Yeast

Single-celled fungi that, without access to oxygen, turn sugar into ethanol and carbon dioxide. Almost every strain used in distilling is a variety of Saccharomyces cerevisiae — so they differ not in species but in what they endure and how cleanly they work.

Baking yeast is cheapest and sits in every shop, but it dies around 8–10% and leaves plenty of fusels behind. People reach for it out of necessity rather than choice. Wine yeast handles 14–16%, works cleanly and copes well with fruit — the natural pick for cider or wine. Distilling yeast, sold as "turbo", reaches 18–20%, except that it pays for the speed with more by-products. Fast does not mean good here.

Temperature ruins the most. Most strains want 18–24°C. Too cold and fermentation drags or stops outright. Too warm and it starts briskly, but the yeast then makes far more fusels, and you can hear about it later in the finished spirit.

They also need nitrogen. With a plain sugar wash, which has no natural nutrients, you have to add yeast nutrient — otherwise fermentation stalls halfway and nobody knows why.

Dried yeast is first dissolved in a little lukewarm water and left a few minutes until it starts working. Tipping it straight into a cold wash delays the start, and bacteria can get there first.