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

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resolution separations are often linked to mass spectrometric detection with

the MS operating in electron impact mode.

Maturation congeners

Whisky maturation at a molecular level involves processes of congener addi-

tion, congener reduction and congener production (Philp, 1986). During these

processes the pungent and harsh characteristics of new-make spirit (new dis-

tillate) diminish and the smoother, more complex character of mature whisky

develops.

Scotch and Irish whisky maturation casks are normally made from Spanish

and American oak. Bourbon whiskey casks are made only from new charred

American oak and are used only once. This results in a flourishing market for

used American oak barrels, with the Canadian and Scotch whisky industries

being major buyers.

Maturation warehouses in Scotland and Ireland tend to be cool all year

around, while those in the USA and Canada can become very hot in the

summer and very cold in the winter. Some bourbon whiskey companies

heat their war ehouses in winter in order to accelerate their maturation process.

As the character of the spirit develops, the volume in each cask decreases by 1

to 2 per cent per year due to evaporation.

The process of congener addition is one of extraction from the oak cask into

the liquid through interaction of ethanol with wood lignin (Reazin, 1979). The

resulting congeners are known as lignin degradation products (LDPs) and

polyphenolics, which may be determined by HPLC (Lehtonen, 1983a, 1983b)

and oak lactones, which may be determined by capillary column gas chroma-

tography. Whilst new-make whiskies are clear liquids prior to the start of

maturation, they acquire varying amounts of colour and particles of charred

wood from the oak casks during the maturation process.

Direct-injection reversed-phase gradient elution HPLC with ultraviolet

and/or fluorescence detection is the most commonly used analysis within

the maturation process (Aylott et al., 1994). This analysis can be used to

quantify LDPs, which include gallic acid, vanillic acid, syringic acid, vanillin,

syringealdehyde, coniferaldehyde, ellagic acid and scopoletin, as well as

furfural (found in Scotch malt spirit) and 5-hy droxymethyl furfural (which

is mainly associated with spirit caramel). A typical chromatogram is shown

in Figure 9.3.

The congener reduction process involves volatilization (such as the loss of

dimethyl sulphide), adsorption onto the charred wood, and oxidation of car-

bonyl compounds. The congener production process involves oxidation to

form acetals, esterification to form more esters, and hydrolysis to form qui-

nones.

Sulphur-containing congeners, such as the volatile dimethyl sulphide

(DMS), dimethyl disulphide (DMDS) and less volatile dimethyl trisulphide

(DMTS), may be determined by temperature programmed capillary column

gas chromatography with flame photometric detection (Beveridge, 1990).

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DMTS is an important flavour compound in Scotch malt whiskies. These

ultra-trace compounds were concentrated from the test sample by a

dynamic headspace concentration technique onto Chromosorb 1 101 prior

to thermal desorption, chromatographic separation and detection at the low

mg/l level.

Whisky age

The concentrations of oak-derived congeners in a given cask of whisky

increase with maturation time. It is therefore possible to start to draw a

graph plotting maturation congener concentrations against age. However,

there are a great variety of barrel types in use, from the new charred oak

used in the bourbon whiskey process (which will produce relatively high

concentrations of addition maturation congeners) to the second or third refill

cask used for a Scotch grain whisky (which will result in less congener addi-

tion). Casks may also be rebuilt and rejuvenated. This means that age deter-

mination on the basis of maturation congener concentrations is imprecise.

However, the chromatographic profiles for maturation congeners are consis-

Chapter 9 Whisky analysis 289

Figure 9.3

A high performance liquid chromatogram of cask-related and other congeners in

blended Scotch whisky with (A) fluorescence detection (excitation at 345 nm and

emission at 450 nm), and ultraviolet detection at 280 nm (B) and 340 nm (C). Peaks

1, gallic acid; 2, 5-hydroxymethyl furfural; 3, furfural; 4, vanillic acid; 5, syringic acid; 6,

vanillin; 7, syringealdehyde; 8, internal standard; 9, coniferaldehyde; 10, synapaldehyde;

11, ellagic acid; and 12, scopoletin. (Aylott et al., 1994, reproduced by permission of the

Royal Society of Chemistry.)

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tent, and can prove useful in checking false authenticity claims in spurious

products where use of wood flavour additives is suspected.

