Quoted from the journal article
background to wine distillation
The Australian Grapegrower and Winemaker
Issue # 303, pages 14-15
various jargon terms sre traditionally used in distillation, such as heads
or foreshot, which is the most volatile fraction consisting of low-boiling
compunds like acetaldehyde; oils of wine consisting of various esters; and
tails or fusel oils consisting of higher-boiling amyl and butyl ***s.
The proportions of heads, oils and tails in the distillate influences its
composition and taste. In pot-still brandy where most of the vapours are
collected, the spirit is more complex in composition and taste than that
from a fractionating column. In principle, the more plates in the
fractionating column the more pure should be the *** obtained, but
*** and water actually form a constant-coiling mixture at 96.4%
*** by volume, which cannot be seperated by further fractionation. The
fusel oils tend to concentrate on the trays at a temperature of about 86C
where the *** content is 35-45% by volume, while the oils of wine
concentrate at about 81C and 75-85% by volume.
So, if you want pure ethyl ***, discard the heads and tails. A
reasonable figure is first 15% and last 15%.
Hope this helps.