How does a wine come to be sweet enough to be dessert in itself? The process starts with grapes that are very high in sugar because they were:
1. Picked after the regular harvest when their sugar content is very high;
2.Picked, laid out on mats, and allowed to raisin ate, thereby concentrating their sugar;
3.Permitted to freeze on the vine (as in eiswein) so that water can be separated from the sugary juice;
4. Attacked by the fungus Botrytis cinerea (the noble rot of French Sauternes), which consumes some of the water in the grapes and helps more to evaporate, again concentrating the sugar.
All of these processes are extremely risky- animals may eat the sweet grapes. The grapes may be attacked by unfavourable moods or diseases, weather may destroy the grapes before the crop can be picked, and so on. Moreover, each of these processes is very labor intensive. Sweet wines, as a result, are almost universally rare and expensive.