plate is not necessarily the level with the highest concentration of ethanol, but

higher plates with a higher ethanol concentration are seen (Figure 6.10) to have

an unacceptable level of volatile type A congeners – mainly acetaldehyde and

any sulphur compounds that have escaped reaction with the copper in the

system. The flavour of the spirit is more important than its strength, and

eventually the ethanol condensed at higher levels will find its way back to

the spirit plate take-off point. Concentrations of all compounds are shown as

percentages by volume in Figure 6.10, but congeners are normally measured in

mg/l and affect quality at well below 1 per cent. Therefore all of the com-

pounds shown in Figure 6.10 have an important effect on flavour for several

plates above and below their disappearance from the graph. Table 6.3 shows

typical concentrations of flavour congeners in new grain spirit: all classes of

congener are present in much lower concentration than in malt spirit.

Chapter 6 Grain whisky distillation 203

Table 6.3

Comparison of flavour congeners of new-make

grain and malt spirit, diluted for cask filling (data

provided by United Distillers Ltd)

Grain spirit Malt spirit

Ethanol (% by volume) 68 63

Total higher alcohols (ppm) 800 2500

Total esters (ppm) 50 650

Total acids (ppm) 0 100

Total aldehydes (ppm) 0 50

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Although iso-amyl alcohol is by far the most abundant of the higher alco-

hols, all three shown in Figure 6.10 must be continuously draw n off at constant

rate, otherwise an increasing amount would collect in the spirit over the dura-

tion of the run. Inspection of Figure 6.10 suggests that at some point between

plates 10 and 15, where all three overlap, they could be drawn off together, but

in practice it has been found preferable to operate several take-off points as

feed for the fusel oil still. For steady-state conditions, these congeners must be

drawn off at the same rate as their introduction in the wash, and this is most

easily achieved as separate side streams. They can be adjusted independently,

but cautiously, to control the flavour of the spirit if necessary. For example, to

increase specifically the butanol content of the spirit, it is obvious that less

butanol should be drawn off as side stream. In practice, the situation is more

complex. If the original removal of propanol is maintained, as the amount of

butanol increases the propanol ‘bulge’ in Figu re 6.10 will move upwards,

increasing the amount of propanol at the level of the spirit plate. Therefore

the rate of removal of propanol to the fusel oil still should be increased to

maintain the original propanol content in the spirit. This effect will be

enhanced if iso-amyl alcohol is also removed more slowly. As that less volatile

compound accumulates at lower levels, it will further encourage the move-

ment of butanol (and propanol) up the column. However, there are practical

limits to this type of manipulation, and an earlier discussion of Figure 6.12

drew attention to the destabilizing effect of unusually high levels of i-amyl

alcohol.

The earlier sec tion on feedstock for distillation mentioned the importance of

a constant alcohol concentration in the wash for stable operation of the still.

Variation in alcohol content also affects flavour, since the changes in alcohol

strength over the height of the rectifier affect the separation of the congeners of

type B volatility (Figure 6.13).

Energy and utilities

A continuous process is inherently more energy-efficient than batch operation

(Whitby, 1995), but nevertheless the production of grain spirit is an energy-

intensive process. The principal energy requirements are steam for cooking

and distillation, and heat for concentration and drying of spent wash as animal

feed. There is also a substantial use of electric power in a grain whisky dis-

tillery, although only a relatively small proportion of the overall demand is

specifically required for the distillation. At least three pumps are required for

operation of the stills: the wash pump, hot feints pump and feed pump to the

fusel oil still. Depending on the layout of pipework, other process flows may

operate by gravity or additional pumps may be required – for example, for

transfer of the top product of the fusel oil still or for removal of spent wash.

Centrifugal pumps have suitable characteristics for all of these duties, but it is

important to be aware of the significanc e of a net positive suction head in

pumping hot alcoholic solutions (Genereaux et al., 1984). Recycling of hot

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feints in particular (high temperature and high ethanol concentration) needs

careful design of the pump and pipework system to prevent cavi tation in the

pump.

Stable contin uous distillation requires a constant rate of supply of steam.

