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Botrytis (noble rot) - how mould makes the best sweet wines

It sounds like a joke: the most prized, most expensive sweet wines in the world are made from grapes attacked by mould. And yet it is true. Noble rot, that is Botrytis cinerea, is a fungus that in the right conditions attacks ripe grapes, dehydrates them and concentrates the sugar and flavour in them, giving wines of a remarkable, honeyed sweetness. From it come legends: the French Sauternes, the Hungarian Tokaji and the German Beerenauslese. The same thing that in bad conditions would be ordinary, destructive rot becomes, in the right ones, a noble gift. Here is a guide to botrytis: how this mould works, in what conditions it forms, what flavours it gives and which wines it is famous for.

What noble rot is

Noble rot is the common name for the fungus Botrytis cinerea, when it attacks ripe grapes in a beneficial way. It is a mould, in this context useful, which grows on ripe berries and dehydrates them, leaving shrivelled, raisin-like fruit of concentrated sugar and flavour. The key is the duality of this fungus: the same Botrytis cinerea can be a blessing or a curse, depending on the conditions. In wet, unfavourable conditions it gives destructive grey rot, which spoils the crop. But in the right ones, described further on, it becomes noble rot, which creates great wines. This is why the same mould stirs in winemakers either fear or joy. Understanding that botrytis is a fungus of two faces is the starting point for the rest. It is one of the most fascinating paradoxes of winemaking. We cover sweet wines more in dry versus sweet wine.

The conditions: mist and sun

The key to noble rot is very specific weather conditions, balancing on a knife edge. Botrytis becomes noble, rather than destructive, when warm, misty mornings alternate with dry, sunny afternoons. The morning moisture and mist let the mould develop on the berries, and the afternoon sun and dry air stop it from turning into destructive rot. This alternation is key: too much moisture is grey rot, too little is no mould at all. This is why noble rot forms only in particular places of an ideal microclimate - usually where a river or lake nearby gives morning mists. The Tokaj region is famous for the dense fogs covering the vineyards in the mornings during the season, which favours botrytis. It is a delicate balance of nature, impossible to force. Understanding these conditions explains why great botrytised wines are so rare. It is a gift of capricious weather.

How the mould concentrates the wine

The heart of how noble rot works is concentration. The fungus penetrates the skin of the berry and causes the fruit to lose water through evaporation - up to 60 percent of its mass. It is like a natural drying of the grapes on the vine. The effect is dramatic: as the water is lost, the sugars, acids, glycerol and unique flavour compounds in the berry rapidly concentrate. From the shrivelled, raisin-like berries very little juice is therefore pressed, but it is extraordinarily concentrated and sweet. This is why botrytised wines are so dense, sweet and intense - and so expensive, because a large amount of grapes yields little wine. The mould does not add sugar but removes water, concentrating what is already there. Understanding this mechanism - dehydration and concentration - is the key to understanding these wines. It is mould as a natural dryer, concentrating the essence of the fruit.

The flavour: honey and apricot

Noble rot not only concentrates but also creates its own, characteristic flavours. Fascinatingly, the fungus induces the production of aroma precursors for stone fruit, like apricot and peach, and the honey notes are attributed directly to the botrytis itself. This means that part of the characteristic flavour of the wine comes directly from the action of the fungus, not only from the concentration of the fruit. Young botrytised wines are characterised by notes of honey, orange, citrus zest, apricot, ginger and almond. It is a rich, complex, sweet palette, impossible to get without the mould. The characteristic note of honey is downright the signature of botrytis, the hallmark of these wines. This is why botrytised wines have such an unrepeatable, luxurious profile - it is not only sweetness but also a depth of aromas created by the fungus. Understanding that the mould creates flavour, and not only concentrates it, completes the picture. It is the flavour of noble decay.

A table: three famous wines

Let us gather the most famous botrytised wines in one place:

The table shows that noble rot creates great wines in several regions of the world of an ideal microclimate. Each has its own tradition, but they are joined by botrytis and a honeyed character.