Natural

C in the atmospheric carbon dioxide is absorbed by metabolism

into all plants, including the cereals used for whisky manufacture. The natural

levels of

C increased between 1945 and 1960, corresponding to the start and

end of atmospheric nuclear testing (Baxter and Walton, 1971), and have now

fallen back to near the pre-1945 levels. Analysis of the

C levels in ethanol

concentrated from whiskies samples has been used to estimate the year in

which the cereal was grown, and then to relate this year to age. Analytical

precision is limited, and this obviously reduces the accuracy of the resulting

date.

pH, residues, ash, anions and cations

The natural pH of whisky at 40 per cent vol. bottling strength is normally in

the range 4–4.5. This mild acidity is as a result of trace organic acids, with

acetic acid being the main component. Calibration solutions for the pH meter

should be prepared in 40 per cent ethanol. The pH of whisky can be made

higher wh en the maturing spirit has been reduced to bottling strength with

softened rather than demineralized water.

Whisky residues are low (typically < 0.2 g/100 ml), and generally represent

non-steam volatile material derived from the cask during maturation. Samples

are prepared for this analysis over a steam bath. Ash values are much lower

(typically < 0.02 g/100 ml), and represent involatile inorganic compounds

remaining when the test sample is taken to dryness in a furnace. The ash

will include trace metals such as calcium, magnesium, sodium and potassium

(all typically at low mg/l levels), which are derived from the water and casks

used in the whisky process. These trace metals may be analysed in whisky

directly by atomic absorption spectroscopy with flame atomisation, or by ion

chromatography with electrochemical detection.

Various trace sugars are also present, and are derived as cask extracts

and low molecular weight carbohydrates present in spirit caramel (when

used). Concentrations in Scotch whisky are typically considerably less

than 200 mg/l. A twelve-year-old bottled deluxe blended Scotch whisky

was found to contain 50 mg/l glucose, 50 mg/l fructose and 20 mg/l

sucrose. Test samples may be analysed by direct injection ion exchange

chromatography with conductivity detection, or by gas chromatography

(after taking the sample to dryness and separation of the sugars as their

trimethyl silyl ethers derivatives). Other sugars derived from carbohydrate

extracted from oak wood may be detected at much lower relative con-

centrations.

Volatile phenolic congeners

A range of volatile phenolic compounds is found in whiskies where peat is

used in the kiln-drying of the malted barley. These flavour congeners are

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present in many Scotch malt whiskies, particularly those from Islay, an island

with eight distill eries off the west coast of Scotland. In addition, volat ile phe-

nols are found in many of the blended Scotch whiskies made with such malt

whiskies.

Volatile phenol congeners are best determined by direct-injection reversed-

phase gradient elution HPLC with fluorescence detection in the presence of an

internal standard such as 2,3,5-trimethyl phenol (Aylott et al., 1994). This ana-

lysis detects phenol, guaiacol, isomers of cresol and xylenol, eugenol, and

other phenol derivatives (Figure 9.4). Alternatively, many of these compounds

may be separated by capillary co lumn gas chromatography and enhanced

sensitivity and selectivity obtained as their 2,4-dinitrophenyl derivatives

(Lehtonen, 1983b).

Sensory analysis

Sensory analysis is the key quality control technique for the whisky blender

and other process colleagues. Researchers investigating those congeners

responsible for the unique characteristics of each whisky have also developed

sophisticated sensory methods. Finally, the informed whisky enthusiast seeks

an understanding of the complexities of whisky flavour (Hills, 2000).

Chapter 9 Whisky analysis 291

Figure 9.4

A high performance liquid chromatogram of volatile phenolic congeners in blended

Scotch whisky with fluorescence detection (excitation at 272 nm and emission at

298 nm). Peaks 1, phenol; 2, guaiacol; 3, m-, p-cresol; 4, o-cresol; 5, 3,5-xylenol; 6,

4-ethyl phenol (2,5-xylenol); 7, 4-ethyl guaiacol; 8, 2-ethyl phenol; 9, eugenol; and

10, internal standard. (Aylott et al., 1994, reproduced by permission of the Royal

Society of Chemistry.)