Cereal cooking requires a large amount of steam intermittently for short per-

iods, and drawing steam for the cookers directly from the boile r that supplies

the analyser column would cause substantial fluctuation in steam pressure

and destabilize the still. The use of separate boilers for cookers and stills is

an expensive solution, and it is more economical to use a constant flow of

steam from the main boiler to superheat water in an accumulator pressure

vessel, which in turn provides cooker steam as necessary. This ensures a

constant flow of the remaining steam to the stills, or to the thermocompressor

that recovers heat energy for the stills (Figure 6.8). Two separate heating

systems are required for the complete distillation system (analyser and fusel

oil still), but the reboiler of the fusel oil still is a light duty in comparison with

the steam requirements of the analyser.

A simple system of one analyser, one rectifier and one fusel-oil still requires

five separate cooling units in total:

1. A spirit cooler, for the hot alcohol from the spirit plate

2. A top condenser (cold feints condenser) above the rectifier

3. A hot feints condenser, to recover ethanol from aeration of the hot feints

4. The heads condenser of the fusel oil still

5. The fusel oil cooler.

However, a larger installation does not require multiplication of all of these

facilities.

Unless a mains supply of drinking water quality is used for all purposes, the

water supply for a grain distillery requires careful consideration. Three differ-

ent grades of water quality may be used. The criteria for condenser cooling

water are a low supply temperature and low levels of the hardness salts,

which could cause problems by scale formation in heat exchangers.

Provided routine maintenance is adequate to insure agains t leaks, other che-

mical, microbiological and physical properties are irrelevant. A proportion of

the water used for mashing, dilution of wash and start-up and close-down

procedures ultimately becomes incorporated in the spirit and must be free

from flavour taints, but since it is sterilized in the course of the cooking and

distillation processes it does not require the microbiological quality of drink-

ing water; only the flavour quality is mportant. Water for steam raising is also

included in this category, and therefore boiler water-treatment compounds

must be carefully chosen to avoid flavour problems from the steam injected

into the cooker vessel and analyser column. Only the dilution water used at

cask filling will persist untreated to be consumed with the final whisky, and

therefore has an absolute requirement for a drinking-water standard of micro-

biological purity. However, a drinking-water supply could legally contain

levels of calcium and iron salts that are unacceptable in whisky spirit, and

these ions must be removed – usually by ion exchange.

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Acknowledgements

Little practical information on operation of grain whisky distilleries is avail-

able in the general scientific literature. Much of the information in this chapter

has been obtained in the course of personal visits or while accompanying

industrial visits of Brewing and Distilling students of Heriot-Watt

University to various grain whisky distilleries, including some sadly no longer

in existence. I am most grateful to the staff of these distilleries who passed on

their experience. Also, I am greatly indebted to United Distillers Ltd, subse-

quently United Distillers & Vintners Ltd, for making available their company

training manuals, which have been used to prepare parts of the text and

several of the figures. Finally, I wish to thank Mr Alastair Munro of North

British Distillery Ltd for checking the manuscript, not least to ensure that the

UDV influence was not too strong!

References

Cosaric, N., Wieczorek, A, Cosentino, G. P. et al. (1983). Ethanol fermentation. In:

Biotechnology, vol. 3 (H. Dellweg, ed.), pp. **. Verlag Chemie.

Coulson, J. M., Richardson, J. F., Backhurst, J. R. and Harker, J. H. (1991). Chemical

Engineering, vol. 2, 4th edn. Pergamon Press.

Genereaux, R. P., Mitchell, C. J. B., Hempstead, C. A. and Curran, B. F. (1984). Transport

and storage of fluids. In: Perry’s Chemical Engineers’ Handbook, 6th edn (D. W.

Green and J. O. Maloney, eds), pp. **. McGraw-Hill.

Korhola, M., Harju, K. and Lehtonen, M. (1989). Fermentation. In: The Science and

Technology of Whiskies (J. R. Piggott, R. Sharp and R. E. B. Duncan, eds), pp. **.

Longman Scientific and Technical.

Nykanen, L. and Suomalainen, H. (1983). Aroma of Beer, Wine and Distilled Beverages.

Riedel.

Panek, R. J. and Boucher, A. R. (1989). Continuous distillation. In: The Science and

Technology of Whiskies (J. R. Piggott, R. Sharp and R. E. B. Duncan, eds), pp. **.

Longman Scientific and Technical.

Seader, J. D. and Kurtyka, Z. M. (1984). Distillation. In: Perry’s Chemical Engineers’

Handbook, 6th edn (D. W. Green and J. O. Maloney, eds), pp. **. McGraw-Hill.

