Scotch Whisky

Iain Campbell International Centre for Brewing & Distilling, Heriot-Watt University, Edinburgh EH14 4AS

Scotch whisky distilleries produce either ‘malt’ or ‘grain’ whisky. The only permitted cereal for malt whisky is malted barley, and the fermented wash must be distilled batchwise in pot stills. Grain distilleries may use any cereal carbohydrate source provided it is hydrolysed by malted barley, and the alcohol may be recovered by continuous distillation. The majority of Scotch whisky brands are blends of malt and grain whiskies, but there is an increasing demand for malt whisky.

Definition of Scotch whisky

The quality of both malt and grain whiskies is maintained by a strict internationally- agreed definition.

1. Produced at a distillery in Scotland from water and malted barley, to which only whole grains of other cereals have been added, all of which have been: a. processed at that distillery into a mash; b. converted to a fermentable substrate only by the enzymes of the malt; c. fermented only by the addition of yeast.

2. Has been distilled at less than 94.8% alcohol by volume.

3. Has been matured In an excise warehouse in Scotland, in oak casks not exceeding 700 litres capacity, for not less than 3 years.

4. Retains the colour, aroma and taste derived from the materials used in its production and maturation.

5. No substances other than water and spirit caramel have been added.

6. Has a minimum alcoholic strength 40% by volume.

Malting, Mashing and Fermentation (Malt Whisky)

Steeping and germination are essentially the same procedure as for production of brewers' malt, but to preserve maximum enzyme activity, distillers' malt is dried at a lower temperature, probably not more than 60 °C, and often only to about 14% moisture. For some distilleries, the malt is dried in the smoke of burning peat to contribute to the flavour of the whisky.

1 To maximise extract and spirit yield, the malt is ground more finely than in a brewery, and mashed initially at lower temperature, for example, 62-65 °C. Subsequent sparging is at increasingly higher temperatures, but only the initial mash run-off and first sparge are used to provide wort, of specific gravity about 1.060. Higher-temperature second and third spargings are kept as ‘first water’ for the next mash. Thus, the optimum temperatures of all amylolytic enzymes are used in the course of the mashing program.

Particles of starch which pass to the fermentation vessel are hydrolysed by continuing enzyme action during fermentation. Yeast is pitched into wort at 20-22 °C, the temperature increases over 24 h to 33-35 °C and fermentation is over by 48 h (specific gravity about 0.998).

The effect of fermentation variables on production of flavour compounds is well known in beer and wine fermentations. The same principles apply to flavour production in whisky fermentations, but it is not possible to change the composition of the wort. The fermentation temperature is not controlled, it varies with ambient temperature, being a few degrees higher in warm weather. Different yeast strains produce different volatile metabolites, and this provides an opportunity to vary the flavour.

Wash Distillation

In Scotland. two stills are normally used, the wash still and spirit still. The initial slow heating of the charge in the wash still prevents wort solids (malt debris and yeast) baking on the heating coil. When boiling begins, heating must be reduced to prevent frothing (due to dissolved CO2 and surface-active fatty components of yeast and malt), breakdown of esters and formation of ethyl carbamate in the first 15-20 min. Frothing persists longer, and limits the rate of distillation.

The wash, containing 7-8% alcohol, begins to boil at about 92 °C giving a distillate of about 55% abv. Distillers try to recover as much as possible of the alcohol in the wash, but in practice it is uneconomical below about 0.1%, corresponding to 1% in the distillate, so the final alcohol content of the ‘low wines’ (LW) is about 20% abv. The cloudiness and surface film developing in the LW is due to the volatile oils and waxes in the wash, mainly from the yeast.

The pot ale remaining in the still contains all non-volatile components of the wash, as well as a few ppm Cu dissolved from the still. After passing though a heat exchanger for heat recovery, commercial pot ale is disposed of by one of (a) a dedicated high BOD effluent treatment plant at the distillery, (b) spraying on farm land, (c) discharge to sea if tidal currents are suitable (the BOD is too high for a river), or (d) mixing with spent grains and drying to animal feed.

2 Spirit Still

The second distillation concentrates and purifies the LW, and according to operating conditions, controls the flavour of the spirit. The first distillate, 80-82% abv, is contaminated by the oily, waxy material which condensed on the inner surface of the neck and lyne arm in the later stages of the previous distillation but is dissolved by the strong alcohol giving a clear solution. This is collected as foreshots (FS), until no cloudiness occurs on dilution with an equal volume of water. In some distilleries, operators also assess for unwanted volatiles, for example, CH3CHO or (CH3)2S2. The alcohol content of the collected spirit must be at least cask-filling strength, which is usually about 63%, but in practice an exact density of distillate (= exactly known abv) is selected as cut point. The remaining alcohol is collected as ‘feints’, and recycled to purify and concentrate the alcohol. Fig. 1 is a flow diagram of batch distillation.