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At the process end, new-mak e spirits (also known as ‘new distillate’ in the

USA) are monitored for conformance to speci fication before being put into

casks for maturation. Similarly, as single casks of mature whisky are selected

for disgorging (‘dumping’ in the USA), the blender will check that each spe-

cific cask is contributing the correct charact ers for the blend. These samples are

normally assessed by nose only. New-make spirit or whisky (25 ml) is typically

placed in a tulip-shaped nosing glass with an equal volume of demineralized

or distilled water in order to bring alcoholic strength down to approximately

20 per cent vol. The nosing glass is covered by a watch glass and is left to

equilibrate for about 30 minutes to allow a build-up of congeners in the head-

space above the liquid. The samples are then nosed by the blender and/or a

trained sensory panel, and their colour checked. Any abnormalities are

recorded and action is taken to resolve any unsatisfactory casks of whisky.

At this stage the sensory language used (in terms of the number of descriptors)

is normally limited and specific to the particular process being monitored.

Sensory assessment after blending is designed to ensure that the product

has consistent characteristics that meet the requirements of the brand and

correspond to a reference standard. A trained panel will nose and sometimes

taste the samples, often using triangle and duo–trio sensory tests in order to

validate the results statistically.

At the research end, descriptive terminology has been developed and

specific molec ules/congeners associated with specific character (Swan et

al., 1981). This important work at the Scotch Whisky Research Institute

(known at that time as Pentlands Scotch Whisky Research) resulted in the

Scotch whisky flavour wheel. Furthermore, the effluent from a capillary gas

chromatographic column may be spilt into two streams, with one stream

going to a mass spectrometer (or other detector) and the other to a nosing

point, where a trained panellist makes sensory comments. This method is

commonly known as GC–sniff analysis, and can help identify those conge-

ners contributing to particular odour characteristics. GC-sniff can also be

useful in detecting trac e contaminants whose peaks are hidde n in complex

chromatograms of whisky and other multi-component matrices.

The Scotch whisky flavour wheel has recently been revised, and reference

compounds have been defined for assessor training purposes against each

flavour term (Table 9.3) (Lee et al., 2001b). The revised flavour wheel is

shown in Figure 9.5. The primary tier terms include a range of common

descriptors for whisky, including: peaty, grainy, grassy, fruity, feints,

woody, sweet, stale, sulphury, cheesy, and oily.

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Chapter 9 Whisky analysis 293

Figure 9.5

The revised Scotch whisky flavour wheel. (Lee et al., 2001b, reproduced with permission from Jo urnal

of the Institute of Brewing.)

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Table 9.3

Whisky descriptors and reference compounds (Lee et al., 2001b,

reproduced with permission from Journal of the Institute of Brewing)

Code Attributes Reference compounds Concentration (mg/L)

N.1 Pungent Formic acid

10  10

A.1, 2 Burnt/smoky Guaiacol

A.3 Medicinal o-Cresol

1.75

B.2 Malty Malted barley

2-and 3-Methyl butanal,

4-Hydroxy-2(or 5)-ethyl-2

(or 5)-methyl-3(2H) furanone,

4-hydroxy-2,5-dimethyl-3

(2H) furanone

–

0.6 (2-methyl butanal)

1.25 (3-methyl butanal)

C.1 Grassy Hexanal

cis-3-Hexen-1-ol

1:00  10

D.1 Solventy Ethyl acetate

2-Methyl propan-1-ol

1:12  10

1:00  10

D.2 Fruity (apple) Ethyl hexanoate

D.3 Fruity (banana, pear-drop) iso-Amyl acetate

D.5 Berry

Catty

Thiomenthone

Sodium sulphide + mesityl oxide

3  10

3

1.26

100 each

E.1 Floral (Natural – rose

– violet)

Phenyl ethanol

1:52  10

>3  10

3

Floral (Artificial –

scented, perfumed)

-, -Ionone

Geraniol

G.5 Nutty (coconut) Whisky lactone

266

Marzipan Furfural

839

G.6 Vanilla Vanillin

G.7 Spicy 4-Vinyl guaiacol

Spicy (clove) Eugenol

a,b

1–55

G.8 Caramel (candy floss) Maltol

1:14  10

G.10 Mothball Naphthalene >8  10

G.11 Mouldy 2,4,6-Trichlorocanisole

Earthy, musty Geosmin, 2-methyl iso-borneol

G.12 Vinegary Acetic acid

5:32  10

I.1 Cardboard 2-Nonenal

0.08

J.1,6 Stagnant, rubbery Dimethyl tri-sulphide (DMTS)

