Steam Drain

Fig. 6.2 A Coffey still.

Whiskies are matured in oak casks. Whereas American bourbon and rye whiskies are put into new oak casks, Scotch, Irish and Canadian whiskies are filled into casks that have previously been employed for Bourbon or for sherry. For the most part they comprise 50 L butts. Whisky casks are either of American white oak (which are used for Fino and Amontillado Sherries) or Spanish Oak (used for Oloroso Sherry). The bourbon casks used for Scotch whiskies must be filled at least once with bourbon and the whiskies must have been in the cask for at least 4 years. Ageing of whisky in most countries must be for at least 3 years. There is a significant loss of alcohol by evaporation in this time, referred to as the 'angel's share'. In the maturation there is the development of mellowness and a decrease of harshness. Flavours associated with mature whisky are vanilla, floral, woody, spicy and smooth. The undesirable flavours that dissipate are sour, oily, sulphury and grassy. Various components are extracted from the wood, including those developed by wood charring. The major flavour components of whisky are listed in Table 6.1.

Usually the lighter bodied spirits generated on a continuous still are blended with a range of heavier bodied spirits coming either from batch stills or by distillation to lower ethanol concentrations in column stills. In the decantation process, the various whiskies are decanted into troughs by which they flow

Table 6.1 Flavour constituents of whisky.

Main congeners

Ethyl octadecanoate


Ethyl octanoate

Ethyl acetate

4-Ethyl guaiacol





2-Methyl butanol

Ethyl undecanoate

3-Methyl butanol




Furfuryl formate

Other congeners

Gallic acid

Acetyl furan







5-Hydroxymethyl furfural

m-, o-, ^-Cresol

Isoamyl acetate

Decanoic acid

Isoamyl alcohol


Isoamyl decanoate


Isoamyl octanoate

Diethyl succinate

Cis-Oak lactone

Dimethyl disulphide

Trans-Oak lactone

Dimethyl sulphide


Dimethyl trisulphide


Dodecanoic acid



Phenylethyl acetate

Ellagic acid

Phenylethyl butanoate



Ethyl butanoate


Ethyl decanoate


Ethyl dodecanoate

Syringic acid

Ethyl hexadecanoate

Tetradecanoic acid

Ethyl hexadecenoate


Ethyl hexanoate

Vanillic acid

Ethyl lactate


Ethyl nonanoate


to a blending vat wherein they are mixed by mechanical agitator and compressed air. Then 'de-proofing water' is added to take the product to its final strength.

In Scotland, the final products may be a blend of whiskies from more than ten grain distilleries and up to a hundred malt distilleries. There is an astonishing interaction and cooperation between separate companies to enable this. The blending is deliberately complex so that the unavailability of one or two whiskies in any single blending will not be noticeable. In other countries where there are far fewer distilleries, batch-to-batch variation must be achieved by varying conditions within the distilleries themselves - for example, the grist or the fermentation and distillation conditions.

Most whisky is filtered. Insoluble fractions, notably lignins and long chain esters of fatty acids, are removed by cooling to as low as -10° C and filtration, typically in plate and frame devices with diatomaceous earth as filter aid.

Whiskey variants

Bourbon (United States) is made principally from corn (maize) plus added rye and barley and is aged in charred barrels. A close relative is Tennessee whiskey (United States), which is produced using a sour mash process. Canadian whisky (Canada) is a light product from rye and malted rye, with some corn and malted barley. Corn whiskey (United States) is from maize and is aged in barrels that have not been charred. Rye whiskey (United States) is from rye mixed with corn and barley and is aged in newly charred oak barrels.

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