In Bordeaux, under the dry Bordeaux Blanc and Graves AOCs, Sémillon is customarily blended with Sauvignon Blanc and Muscadelle, typically constituting less than half the blend. The reverse is the case in the sweet wines of Sauternes and Barsac, where it may account for more than 90 percent of the blend. The phenomenon that creates these wines is a type of rot called Botrytis cinerea, which is encouraged by the autumn morning mists drifting off the cold Ciron river. The fungus pierces the skin, allowing loss of water to concentrate the sugar and acidity, and lends its own smoke and honey flavors to the fruit. The shriveled grapes are harvested in successive passes through the vineyard, and when fermentation has brought alcohol levels to 14 percent, the yeasts die off, leaving considerable residual sugar in the wine.
Dry Sémillon is mostly identified with Australia’s Hunter Valley, where it makes a lean wine and offers more restrained fruit flavors. Sémillon is highly compatible with oak, which enhances fruit richness and adds silky notes of vanilla to the wine.
Sémillon is a cool climate vine of high vigor and potentially high productivity that is particularly susceptible to rot and botrytis. It ripens relatively early and benefits from moderate sunlight and a degree of humidity. Sémillon requires soil that is not too arid; it is suited to sandy gravel, chalky clay and crumbly loam. The large, thin-skinned berries are high in extract, sugar and potential alcohol and low in aroma and acidity, with a tendency to oxidize. Careful cultivation to restrict yield and preserve acidity makes the difference between a good wine and an indifferent one.
The Sémillon vine almost certainly originated in Bordeaux – most likely in the Sauternes area – before spreading to other parts of the region.