You can increase the final concentration of ethanol by re-distilling, and this is exactly what is done in practice. Two or even three distillations are done before the product is ready for storage and maturation. This traditional process works well, but is time and labor intensive. Re-designing the still can save a considerable amount of both, by creating reflux.
Whiskey stills are large copper pots topped by lovingly crafted copper alembics. Each distillery has its own unique alembic design, perfected through years of trial and error. These designs are carefully maintained and duplicated, even down to all the dents and blemishes. In the complex environment of a still, very small changes can have very large effects.
Tradition and beauty are not the only reasons for using copper. Several distilleries have tried substituting stainless steel for copper, with bad-tasting results. Changing some of the parts back to copper restored the proper taste. Both physics and chemistry are involved in the process.
The alembic domes are not insulated, and are deliberately left open to the air for cooling. Vapor condenses on the inside surface to form liquid reflux, and this in turn re-evaporates, with the result that vapor becomes stronger and the reflux weaker. This process, called "fractionating", can repeat itself many times in a good whiskey still.
After making its way to the top of the alembic, the vapor flows through a special tube called a Lyne arm or Swan's Neck to the condenser. The size and shape of the Swan's Neck help establish the rate of production and the amount of turbulence in the alembic dome, controlling the amount of reflux processing that occurs. The shape of the alembic, the height of the still, and the angle of the Swan's Neck all have a marked effect on the quality of the product.
Whiskey is usually distilled twice, sometimes three times. The first distillation is performed in a large Wash Still, heated directly by fire or steam. The entire fermented mash (including the grains) is boiled, producing a crude and fiery distillate known as low wines. Boiling the mash produces many of the compounds that define the flavor and character of a whiskey. The spent mash contains a lot of protein, and is usually fed to animals.
The low wines are processed in a smaller Spirit Still, which is the traditional whiskey still discussed above. Three distinct fractions are collected from the Spirit Still. The foreshots, containing the most volatile congeners, are the first material to appear. The main part of the distilling run produces primarily ethanol and water, while the feints appear last and contain the fusel alcohols and other low-volatility congeners. Small portions of the foreshots and feints are sometimes added to succeeding batches of low wines to balance the flavor. The spent lees remaining in the still are sent to the sewer.
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