Travis, G. L. (1998). Ferment, 11, 341.

Walker, M. W. (1988). Ferment, 1, 45.

Watson, J. G. (1989). Energy requirements. In: The Science and Technology of Whiskies (J.

R. Piggott, R. Sharp and R. E. B. Duncan, eds), pp. **. Longman Scientific and

Technical.

Whitby, B. R. (1992). Ferment, 5, 261.

Whitby, B. R. (1995). Designing a distillery for the future. In: Proceedings of the Fourth

Aviemore Conference on Malting, Brewing & Distilling (I. Campbell and F. G. Priest,

eds), pp. **. Institute of Brewing.

Wright, S. A. (1998). Ferment, 11, 345.

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Introduction

In the production of Scotch whisky many factors are known to influence the

final quality of the product, including the water, barley type, extent of peating,

still type and distillation conditions. The most important contributor, however,

is undoubtedly the oak container in which maturation takes place. During the

time that the spirit spend s in this container, major changes occur in its che-

mical composition. The pungent, feinty aromas of the new distillate transform

into the typical mellow characteristics of a mature whisky. The colour of the

spirit also changes, from virtually clear to golden brown.

Historically the origins of maturation are obscure, with the majority of

whisky distilled in the eighteenth and nineteenth centuries probably drunk

unmatured. However, the making of whisky was traditionally a seasonal pro-

cess and therefore involv ed a period of storage, and this storage would inevi-

tably have used wooden casks, the nature of which depended on the

availability of suitable timber. In the USA, where there were large reserves

of white oak, storage in new casks was the norm. In Scotland suitable supplies

were limited, and second-hand containers such as old sherry casks or brandy

barrels were used. With time the benefits of this storage were realized, and a

period of maturation became an accepted part of whisky production and was

incorporated into the legal definitions of whisky that exist throughout the

world.

Outwardly the maturation process appears simple. Casks are filled with

spirit and set aside in a warehouse to mature. However, a wide range of

variables can influence the quality of the final product. The type of cask

used, its method of manufacture and even the climatic conditions during

storage all affect the maturation process, and are described in detail in this

209

Chapter 7

Maturation and

blending

John Conner, Ken Reid and Frances Jack

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chapter. When selecting and controlling maturation variables, the traditions

and product expectations for the particular whisky have to be consi dered.

American bourbon and rye whiskies are matured in new charred-oak casks,

while whiskies produced in Scotland, Ireland and Canada are matured in oak

casks previously used for the maturation of bourbon, or for the fermentation

and shipment of sherry (Booth et al., 1989). Consequently, while maturation of

a Scotch malt whisky in a new charred-oak cask may produce a well-matured

whisky, it may not be readily identifiable as Scotch (Clyne et al., 1993).

This chapter aims to give a detailed overview of the current technical under-

standing of the matu ration process and indicate the main variables that influ-

ence product quality. Current control of the maturation process is achieved by

the careful selection and sourcing of casks and their re-use. This ‘wood policy’

varies from company to company, and even within a company for individual

products, and is used to ensure a continued supply of quality and diverse

whiskies to the blender, who can then assemble the finished products.

Most bottled whiskies are blended from different production batches and

cask types. However, blended whisky generally refers to a comb ination of

mature whiskies from several different distilleries. The reasons for blending

whisky, the major considerations in formulating a blend and the practicalities

of blend manufacture are also covered in this chapter.

Finally, the authors’ expertise has been gained from working within the

Scotch whisky industry and for this reason the chapter focuses mostly on

the maturation and blending of Scotch whisky. The production of other

whiskies around the world uses a similar range of cask types to that used in

Scotland. The excepti on is American straight whiskies, most notably bourbon,

which are matur ed in new charred-oak casks. Again the process of blending is

similar throughout the world, with ‘light bodied’ whiskies distilled at high

ethanol concentration in column stills being combined with ‘heavier bodi ed’

whiskies produced in pot stills or distilled to a lower ethanol concentrations in

column stills. Consequently, the basic principles described can be widely

applied to all whiskies.

Cooperage oak wood

A considerable proportion of the production costs of Scotch whisky is due to

investment in cooperage oak. The majority of this is in the form of ex-bourbon

barrels imported from the USA and, to a lesser extent, sherry butts imported

from Spain. These wooden containers are constructed usin g the heartwood of

several species of white oak that are grown in the forests of North America

and Western Europe.