J.2 Yeasty

Rotten egg

meaty

Hydrogen sulphide (H

Hydrogen sulphide

Methyl (2-methyl-3-furyl) disulphides

> 0:02

> 0:14

J.3 Vegetable (sweet corn,

cooked cabbage)

Dimethyl sulphide (DMS)

>0:6

J.5 Gassy Ethanethiol

>0:072

3-Methyl-2-butene-1–thiol

>7:2  10

4

K.1 Rancid n-Butyric acid/ethyl butyrate

Sweaty Iso-Valeric acid

L. Oily Heptanol

L.1 Soapy Ethyl laurate

1-Decanol

100

L.2 Buttery Diacetyl

0.1

in 23% ethanol solution;

in 23% grain whisky;

in lager;

>threshold;

thresholdinbeer

For reference papers to standards please see the original paper ‘‘Origins of Flavour in Whiskies

and a Revised Flavour Wheel: A Review’’ by K.-Y.M. Lee, A. Paterson, J.R. Piggott and G.D.

Richardson, published in 2001 in the Journal of the Institute of Brewing, Vol 107 (5) pp 287^313.

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Quality assurance and analysis in the whisky production process

Malting, fermentation, distillation and maturation

Distillers require cereals with specific properties, such as low nitrogen content,

known moisture content and appropriate malting characteristics. Malting bar-

ley varieties may require authentication. Potential distillery yield may be pre-

dicted by testing fermentation efficiencies in laboratory trials, and an array of

techniques are available for distillery process troubleshooting (Hardy and

Brown, 1989). Further routine process analyses during fermentation, distilla-

tion and maturation are normally limited to sensory assessments and alcoholic

strength measurement.

Blending and bottling

The main analysis (along with sensory assessment) in the blending and bot-

tling process is alcoholic strength measurement, and modern facilities employ

electronic density measurement.

The quality of the water used for final reduction to bottling strength is very

important. The town water supply is normally used, but it is usually subjected

to a demineralization and carbon and ultraviolet light treatment process at the

bottling plant in order to reduce trace anions and cations to a minimum and

eliminate the risk of any off-odours. Conductivity measurement is used to

monitor the water treatment process and the characteristics of bottled product

in association with pH measurement.

Colour consistency is also an important quality parameter, particularly to

those whiskies that include addition of trace spirit caramel for colour standar-

dization. Colour is normally monitored by visible spectrophotometry at a

wavelength of around 500 nm. The resulting ‘tint’ is usually quoted as the

absorbance at a chosen fixed wavelength  100.

Whiskies are usually filtered through both coarse and fine sheets or car-

tridge filters in order to ensure a clear, bright product. Chill filtration is often

used, where the liquid is chilled before filtration in order to achieve enhanced

stability under prolonged cold environmental storage conditions. Clarity or

turbidity is measured on a nephelometer using instruments made by Sigrist 1

and Hach 1.**49

Whisky stability

Once in the bottle, whisky is a very stable product. Providing the closure gives

a good seal on a glass bottle, alcoholic strength will remain constant and the

product character will not change. Storage at warm temperatures can result in

slightly elevated acetaldehyde and ethyl acetate concentrations. If the closure

is loose, then ethyl acetate and acetaldehyde concentrations will diminish

through volatilisation, and alcoholic strength will be lost. Stability testing

Chapter 9 Whisky analysis 295

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with Scotch whisky packaged in polyethylene terephthalate (PET) 5-cl bottles

over a two-year test period showed that while sensory properties remained

stable, alcoholic strength slightly increased due to prefe rential migration of

water through the bottle wall. Ethylene terephthalate cyclic trimer was also

detected by HPLC as a trace migrant from PET into spirit at the sub-mg/l level

(Aylott and McLachlan, 1986).

Whilst filtered whisky is normally a clear, bright product, two forms of

flocculation have occasionally been experienced in Scotch whiskies. The first

form is known as ‘reversible floc’, and can form when whisky is stored for

prolonged periods at very cold temperatures (as may be encountered in cold

winter transit). The whisky develops a haze, which disappears when the liquid

is warmed and shaken. The main congeners detected in reversible floc are the

ethyl esters of long chain fatty acids and larger alkyl esters, both detectable by

capillary column GC–MS. As whiskies are produced for distribution around

the world to a wide range of climatic co nditions, it has been found that rever-

sible floc formation may be minimized by chill filtering whisky prior to bot-

tling. This process reduces the concentrations of reversible floc-forming

material, and has no effect on product character.

