Mash

What goes into the boiler before distilling: water, sugar in one form or another, and yeast, after fermentation has run.

The simplest is a sugar wash — sugar, water, yeast, done. The distillate comes out neutral, with no character of its own. A grain mash takes more work, because the starch in rye or wheat has to be broken down into sugars first, by malting or by adding enzymes; that step is where the word "mashing" comes from. A fruit mash has the sugar ready and brings aroma with it, which carries through into the distillate.

You cannot push the strength as high as you like. It usually stops in the teens, because the yeast dies in its own alcohol; even distilling strains rarely pass 18–20%. Which is exactly why distillation is needed at all — fermentation alone will not reach vodka strength.

It is ready when fermentation stops: the airlock goes quiet, the liquid begins to clear, the yeast settles. Distilling an unfinished mash means pouring away sugar that could still have become alcohol.

One thing worth doing just before distilling: rack the mash off the sediment. Sediment scorches when heated and can ruin the smell of the whole batch.