In a common distillery layout, the washback (fermentation vessel) holds 2´ the volume of the wash still, and two successive 4 hour wash distillations provide the charge for a spirit still distillation (two batches of LW and one of FS/feints) of up to 8 h. Both stills are approximately the same size. Alternatively, there are pairs of wash and spirit stills, each wash still producing LW for its smaller partner spirit still which is running at the same time. Normally, LW and FS/feints from both wash and spirit stills are collected in a single receiver, with a consistent fall in alcohol content with time as the synchronised distillations proceed. Operation with wash and spirit stills out of phase causes flavour problems from varying % alcohol and different separation of waxes and fusel oil during successive ‘unbalanced’ distillations. Spent lees from the spirit still has, in comparison with pot ale, a low organic content and BOD. After heat recovery, it may be discharged directly to drain.

Flavour Development in Pot Distillation

Since the whole distillate of the wash still is collected as a single batch, there are only limited possibilities for varying the flavour composition at that stage. All volatile components, ‘congeners’, of the wash are collected in the LW. The design and operation of the spirit still, however, are the principal influences on the flavour of the new spirit and are extremely important to the final whisky.

There are three classes of congener: (a) more volatile than ethanol, (b) similar volatility to ethanol and (c) less volatile than ethanol. This affects their distribution between foreshots, sprit and feints fractions as shown in Fig. 2. Different congeners of type C reach their maximum level in the distillate at different times, shown as C1 and C2 in Fig. 2. Therefore the ‘cut point’ between spirit and feints fractions has an important influence on flavour: compare the levels of congeners C1 and C2 with cut points of 60% or 50% abv. With a cut point of 50% abv, the final spirit has a stronger flavour of congeners although it is weaker in terms of ethanol.

3 Reflux in the still also affects flavour. The greater the reflux effect, the more the low- volatile but strongly flavoured components of the vapour condense and return to the pot. With less reflux, more of these components pass over to the condenser and to the spirit. Reflux is affected by the shape of the still (Fig. 3), where the series A to D represents increasingly strong flavour, but also by boiling rate (which can be controlled) and ambient temperature and barometric pressure (which cannot).

Another aspect of flavour is the effect of a series of distillations after some change in the system. Suppose a new yeast strain is used which produces higher levels of esters, at present considered to be a desirable quality. Although all ester which is sufficiently volatile to distil will be collected in the LW, the situation is more complex in the spirit still. Not all components of LW are distilled in the spirit still, which for most of its run is operating at a lower temperature than the wash still. Also, up to half of the charge of the spirit still is recycled FS/feints, so only half of the higher level of ester is immediately available in the spirit still. Of that ester content, only part appears in the spirit itself if the esters collect in the foreshots or feints fractions. These are always returned to the next distillation, but it may be many distillation cycles before a higher level of ester in the wash is present in the spirit.

Finally, although the standard Scottish system is double distillation, there are some examples of triple distillation. This produces a stronger spirit, but it is diluted for maturation, and later for bottling, so flavour is diluted. Also, the additional distillation stage removes more of the flavour volatiles, giving a ‘lighter’ spirit (in flavour, but not alcohol).

Grain Whisky

The principal source of fermentable sugar for grain spirit is unmalted cereal, usually either maize or wheat. The cereal slurry must be heated to at least its gelatinisation temperature but many distilleries pressure-cook unmilled grain at 3 bar; release of pressure disrupts the grain, which is run into the mash tun and mixed with ground malt. A typical recipe is 90% grain:10% malt. Malt of high diastatic power is essential for hydrolysis of up to 90% unmalted cereal: either lightly kilned or ‘green’ unkilned malt is used. In many distilleries ‘mashing’ is very brief, and the entire slurry is transferred to the washback and pitched with a pure culture of distillers' yeast; starch is hydrolysed during fermentation. As in malt distilleries, washbacks normally have no temperature control, the temperature rises to 33-35 °C and the fermentation is complete by 48 h. CO2 is collected as an important by-product. The wash, including grain debris and yeast, is heated in the rectifier column of the continuous still before discharge at the top of the analyser, in which volatiles are stripped out by steam and condense at the appropriate level in the temperature gradient of the rectifier (Fig. 4). Ethanol at 94% abv is collected near the top of the rectifier. This is not pure enough for gin or vodka spirit, which must be distilled again, but for grain whisky, the spirit is reduced to 63-70% and matured for at least 3 years in oak casks.

4 Flavour Development in Continuous Distillation

Although Fig. 3 refers to pot distillation, the effect of the three different types of volatility is still relevant to continous stills. The mode of operation ensures that compounds which are more volatile than ethanol, type A in Fig. 3, will be stripped from the ethanol stream. Type C compounds, less volatile than ethanol, will condense in the lower section of the analyser column for collection as feints. Compounds shown as type B in Fig. 3, less volatile than ethanol at high concentrations but more volatile at low concentrations, tend to behave as type C in a continuous still. Therefore in comparison with pot stills, only small amounts of flavour congeners are present in the spirit from a continuous still, but sufficient to impart a whisky character. The principal flavour compounds of grain spirit are higher alcohols. Fig. 5 shows the profile of alcohols at different levels of a continuous still.

