On Food and Cooking

Selecting Desirable Volatiles Distillers must
therefore control the composition of the
distilled liquid. They do this by subdividing
the vapor into fractions that are more and less
volatile, and collecting mainly the fraction
that is richest in alcohol. The fraction more
volatile than alcohol, often called the “heads”
or “foreshots” because it evaporates earlier
than alcohol, includes toxic methanol, or
wood alcohol, and acetone. The fraction that’s
less volatile than alcohol, the “tails” or
“feints,” includes a host of aromatic
substances that are desirable. Among these
“congeners” (substances that accompany
alcohol) are esters, terpenes, and volatile
phenolics, along with some substances that
are desirable in limited amounts. The most
notable of the latter are the “higher” alcohols,
whose long, fat-like chains can give spirits a
full, almost oily body, but also contribute a
pronounced harsh flavor and unpleasant
aftereffects. They’re often called fusel oils.