What Is Fermentation?

Fermentation is the process where yeast converts sugar into alcohol and carbon dioxide, turning a sugary liquid into a low-alcohol "wash" ready for distillation.

Fermentation is the biological engine behind every spirit — distillation can only concentrate alcohol that fermentation has already created.

The basic chemistry

Yeast consumes sugar and, through its metabolism, produces ethanol (alcohol) and carbon dioxide as by-products. Written simply:

sugar + yeast → alcohol + carbon dioxide (+ flavour compounds)

What gets fermented

• Grain-based spirits (whisky, vodka) ferment the sugary wort produced by mashing (see "What is mashing?").
• Rum ferments molasses or cane juice directly, since those already contain fermentable sugar without needing a starch-conversion step.
• Gin's base spirit goes through the same grain fermentation process as vodka, before botanicals are ever introduced.

How long it takes

Fermentation typically runs anywhere from a couple of days to a few weeks, depending on the yeast strain, temperature, and the flavour profile the distiller wants. Faster fermentations tend to be cleaner; slower fermentations often develop more complex, sometimes fruitier or funkier flavour compounds (see "How does yeast affect flavour?").

What comes out the other end

The result is a low-alcohol liquid — often called a "wash" (for whisky/grain spirits) or simply fermented wort/juice — typically in the range of 6–10% ABV, similar in strength to a beer or cider. This wash is what then gets distilled to concentrate the alcohol and refine the flavour into the final spirit.