Late harvest

Late harvest wine is wine made from grapes left on the vine longer than usual. Late harvest is usually an indication of a sweet dessert wine, such as late harvest Riesling. Late harvest grapes are often more similar to raisins, but have been naturally dehydrated while on the vine.

Botrytis cinerea, or noble rot, is a mold that causes grapes to lose nearly all of their water content. Wines made from botrytis-affected grapes are generally very sweet.

Botrytis cinerea is a fungus that affects many wine grapes and causes them to shrivel into moldy raisins. The fungus responds to the humidity and warmth in the climate and attacks the grapes. As the mold penetrates the skin its spores begin to germinate, causing the water inside to evaporate and the grape to dehydrate. With the absence of water, the sugar becomes more concentrated and the botrytis begins to alter the acidity within the grape. Typically botrytis infection begins to take place in late September and can last till late October. In some years desiccation may occur leaving tiny amount of sweet liquor like juice within the grape.

The infection rate of botrytis is sporadic with vines and bunches achieving full rottenness at different times. This requires harvest workers to go through the vineyards several times between October and November to hand-pick the full rotted grapes. In some occasions, the usable grapes from a single vine may only produce enough juice for a single glass.

Sauternes, such as Château d'Yquem, are produced in the Sauternes region (including Barsac) south of Bordeaux. They are made from botrytis infected Semillon and Sauvignon blanc grapes. Semillon is preferred due to the grape's thin skin and susceptibility to the botrytis which gives the grape a high sugar content. These wines are noted for the balance that complements the honeyed sweetness.

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Wikipedia