Noble rot

Botrytis breaks down the grape skin and makes it porous. Water evaporates through it, the berry shrivels, and the sugar, acid and flavour left behind get more concentrated.

But it's not just concentration. The fungus rewires the grape's chemistry, generating new aromatic compounds and altering the ones already there.

That's why a botrytised wine never tastes like a sweeter version of the same grape. Expect honey, beeswax, dried apricot, marmalade, ginger and saffron, layered over whatever the grape itself brings.

Botrytised wines can carry huge amounts of residual sugar. The good ones still don't taste like syrup. Acidity is why.

As the grape loses water, everything inside it concentrates, acid included. Varieties with naturally high acidity, Riesling and Furmint especially, hold onto that freshness even at extreme sugar levels.

This tension is what makes botrytised wine work. Sugar brings weight and richness. Acid keeps it alive and makes you want the next sip.

Botrytis doesn't move through a vineyard in one neat pass. One bunch is ready while its neighbour isn't. Even inside a single bunch, some berries hit the ideal stage while others are still healthy, or already rotting the wrong way.

So pickers go through the vineyard repeatedly, called tries in Sauternes, taking only the berries that are ready. At the most exacting estates, that means picking individual berries and coming back to the same vines again and again over weeks.

It's slow, expensive work, and a big part of why these tiny bottles cost what they do.

Botrytised berries are loaded with sugar and flavour and short on one thing: water.

Yields drop hard. After all that careful, repeated picking, growers end up with shrivelled grapes and not much juice to show for it.

The must that results is also a pain to ferment. High sugar stresses the yeast, fermentation slows, and it can stall before all the sugar converts to alcohol. Whatever's left over is residual sugar, and that's the sweetness in the glass.

Noble rot can happen almost anywhere, but only a few regions get the geography and climate to make it something close to reliable.

• Sauternes & Barsac, France - The cool Ciron river meets the warmer Garonne here, and the autumn mists that result are exactly what Botrytis wants. Sémillon leads, usually with some Sauvignon Blanc alongside. Expect citrus, apricot, honey, saffron and beeswax.

• Tokaj, Hungary - Botrytised aszú berries have defined Tokaj for centuries. Furmint is the backbone, bringing the acidity that keeps Tokaji Aszú from tipping into cloying.

• Germany - Home to some of the most concentrated botrytised wines anywhere, particularly Beerenauslese and Trockenbeerenauslese. Riesling's natural acidity makes it a near-perfect match for the style.

• Burgenland, Austria - The shallow Neusiedler See generates the humidity Botrytis needs. The vineyards around it are known for their own Beerenauslese and Trockenbeerenauslese.

Some grapes just take to noble rot better than others.

Sémillon has thin skins and is highly susceptible to Botrytis. It's the backbone of Sauternes, and with age develops honey, wax, apricot and citrus.

Riesling pairs Botrytis with its own naturally high acidity. Even pushed to extreme sweetness, it stays fresh and precise.

Furmint does something similar in Tokaj. High acidity, high susceptibility to Botrytis, a natural fit for concentrated sweet wine.

Sauvignon Blanc and Chenin Blanc can also turn out excellent botrytised wines, among others.

Botrytis leaves a recognisable signature, though the grape, the region and the winemaking still shape the final wine.

Look for honey, beeswax, apricot, marmalade and candied citrus, often with ginger, saffron and other sweet spices in the background. Older examples pick up dried fruit, caramel, nutty and savoury notes.

The grape doesn't vanish underneath all that. A botrytised Riesling still doesn't taste like a Sémillon-based Sauternes, or a Furmint-based Tokaji. Botrytis adds a layer. It doesn't replace what's already there.

Noble rot is one way to make sweet wine, not the only way. Grapes can be dried after picking, frozen on the vine, harvested late, or fermented so residual sugar remains. None of that requires Botrytis. Even a wine labelled late harvest isn't automatically botrytised.

What sets noble rot apart is that the fungus changes the grape, physically and chemically, before it ever reaches the winery. Sweetness is just one part of what it leaves behind.

Growers choose their grapes, tend their vines, decide when to pick. What they can't do is control the weather. Noble rot needs ripe grapes, the right fungus, moisture at the right moment, dry weather right after, and enough time for the berries to shrivel without grey rot taking over instead. Line all of that up, and grapes that look almost ruined can produce something extraordinary.

Miss it and there's no noble rot at all, just rotten grapes.That's the strange part. The thing that makes these wines special isn't anything added in the winery. It's a fungus, given exactly the right conditions to do something remarkable.