As mentioned before, the fermentation of sugars derived from grapes, barley, corn, potatoes, molasses, milk or any other source produces a wide variety of chemicals, the major one being ethyl alcohol (ethanol). Minor constituents will be methyl, propyl, butyl and amyl alcohols, aldehydes, ketones, esters, and a host of other organic compounds in small amounts. Analytical methods such as chromatography reveal that there are literally hundreds of compounds present after a fermentation. These minor constituents are the congeners and the amount of each will determine the flavour, bouquet and colour of a particular beverage. They are also responsible for unpleasant side effects such as headaches and hangovers since many of them are very poisonous. The type of still used for making whiskies, brandies, rums and so on, all of which require that a percentage of taste-giving congeners remain, are called pot stills.
To make brandy (as an example of a distilled spirit) the fermented liquor (wine in this case) is brought to the boil and the vapours led over into the condensing section. This section contains a cooling coil with water running through it where the vapours are condensed to liquid. The first vapours to come over will be rich in the more volatile components such as acetone and methanol. This first fraction is referred to as the "heads". There is no sharp separation so, long before the heads are completely exhausted, the ethanol begins to appear and is collected, even though it would be somewhat contaminated with heads. Later, when ethanol production is tapering off, the "tails" begin to emerge. These are the least volatile components of the mixture and include propyl, butyl and amyl alcohol. These three alcohols are known as "fusel" oils. Thus, in a simple distillation using a pot still there are three main fractions — the heads, the tails, and the middle fraction of ethanol contaminated with a little heads and tails, the amount of each depending on just where the cut-off is made.
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