Today the majority of ex-bourbon barrels are imported from the USA as

standing barrels (180 l). The practice of disassembling barrels for transporta-

tion is in decline. These stave ‘shooks’ were commonly re-built to form a

hogshead, using a greater number of staves and new plain oak ends to give

a larger capacity (250 l). The other traditional source of cooperage timber is

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Spain, in the form of sherry butts (500 l). These are much sought after due to

their excellent reputation for maturing whisky and for their extended lifespan

compared to that of a typical American oak container. The Spanish sherry

industry, however, can only supply a fraction of the cooperage oak wood

required by Scotland, and therefore the Scotch whisky industry will continue

to look to the USA for its main supply of cooperage wood.

In the past, many types of wood have been used for the production of tight

cooperage containers. However, most woo d types either contribute undesir-

able flavours to the maturing alcoholic beverage or have porosity problems

that render them unsuitable for the prolonged storage of liquid. Only a limited

number of oak species are completely suited to the production of casks for

wine and spirit maturation.

Tight cooperage production from European oak uses two wood species

preferentially, namely Quercus robur and Quercus sessilis, both of which are

widely grown throughout the co ntinent (Singleton, 1974). There is a large

degree of similarity between the two species, and indeed Q. sessilis has been

considered as a variety of Q. robur. For the production of Spanish sherry casks

the European oak, predominantly Q. robur, is harvested mainly from the

regions of Galicia, Asturias, Cantabria and Pais Vasco in Northern Spain.

White oak wood imported from the USA is also used to produce sherry

casks, and is generally the preferred wood of the sherry maker.

The list of American oak species used for tight cooperage is, however, con-

siderably larger than that used in Europe, and includes Q. alba, Q. bicolor, Q.

muehlenbergii, Q. stellata, Q. macrocarpa, Q. lyrata and Q. durandii. Q. alba

accounts for approximately 45 per cent of the timber used. The main areas

of bourbon barrel production in the USA are Kentucky and Missouri. Barrels

produced in these areas would probably contain Q. alba, Q. bicolor and Q.

macrocarpa, with small amounts of Q. lyrata (Singleton, 1974).

Structure of wood

The structural features of certain oaks that make them ideal for tight cooperage

are their medullary rays and tyloses. Medullary rays are thin, flat structures

that lie along the radius of the tree trunk from its centre to the bark. They are

much harder than the structure of the rest of the tree, and are considered to be

impervious. In most trees the medullary rays are only one cell wide, but in

oaks and some other hardwoods they can be several cells wide (multiseriate).

In oak species, these unusually large rays represent between 19 and 32 per cent

of the wood volume. In Quercus alba, the most common species of oak wood

used in American tight cooperage, these rays represent 28 per cent of the wood

volume and contribute significantly towards the strength and flexibility of the

wood (Schahinger and Rankin, 1992).

White oak also contains conductive vessel elements or tubes that can be as

large as 0.3 mm in diameter in the spring portion of each annual ring. The

growth of tyloses, which are formed by the ballooni ng of the cell walls during

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the conversion of sapwood into heart wood, seals the channels of the vessels

and consequently stops the cask from leaking at the ends. Tylose formation

only occurs in approximately twenty species of oak (Schahinger and Rankin,

1992).

Wood tissue is composed of cellular and intercellular material. The cell wall

structure is constructed from the macromolecular components cellulose, hemi-

cellulose and lignin, whereas the intercellular region consists mainly of lignin.

In addition to the main polymeric fraction of the wood, low molecular weight

components (extractives) are present in relatively small amounts. However

these are not an integral part of the cell wall structure.

Cellulose is the most abundant component of wood, accounting for approxi-

mately one-half of the total content. It can be described as a linear polymer

with a uniform chain structure that is composed of anhydro glucopyranose

units. These units are bound by b(1-4)-glycosidic linkages formed by the elim-

ination of one molecule of water between hydroxyl groups at C1 and C4 of two

glucose units. Hydrogen bonding between hydroxyl groups on adjacent cellu-

lose molecules results in the formation of fibrils, which go together to make the

cell wall layers and consequently produce a superstructure for the remaining

wood components to build upon (Fengel and Wegener, 1983).

