Chapter 11 Production of Scotch and Irish whiskies: their history and ...

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154 T.P. LyonsTable 4. Effect of the level of bacterial infection on the loss of spirit incurred following fermentation of a scotchmalt whisky mash.*Infection Bacteria/ml in 30 hr old Approximate loss in Approximate financial loss inrating mash (million) spirit yield (%) US dollars (thousands)At fillingDuty paida 0 - 1 131 >1844*Dolan, 1976.The effect of low insoluble solids content is afactor relevant to congener levels in malt whiskyfermentations. In grain whisky production, where‘all-grains-in’ fermentations are generally used,the degree of rectification during distillation isthe principle determinant of higher alcohol levelsin the spirit.DistillationWhether the fermented beer is distilled in a potstill, as in production of Scotch malt and Irishwhiskies, or in a continuous still based on theCoffey design as in the manufacture of Scotchgrain whiskies, the objectives are the same:selective removal of the volatile compounds(particularly the flavor-producing congeners)from the non-volatile compounds and to createadditional flavor-producing compounds as aresult of chemical reactions that take place in thestill. Nevertheless, it is still most convenient todiscuss whisky distillation under the separateheadings of pot still and continuous distillation.Pot still distillationThe copper pot still, which is the featuredominating any Scotch malt or Irish whiskeydistillery, has changed hardly at all over thecenturies, except of course in size. Traditionally,the onion-shaped stills were fired from beneathand had the vapor pipe or ‘lyne arm’ from thestill projecting through the distillery wall toconnect with a condenser in the form of a coilimmersed in a water tank fed from a local stream(Figure 9). Internal steam-heated calandria arenow preferred to direct firing because thisdecreases the extent of pyrolysis of the stillcontents and results, for example, in a lowerconcentration of furfural in the whisky. VariationsTable 5. Production of higher alcohols (mg/100 ml).Insoluble solidscontent 2-methyl butanol 3-Methyl butanolMash (mg/100 ml) n-Propanol Isobutanol (amyl alcohol) (isoamyl alcohol) Total1 0 1.5 4.2 3.0 8.0 16.725 1.5 6.5 4.4 8.8 21.22 0 2.8 4.5 2.7 9.5 19.535 3.3 6.6 3.7 10.1 23.73 0 1.7 4.8 3.1 8.0 17.650 1.7 6.8 4.6 9.5 22.6
Production of Scotch and Irish whiskies: their history and evolution 155in still design include expansion of the surfacearea of the column into a bulbous shape, waterjacketing and return loops from the first stage ofthe condensation. (Nettleton (1913) provided avaluable account of early still design). In manydistilleries, condenser coils have been replacedby tubular condensers that have an advantagein that they are designed to conserve the heatextracted from the distillates. Yet other pot stillsare fitted with ‘purifiers,’ which consist of acircular vessel cooled by running waterinterposed between the neck of the still and thecondenser. In Irish pot stills this purifier functionis effected by a trough fitted around the lyne armthrough which running water is circulated. Potstills in Scotch and Irish distilleries are traditionallyconstructed of copper. The reason for thisadherence to copper is more than tradition. Ithas been established that copper fixes somevolatile sulfur-containing compounds that areproduced during fermentation but undesirablein the distilled spirit.Early whisky distillers realized that althoughthe objective of distilling was to separate volatileconstituents from the beer, collecting thedistillate not as a whole but in several fractionsand combining certain of these fractions gave amuch more acceptable product. Pot stilldistillation in Scotland and Ireland differ not onlyin the size of the stills (25,000-50,000 liters inScotland vs 100,000-150,000 liters in Ireland), butalso in the different ways in which fractions arecollected from the stills.In Scotland, the beer is subjected to twodistillations. In the first, carried out in the beerstill, the beer is brought to a boil over a periodof 5-6 hrs and the distillate is referred to as lowwines. This distillation effects a three-foldconcentration of the alcohol in the beer (Figure10). The residue in the wash still, known as ‘potale’, is either discharged to waste or evaporatedto produce an animal feed (Rae, 1967).Distillation of the low wines in the spirit is stillmore selective. The first fraction, which containslow boiling point compounds, is rejected as ‘foreshotheads’. At a stage determined by continuedhydrometric monitoring, which usually occurswhen the distillate has an alcohol content ofapproximately 70-73 o GL, the distillate is switchedfrom the fore shots tank to the whisky receivertank. This switch has traditionally been made atthe discretion of the distiller; and he has beenFigure 9. Diagram of an Irish distillery pot still. Designs used in Scotch malt whisky distilleries are similar exceptthat the pot is onion-shaped and the still usually has a shorter lyne arm not surrounded by a lyne-arm tank(Lyons, 1974).
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