Yeast

Yeasts are single-celled fungi that break sugar down into alcohol and carbon dioxide during fermentation. They also form aroma compounds such as esters and higher alcohols, which shape the fruitiness and character of a drink. Either cultivated pure-culture yeasts are used, or the yeast naturally present on fruit and in the cellar, known as spontaneous fermentation.

Different strains work at different temperatures, tolerate different amounts of alcohol and produce different aromas. After fermentation the yeast settles as sediment at the bottom. It can be removed, but can also deliberately be left in the drink, as with wheat beer.