Fusel Oils and Congeners:
One of the more widely known groups in the table is the higher alcohols, sometimes called fusel oils. In general, the compounds in this group are a mixture of volatile, oily liquids with a disagreeable odor and taste. Another, somewhat wider, grouping of the compounds listed are called congeners. Congeners include the aldehydes, esters, and primary alcohols such as methanol and isoamyl alcohol. Congener content is significant because they can act as CNS depressants, mucosal irritants, and produce nausea. Taken together, they appear to increase the duration of intoxication, the amount of hangover, and the toxicity of alcoholic beverages.
Not surprisingly, in the beverage industry, congeners and fusel oils are ordinarily allowed to remain in the finished distillation products. They are the major ingredients that differentiate brand name whiskeys by taste.
In many circles, the mark of a poorly distilled spirit is a colossal hangover. Whenever you distill something, the most volatile products come out first. So when you distill a mash, the low boiling point compounds in it (in general the Nitrogenous Substances, Aldehydes, and Esters) will appear in the first distillate. This part of the distillation is commonly called the "Heads". You can prevent them from contaminating the product you are attempting to separate by watching the temperature and discarding (or saving for addition to the next batch) everything that boils off before you reach the boiling point of the target component.
But, depending on the nature of the wash, it's sometimes difficult to isolate the heads by simply monitoring the temperature. It's easy to miss the boiling points of those compounds that vaporize below 70º C when there is an excess of heat input, and the vapors rise up the column quickly to reach the thermometer bulb. Many experienced distillers carefully monitor the taste and smell of the first distillate from the still to insure that all the heads are boiled off before they begin the collection of the body of the spirits. Others simply discard a small (e.g.150 ml) fixed amount, before beginning the collection of the ethanol.
A similar distillation cutoff point is also encountered as the ethanol nears depletion from the distillation. This phase is commonly referred to as the "Tails". The tails contain an increased amount of the higher boiling point compounds, such as the higher alcohols and furfurol. These compounds can also spoil the taste of the spirits if the collection is carried on too long. A cutoff similar to that of the heads should be made.
Again, you can recognize this point by monitoring either the temperature or the taste and smell of the distillate. Many distillers simply limit the collection of the pure spirits to a narrow range of temperatures (e.g. 78.3 - 80 C), and then make the cut. Others sample the specific gravity of the distillate as it nears the end of the run. Still others use the smell and taste indicators.
In any event, there usually is considerable ethanol that can be recovered from that remaining after the tails have been cut. Commonly, the tail collection is saved for inclusion in the next batch.
