First let's take a look at the simplest situation — the events taking place in a pot still when beer is distilled. The vapour is richer than the liquid in the most volatile constituents, i.e. the ones with the lowest boiling points such as acetone and methanol in the above table. When they distil over they are referred to as the "heads". There is no clear-cut separation of the various compounds so the heads will still be coming over when the ethanol starts to appear. Similarly, before all the ethanol has distilled over, the "tails" will begin to appear in the distilate. These tails are the compounds at the lower end of the above table, i.e. those with the highest boiling points such as propyl, butyl and amyl alcohols. These alcohols are known collectively as "fusel oils" and, like methanol and some of the other compounds, are quite poisonous.
In such a system there may be a small fraction in the middle which is pure ethyl alcohol but most of it will be contaminated with either heads or tails. One could discard the first heads and the last tails and re-distil the middle fraction, repeating this process over and over again until the last of the impurities had been wrung out of the ethanol. Unfortunately, as mentioned before, apart from being very time consuming, the loss of ethanol on repeated re-distillation would be such that the final yield of pure alcohol would be virtually zero.
The retention of some of the "impurities" in the original beer when carrying out a distillation with a pot still does not bother many people because they have grown (or have been taught) to like the taste of these impurities. They add character to the alcohol. They add flavours (some of them pretty vile but some quite pleasant). By playing around with the distilling conditions it is possible to retain more or less of these impurities, or "congeners" as they are called, the manufacturers then referring to their product as whisky, brandy, etc., etc.
For those who wish to drink vodka or gin, however, or to obtain pure alcohol in order to make liqueurs, it is necessary to get rid of the congeners and the multiple counter-current distillation procedure described below must then be resorted to.
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