5. The oxidation of unsaturated fatty acids (derived from barley malt) to carbonyl compounds.

B. Grain Whiskey

Grain whiskey is normally produced by continuous distillation in a Coffey (patent) still, which consists of an analyzer column and a rectifier column and which in fact has not greatly changed since it was first developed by Aeneas Coffey in 1831 (Fig. 4).

In this distillation system, the incoming material is the cold wash of 10% ethanol content, and the products from this process are as follows:

1. A low-boiling-point fraction (the heads) rich in aldehydes and esters, from which any ethanol present can be recovered.

2. Raw spirit (main fraction) containing ethanol, approximately 95% by volume (190° proof U.S.). In practice, the whiskey is actually distilled at 180-186° U.S. proof because United States law does not permit distillation at over 190° U.S. proof (166° proof British).

3. A higher-boiling fraction that is rich in fusel oil components (Table 2)

The distillation of aqueous ethanolic mixtures is governed by specific physical properties of water and ethanol, most notably that an aqueous ethanolic mixture cannot be completely separated into ethanol and water. These are liquids that are completely miscible with each other and, therefore, out of all the possible ethanol-water mixture compositions, there is one particular mixture (96% ethanol and 4% water) that has a maximum vapor pressure and that therefore boils at a lower temperature than any other mixture of these two liquids. This mixture is an azeotropic (constant boiling) mixture that is able to distil without changing its composition; that is, the distillate has the same composition as the original liquid. In this case it is a minimum boiling mixture (a particular kind of azeotropic mixture) because the boiling point is lower than for any other mixture of ethanol and water (15).

In the Coffey still, the wash is steam-distilled and the resulting vapor is richer in ethanol than the original wash. However, in the rectifier column, the principal distillate fraction recovered is the azeotropic ethanol-water mixture (raw spirit) containing approximately 95% ethanol.

After distillation, the spirit is diluted with distilled water: the ethanol content (percent by volume) of malt whisky is reduced to 63.5% (20), while that of grain whiskey is reduced to 55% before being placed into oak barrels for maturation.


Figure 4 Coffey still for continuous distillation in the production of grain whiskey. (From Ref. 5.)


Figure 4 Coffey still for continuous distillation in the production of grain whiskey. (From Ref. 5.)

Table 2 Aliphatic Alcohols in Relation to Their Recovery from Continuous Distillation in the Coffey Still


Boiling point (°C)



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