Sauternes, Tokaji, Beerenauslese

Three regions became famous for botrytised wines, each with its own tradition. Sauternes from French Bordeaux is perhaps the most famous classic - a rich, golden wine from semillon and sauvignon blanc, of a honey-apricot profile, able to age for decades. Tokaji aszu from Hungary is one of the cradles of botrytised wines, where morning mists over the vineyards have for centuries favoured the mould, giving the famous, sweet, honeyed wine. Beerenauslese and Trockenbeerenauslese (TBA) from Germany and Austria are the peak of the German pyramid of sweetness, made mainly from riesling attacked by botrytis, intensely sweet and long-lived. Each region has its own grape, style and name, but all are joined by the same noble rot and concentration. They are three faces of one fascinating phenomenon. We cover German sweetness more in German riesling and Pradikat.

Noble rot versus grey rot

It is worth stressing the difference that decides everything: noble rot versus grey rot. It is the same fungus, Botrytis cinerea, but two completely different effects depending on the conditions. In favourable conditions (misty mornings, dry afternoons) the fungus acts gently, dehydrating healthy, ripe berries and concentrating them - this is noble rot, a gift for the winemaker. In wet, unfavourable conditions the same fungus turns into destructive grey rot, decomposing the fruit, spoiling the flavour and ruining the crop. The difference therefore lies not in the fungus but in the weather and the state of the berries. This is why the winemaker in botrytised regions must follow the weather with tension and often hand-select only the nobly attacked berries, berry by berry. It is a risky, labour-intensive art. Understanding this line between gift and disaster explains why these wines are so rare and precious. We cover wine faults more in wine faults.

Why these wines are expensive

Botrytised wines are among the most expensive sweet wines in the world, and there are many reasons for this. First, the yield is dramatically low: the shrivelled, dehydrated berries give very little juice, so from a vineyard that would give cases of dry wine only a few bottles of sweet are made. Second, the labour is enormous: nobly attacked berries are picked by hand, often berry by berry, in many passes through the vineyard, because the mould does not attack all of them at once. Third, the risk is high: the weather must oblige, and botrytis can easily turn into destructive rot, ruining the whole crop. Fourth, the best botrytised wines age for decades and are rare. All of this together makes them a luxury. Understanding these costs explains the prices and helps appreciate how much work and risk hide in a glass of sweet wine. It is an essence worth its price. We cover fortified wines more in port, sherry and madeira.

How to sense it in the glass

A botrytised wine is easy to recognise by its characteristic profile. You sense above all an intense sweetness and a dense, almost oily texture from the concentrated glycerol. In the aroma honey, apricot, peach, orange peel, dried fruit dominate, and often also notes of ginger, almond and wax. Characteristic too is a surprisingly high acidity, which balances the sweetness and makes the wine not cloying but fresh despite its density. The colour is often deeply golden or amber. If a sweet wine smells of honey and apricot, has a dense texture and a lively acidity, that is a good clue that it is botrytis. It is worth trying a glass of Sauternes or Tokaji, best with blue cheese or dessert, to feel this magic. Over time you will start to recognise the signature of noble rot. It is one of the most luxurious experiences in the world of wine.

The essentials in brief

Let us gather it up. Noble rot (Botrytis cinerea) is a fungus that in the right conditions attacks ripe grapes, creating the best sweet wines in the world. It forms when warm, misty mornings alternate with dry, sunny afternoons - otherwise the fungus turns into destructive grey rot. The mould dehydrates the berries (up to 60 percent of mass), rapidly concentrating the sugar, acids and flavour, and on top of that creates characteristic notes of honey and apricot. The most famous wines are Sauternes (France), Tokaji aszu (Hungary) and Beerenauslese (Germany). These wines are expensive because of the low yield, hand labour and risk. They have an intense sweetness, a dense texture and a lively acidity. Now you know how mould makes the best sweet wines and where their honeyed flavour comes from.

Note every wine in GustoNote - including whether it is a botrytised wine and the notes you sense. Over time you will start to recognise the signature of noble rot.