The second form is known as ‘irreversible floc’. This shows itself as ver y

small, hair-like crystals of calcium oxalate, which slowly form and settle in the

whisky when natural low mg/l concentrations of oxalic acid in the whisky

react with similarly low concentrations of calcium ions. Oxalic acid may be

determined by ion chromatography and calcium by flame atomic absorption

spectrophotometry. Irreversible floc formation is eliminated by ensuring that

final calcium concentrations are kept to a minimum by demineralizing the

water used for final reduction. Tennessee whiskies, whose process includes

percolation of new distillate through maple charcoal prior to maturation, also

pick up calcium from the wood, and levels may be reduced using an ion

exchange treatment.

Off-odours as contaminants in whisky

Off-odours are occasionally reported in bottled product. The more common

off-odour issues result from whisky being transported and stored in unsuita-

ble environmental conditions. For example; transit and storage close to very

smelly chemic als can result in odour ing ress into the bottle. Bottles capped

with roll-on pilfer-proof (ROPP) closures are most resistant to odour ingress.

In each of the examples described below, GC–sniff used in parallel with GC–

MS helped to identify the offending contaminants.

Ingress of naphthalene odours (mothball smell) has been detected on var-

ious occasions as a contaminant in consumer complaint samples returned

from distant markets when product had been stored under unsuitable envir-

onmental conditions. The sensory threshold of naphthalene in whisky is very

low (typically at mg/l levels in product), and its presence may be quantified by

HPLC with fluorescence detection or GC–MS with selected ion monitoring.

Musty-smelling compounds can also be encountered as contaminants in

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whisky. The first example is 2,4,6-trichloroanisole (TCA), a particularly smelly

compound that has been reported on many occasions (Saxby, 1996). TCA

contamination at the sub-mg/l level has been found when good quality bottled

product was stored in hot, wet and humid climatic conditions such as found in

southern Asia. It was thought that trace trichlorophenols in the pulp used to

make cartons and cases underwent microbiological degradation to form TCAs,

which then entered the product through the closure/bottle interface. The sec-

ond musty-smelling example is that of geosmin contamination resulting from

microbial activi ty on cereals stored under damp conditions prior to their bein g

taken for fermentation. Trace mg/l concentrations of this musty/earthy-smel-

ling compound were carried through fermentation, distillation and maturation

to contaminate the resulting whisky.

Consumer issues

Analysis can prove very useful in the examination of samples returned by

consumers as complaints . Considering the enormous number of whisky bot-

tles sold globally, the number of consumer complaints is usually very small.

This is due to the inherent stability of whiskies and the very high quality

standards applied throughout whisky manufacturing processes. The first

thing that a quality laboratory has to do on receipt of a consumer complaint

is to confirm its validity and the likely cause. Valid consumer comp laints may

be divided into two categories; those resulting from packaging and/or those

from liquid issues. Complaints in the packaging category normally aris e from

quality issues on the production line or damage caused during distribution.

Packaging line issues may be minimized by employing appropriate stan-

dards and quality assurance procedures in the supply of materials to the

manufacturing process. Modern glass bottles are treated at the hot end of

the glass-making process with tin oxide in order to toughen the glass, and

at the cold end of the process with surfactants and oleic acid in order to give

the glass resistance to scuffing during handling. Over-application of oleic acid

can give rise to contamination by small, oily globules of the acid in product.

Many manufacturers apply codes to their bottles that enable product trace-

ability back to specific production batches, or to pro duction dates and time of

bottling. These lot codes are usually found near the neck of the bottle, on the

side of the bottle near its base, or on the glass behind a label.

Sometimes a consumer may inadvertently contaminate whisky with mate-

rial that renders the whisky unstable. As an example, traces of milk result in a

heavy precipitate of insoluble milk and whisky components. This contamina-

tion can be characterized through the detection of lactose (from milk) in the

supernatant and dairy fats in the precipitate (analysed as their methyl esters

after derivitization with boron trifluoride/methanol).

In a similar way, low mg/l concentrations of iron react with maturation

congeners and cause a green discoloration, which is virtually black at the

10 mg/l iron level. Rust in any post-distillation process can be very damaging,

Chapter 9 Whisky analysis 297