In addition to cellulose, the cell walls contain other polysaccharides known

as hemicelluloses that act as a matrix for the cellulose superstructure (Parham

and Gray, 1984). These compounds are branched heteropolymers that are

much shorter than the cellulose molecules, having a degree of polymerization

of approximately 200.**28 Hemicellulose consists mainly of sugar components

that can be divided into groups such as pentoses, hexoses, hexuronic acids and

deoxy-hexoses. The main chain (backbone) of a hemicellulose molec ule can

consist of one or more sugar units, and attached to this chain are other sugar

components (side groups) such as 4-0-methylglucuronic acid and galac tose

(Fengel and Wegene r, 1984). Oak wood hemice llulose is mainly xylose

based (xylans), and accounts for 15–30 per cent of the dry weight. Its backbone

consists of b-D-xylopyranose units, which are linked by b(1-4)-glycosidic

bonds. Approximately seven xylose units in every ten are substituted at the

C2 or C3 positions with O-acetyl groups. In addition to the acetyl side groups,

4-O-methylglucur onic acid units are attached by a(1-2)-glycosidic bonds at

irregular intervals along the chain (Sjostrom, 1981).

The third major component of wood is lignin, which has a highly branched

three-dimensional structure of high molecular weight. Lignin is located in

both the cell wall and intercellular regions (middle lamella), and serves as a

binding agent for the wood cells. Oak wood contains between 15 and 30 per

cent lignin, of which 70 per cent or more is located in the cell walls themselves

(Parham and Gray, 1984). The complex structure of lignin is built from phe-

nylpropane units that are substituted with hydroxyl and methoxyl groups. In

hardwoods such as oak, lignin is formed by the polymerization of the two

precursors, co niferyl alcohol and sinapyl alcohol, producing ‘guaiacyl-lignin’

and ‘syringyl-lignin’ respectively. However no natural lignin is excl usively

composed of these groups. The mixed polymers are formed by a variety of

ether and carbon–carbon bonds involving both aromatic rings and side chains

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of the sinapyl and coniferyl monomers (Monties, 1992). Chemical bonds have

also been demonstrated between lignin and almost all the hemicellulosic con-

stituents, and are thought to exist between lignin and cellulose. It is not known

what roles these bonds and the physical associations between constituents

play in maintaining the structura l integrity of oak wood (Haluk and

Irmouli, 1998).

In addition to the main macromolecular cell wall components of wood

tissue, oak can also contain up to 12 per cent low molecular weight compo-

nents. These do not contribute to the superstructure of the wood, but are

readily extracted by the maturing spirit and may have a significant influence

on flavour development during maturation. The composition of the extract is

complex, and varies depending on the species and origin of the tree. The main

components of interest for cooperage oak are the hydrolysable tannins and

volatile compounds (Masson and Pue ch, 2000). Hydrolysable tannins are

partly responsible for the bitterness and astringency of oak wood, and may

play a role as catalysts for oxidation during maturation. A variety of structures

based on gallic and ellagic acids have been identified, though these degrade

during heat treatment, with the major constituents extracted by maturing

spirits being the free acids (Mosedale, 1995).

Oak contains numerous volatile constituents, with around 100 peaks

detected by gas chromatography-mass spectrometry (Masson and Puech,

2000). Of interest in the maturation of whisky are organic acids, lactones,

norisoprenoids and volatile phenols. The major organic acids are acetic and

linolenic acids. Linolenic acid is one of the haze constituents in mature

whiskies, and its degradation produces odorous aldehydes and alcohols.

Oak lactone is one of the main volatile compounds in oak wood and plays

an important role in the aroma of mature whiskies, particularly the cis isomer,

which has a much lower perception threshold than the trans isomer.

Norisoprenoid compounds such as 3-oxo-ionol are also detected. Although

3-oxo-ionol has little odour, it may be the precursor for highly aro matic mega-

stigmatrienone isomers. Volatile phenols such as eugenol and vanillin are also

present in small quantities, though these may be increased by the thermal

degradation of oak.

Cask manufacture

Timber processing

Oak wood logs destined for cask production are initially cut into either stave

or heading (for cask ends) lengths, depending upon any def ects that may be

present on the log. These shorter lengths are then quarter sawn with the cuts

being made through the heart of the log, running along its radius. From these

quarters flat stave and heading timber can be cut to the appropriate thickness,

resulting in a reco very of approximately 50 per cent useable timber from the

log. Figure 7.1 illustrates the stave cuts.

Chapter 7 Maturation and blending 213