Maturation

By law, Scotch whisky must be matured for a minimum of 3 years in a bonded warehouse in Scotland. In the cool moist Scottish climate the progress of maturation is different from most other whisky-producing countries. Approximately 2% of the volume is lost annually, and possibly up to 0.5% of the alcohol, so the improvement of flavour has to be balanced against loss by evaporation and the substantial cost of storage. Most distillers would mature longer than 12 years only for a premium product. The age of whisky refers to maturation time in cask; no further changes occur in glass, so the dates of distillation or bottling are not normally stated.

Only oakwood is permitted for whisky casks, in practice either American or Spanish oak. The basic flavour of the new spirit is derived from the flavour congeners of fermentation, and the malt or grain itself. This flavour persists through maturation, but is modified by changes occurring over the period in contact with wood. Changes are complex, but include evaporation of unwanted volatile sulphides and CH3CHO, extraction of colour and flavour compounds from the wood, development of more new flavour compounds which dissolve back into the maturing whisky.

Many maturation reactions are oxidative, using first the dissolved O2 from the turbulence of filling and then, more slowly, from the air diffusing through the wood. In addition, flavour characters are dissolved from the wood itself: wood acids, aldehydes, esters, tannin derivatives and lignin degradation products (for example, syringaldehyde, vanillin) are important contributors to the flavour and aroma of mature whisky (Tables 1, 2). The removal of cereal, sulphury, and/or rubbery off-notes takes place during maturation, whilst colour, fragrance, estery character, astringency (lignin degradation compounds, tannins), mellowness (reduction of pungency) and complexity will increase.

Grain spirit is distilled at up to 94% abv, but that strength would extract unpleasant alcohol-soluble flavour components from the wood. So spirit must be diluted to 63-65%,

5 which experience has shown to give best extraction of desirable flavour components of the wood. Malt spirit is already at the correct % abv.

Blending

The majority of whiskies are blends of malt and grain whisky. Although some cheaper blends exist with lower proportion of the more expensive malt whiskies, standard blends contain 60-65% grain whisky and 35-40% malt. Internationally known blends in this category are Ballantynes and Johnnie Walker Red Label. In de luxe blends (for example, Chivas Regal, Johnnie Walker Black Label) these proportions are reversed, and the component whiskies are older. Age of a blend, if stated (for example, Chivas Regal), refers to the youngest component; it is not an average. Although most Scotch malt whisky is used for blenidng, there is an increasing demand for ‘single malt’ whiskies. ‘Vatted malt’ whiskies are occasionally encountered: they are blends of malt whiskies only, usually selected from the same area.

A standard blend of Scotch whisky will contain at least two different grain whiskies and at least 10 different malt whiskies; many contain more. The blender has a general recipe for a particular named blend, but variations between different distillations of malt whisky, or even between individual casks, mean some alteration of composition each time to achieve the standard flavour associated with that blend. The skill of the blender is in the ability to predict the flavour of the whisky (i.e. the consumer's sense of taste) solely by ‘nosing’ whisky samples diluted to 20% abv. Blenders always nose, and never taste, their samples.

Normally, as a final stage before bottling, cask-strength whiskies are diluted to an alcohol content of 40% or 43% with pure water, often deionised to prevent slow development of inorganic precipitate during storage. Whisky is adjusted to the correct colour if necessary by the addition of flavourless ‘spirit caramel’ (the only legal additive other than water), chilled to -10 °C and filtered through cellulose sheets to remove any haze-forming compounds extracted from the wood during maturation.

Ó Iain Campbell, 1995

6 100 volumes wash at 7-8% abv

Wash Spirit still still

20.5-23 35 10.5-11.5 foreshots low wines spirit and feints at 19-22.5% at 68-70% at 28-30%

65 23.5-24.5 pot ale spent lees charge at <0.1% at <0.1% at 23-28%

Fig. 1. Flow diagram of malt distillation.

80 ethanol

60

40 B C2 C1 20 A

Time

Fig. 2. Congener profiles in the spirit still. A, more volatile than ethanol; B, similar volatility to ethanol; C1 and C2, less volatile than ethanol. A B C D

Fig. 3. Effect of still neck and lyne arm on reflux. The relux is greatest in A, and least in D.

Fig. 4. The Coffey (continuous) still used in the production of grain spirit. 32

ethanol

24

iso-butanol 16

8

iso-amyl alcohol

Relative amount

Fig. 5. Alcohol profiles in the rectifier of a Coffey still.