Sauternes, 40 miles (65km) south of Bordeaux city, is a village famous for its high-quality sweet wines. Although some wineries here produce dry wines, they sell them under appellations other than the sweet-specific Sauternes appellation. The village is surrounded on all sides by vineyards, the best of which produce some of the world's most prestigious, long-lived and expensive dessert wines. A half-bottle of top-quality, aged Sauternes wine from a good vintage can command prices in excess of $1000.

The classic Sauternes wine has an intense golden colour (darker than most other dessert wines), which turns a deep amber as it ages in bottle. The aromas include blossom and stone fruit, with a hint of honeysuckle – the trademark of botrytized wines. The best wines balance sweetness with acidity, concentration with freshness, and power with elegance.

Sauternes' wines are made mostly from the Semillon grape variety, which accounts for about eight in every 10 vines in the local vineyards. Sauvignon Blanc accounts for much of the remaining vineyard area and is the dominant variety in just a small handful of Sauternes wines. Semillon forms a broad, well-structured base with aromas of beeswax and apricot, while Sauvignon Blanc brings its trademark herbal aromatics and sufficient acidity to keep the resulting wine fresh rather than palate-cloying. This pair (which are sometimes complemented by a tiny amount of Muscadelle and Sauvignon Gris) have become the preferred varieties here not only because they are also used to make Bordeaux's dry whites, but because of their susceptibility to a particular kind of fungus, Botrytis cinerea (often just botrytis).

Other than yeasts, without which grape juice could not ferment into wine, one might not expect a fungus to play a key role in winemaking. And yet the distinctive Sauternes wine style is entirely dependent on this particular fungus strain. Under adverse conditions, Botrytis cinerea causes grapes to rot and disintegrate, further exposing their flesh and juice to all manner of other fungi and bacteria. In this form, it is known as "grey rot", and leads to sour, unpleasant aromas in wine. But when botrytis spores land on healthy grapes under favourable weather conditions, they have quite a different effect, and develop into benevolent "noble rot".

Noble rot develops most reliably in areas where morning mists, which allow the fungus to thrive, are followed by warm, dry afternoons that dry the grapes out and prevent the development of grey rot. When repeated over a number of weeks, this process gradually dries the grapes, reducing their water content and naturally concentrating their sugars and flavour compounds. The result is intensely sweet, flavour-rich juice. In autumn, Sauternes and its neighbours Barsac, Bommes, Fargues and Preignac have exactly these climatic conditions, thanks to the warming and cooling of air around the nearby River Ciron.

The Sauternes appellation laws state that grapes may be picked only when their must weight reaches 221 grams per litre (the minimum for regular, dry Bordeaux Blanc wines is just 162g/L). Because not all these abundant sugars are fermented into alcohol, the finished wine contains naturally high levels of residual sugar. In good vintages, nature needs no help getting grapes to such high levels of sweetness, but in poor vintages, winemakers turn to cryoextraction and even chaptalisation to achieve this. Cryoextraction involves freezing the grapes before they are pressed, which reduces the amount of water in the resulting juice. Chaptalisation – the addition of sugar or artificially concentrated grape juice – is permitted only in poor vintages and, even then, only to a limited extent.