Heads, hearts and tails

A distillate does not run the same from start to finish. What boils below ethanol comes out first, what boils above it comes out last. By convention it is split into three parts.

Heads

The first portion. Acetone, acetaldehyde, methanol. It smells sharp, of solvent. It is discarded in full, and there is no argument about it.

Methanol is a poison. It causes permanent blindness and death. It does not differ enough in taste or smell to be recognised, and there is no home method of checking how much is left in the liquid. That is the one reason heads are cut generously rather than grudged by a few dozen millilitres.

Hearts

The proper part, the one you distil for. It runs steadily and smells cleanly of alcohol.

Tails

The end, where the higher alcohols build up. The smell turns heavy and oily, sometimes frankly cheesy — hard to miss. Tails either go down the drain or are collected separately and added to the next run.

Nobody has drawn the boundaries sharply, because one fraction passes into the next gradually. Experienced distillers collect into small vessels and judge each one by nose rather than staring at the thermometer. The better the dephlegmator is set, the sharper the transitions and the easier the cut.