Page 105 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

and converts it into crude alcohol. Fermentation takes about 48 hours and produces a liquid known
as wash, containing alcohol of low strength, some unfermentable matter, and certain by-products of
fermentation.
(4) Distillation
Malt whisky is distilled twice in large copper pot stills. The liquid wash is heated to a point at
which the alcohol becomes vapor. This rises up the still and is passed into the cooling plant, where
it is condensed into liquid state. The cooling plant may take the form of a coiled copper tube or
worm that is kept in continuously running cold water, or it may be another type of condenser.
The first distillation separates the alcohol from the fermented liquid and eliminates the residue of the
yeast and unfermentable matter. This distillate, known as low wines, is then passed into another still,

where it is distilled a second time. The first runnings from this second distillation are not considered
potable, and it is only when the spirit reaches an acceptable standard that it is collected in the spirit
receiver. Again, toward the end of the distillation, the spirit begins to fall off in strength and quality.
It is then no longer collected as spirit but drawn off and kept, together with the first running, for
redistillation with the next low wines.
Pot Still distillation is a batch process.

Grain Whisky

The patent still process by which grain whisky is made is continuous in operation and differs from the pot
still process in four other ways.

a. The mash consists of a proportion of malted barley together with unmalted cereals.

b. Any unmalted cereals used are cooked under steam pressure in converters for about 3½ hours. During
this time the mixture of grain and water is agitated by stirrers inside the cooker.

c. The starch cells in the grain burst, and when this liquid is transferred to the mash tun, with the malted
barley, the diastase in the latter converts the starch into sugar.

d. The wort is collected at a specific gravity lower than in the case of the pot still process.

e. Distillation is carried out in a patent or Coffey still, and the spirit collected at a much higher strength.

Storage and aging of the whisky are also an important part of the overall process but need not be
considered for this problem. Storage occurs in oak barrels that previously stored either sherry or bourbon
(or both, in the case of double-aged whisky). The length of storage in the barrel determines the vintage of
the whisky. Unlike wine, the time after bottling does not count, and so a 15-year-old scotch that was
bought in 1960 is today still a 15-year-old scotch.