How wine stays sweet
A wine is sweet when fermentation stops before all the sugar is eaten, leaving 'residual sugar'. The trick is either to concentrate the sugar so high the yeast gives up, or to stop the yeast deliberately.
- Noble rot (botrytis) — a benevolent fungus shrivels grapes, concentrating sugar (Sauternes, sweet German wines).
- Drying grapes — raisining them on mats or vines (Italy's passito wines).
- Freezing on the vine — water freezes out, leaving sweet juice (Eiswein/Icewine).