Barsac is a commune in the south of the Bordeaux region of France. It is also a very important sweet white wine appellation and is the most distinctive of the five communes that make up the Sauternes title. The other four (Fargues, Bommes, Preignac and Sauternes itself) are permitted to use only AOC Sauternes, whereas Barsac may also use its own Barsac appellation.
The laws for the two appellations are identical in every respect (other than the specific geographical origins of the grapes). Vines must be trellised and pruned with specific techniques, and planted to a minimum density of 6500 plants per hectare (2631 per acre) – although there are some complex exceptions to this rule. The wines must reach a minimum alcohol level of 12% and be derived from grapes harvested with natural sugar levels of 221g/L, leading to a naturally high content of residual sugar in the finished product.
Located on the southern (left) bank of the Garonne river, Barsac is also very close to one of the Garonne's tributaries, the Ciron – a vital factor in its ability to produce high-quality sweet white wines. The presence of the two rivers creates night-time and morning mists as the air cools down after hot summer days. This airborne moisture encourages the development of Botrytis cinerea – the 'noble rot' so essential to the creation of sweet wines. Botrytis concentrates the sugars and natural flavors in the Semillon and Sauvignon Blanc grapes used for Barsac's wines (Muscadelle and Sauvignon Gris are the only other varieties permitted under the strict appellation laws). The development of botrytis is not guaranteed, however, and in some vintages it has been known not to occur at all. Grapes left on the vines without botrytis are susceptible to early frosts or heavy rain, and represent a significant and risky investment.
Across the Ciron to the south is the more famous village of Sauternes. Its hillier topography increases the effects of the mists, leading to richer wines, while Barsac's flatter sand- and limestone-based soils create finer flavors in the grapes.