Fermentation

The anaerobic conversion of sugar into ethanol and carbon dioxide, carried out by yeast. One molecule of glucose gives two of alcohol and two of carbon dioxide.

The word "anaerobic" matters. With air available, yeast prefers to respire with oxygen — it grows faster then, but makes almost no alcohol. It has to be forced into fermenting by cutting off the air. That is the job of the airlock: it lets carbon dioxide out without letting in air or the acetic bacteria that would happily turn the whole effort into vinegar.

It takes from a few days to three weeks, depending on temperature, strain and how much sugar there is. Cider from apple juice usually falls within 14 to 20 days.

You can tell it has finished when the airlock stops bubbling, the liquid begins to clear and the yeast settles. The surest sign is another one though: the density stops falling across measurements taken a few days apart. A quiet airlock alone can mislead, because in the cold fermentation sometimes only slows and picks up again when it warms.

It stops on its own once there is enough alcohol that the yeast can no longer take it. That is why a mash sits in the teens, and why distillation is needed.