Winemaking: Jekyll and Hyde - The Fungal Story

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Sweet Wines and the Mystery of Noble Rot

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Caroline Gilby Caroline Gilby

Scientist and Master of Wine, Caroline Gilby discovers the role of fungus in making some of the world's best sweet wines.


Like the Victorian doctor in Robert Louis Stevenson's horror tale, Jekyll and Hyde, Botrytis cinerea is a fungus with two very different faces. It's a ubiquitous presence in vineyards (and other fruit crops) across the world and in its grey rot form is estimated to cause anything from 15 to 40% crop losses, depending on the weather, each year (these are data for France).

It causes off-flavours, early browning and oxidation, while allowing other fungi to infect berries too, adding mouldy flavours and making virtually undrinkable wine. So grape growers are constantly battling with agrochemicals and vineyard management techniques such as leaf plucking and bunch aeration to keep it at bay.

Sweet success: the positive effects of fungi

However, exactly the same fungus is responsible for the phenomenon of 'noble rot' which is an essential ingredient in some of the most famous sweet wines: Tokaji Aszú, Sauternes, Trockenbeerenauslese, Coteaux du Layon and Alsace Sélection des Grains Nobles to name just a few.

Making eiswein, an even more costly business
Making eiswein, an even more costly business

But, there are other ways of making sweet wines: by freezing to make eiswein; by drying grapes to make passito style wines like Vin Santo or recioto; and by stopping fermentation before all the sugar has been fermented (filtration, chilling or fortification are the techniques used here). However, no other method makes unfortified sweet wines that reach the ethereal heights of wines made with the help of noble rot.

Mysterious mould

Less is known about this fungus than perhaps should be, though it has recently been sequenced in the hope of understanding better how to control it - both for good and bad. It seems that to become noble rot, it needs ripe or nearly ripe grapes that have made it through the season undamaged by insects and without split skins due to rain.

Then you need weather that alternates between humid periods to allow the fungal spores to grow, followed by drier days that prevent the fungus from going too 'rogue'. Spores of Botrytis cinerea will be all over the vineyard and if there's enough moisture around they can start to germinate, infecting the grape through tiny, microscopic-scale fissures in the skin or via tiny pores called stomata that allow air exchange. Once under the skin, the fungus grows a mycelium (a network of tiny threads called hyphae), turning the grape skin brown and secreting enzymes that break down cell walls, and feeding itself from the sugars inside the grape itself.

Finally, the fungus bursts back out of the grape skin, and as the surface (epidermal) cells are dead, they are no longer under control of the vine to stay hydrated and they start to shrivel up.

Changes within the grape too

Alongside all this, there are significant changes within the grape - sugar gets used up by the fungus, (which prefers glucose, leaving more of the sweeter-tasting fructose behind) and grape acids (especially tartaric acid) get broken down too, but these losses are made up for by the water evaporation and concentration of berry pulp.

At the same time, the fungus breaks down compounds that give varietal character to some grapes like muscat and produces new compounds of its own. These include higher levels of glycerol, which gives smooth texture to so many sweet wines and Sotolon, a compound that gives notes of honey, sweetness and caramel. And there are at least 20 more that haven't been analysed fully. Some grapes like riesling and semillon gain more complexity from noble rot than they lose in the breakdown of the esters that give their simple fruity varietal characters.

Some grapes more prone to rot than others

Santo Grapes

It seems that some grapes are more prone to infection by botrytis than others. This may be due to factors such as thickness of skins, number of stomata (those tiny breathing pores on the surface of plant tissues) and the plant's ability to produce antifungal compounds (the famous Resveratrol that is claimed by some researchers to give health benefits to wine is one such).

Who were the first to make wines from nobly rotted grapes?

Historically speaking, botrytis cinerea would have been a widespread challenge to winemakers ever since wine was first made. But evidence suggests it may have been the Hungarians who first documented deliberate use of noble rot in winemaking in Tokaj in the 16th century. The Germans were also early adopters with the first records at the famous Schloss Johannisberg around 1750 while it was well established in Sauternes by 1830.

Who were the first to make wines from nobly rotted grapes?

Historically speaking, botrytis cinerea would have been a widespread challenge to winemakers ever since wine was first made. But evidence suggests it may have been the Hungarians who first documented deliberate use of noble rot in winemaking in Tokaj in the 16th century. The Germans were also early adopters with the first records at the famous Schloss Johannisberg around 1750 while it was well established in Sauternes by 1830.

Picking berry by berry

Each of these regions has developed subtly different styles of wine and winemaking though a common feature is the need to pick individual nobly rotted berries separately by hand, often requiring several trips through the vineyard as the fungus develops.

This makes sweet winemaking expensive and extremely labour intensive as well as giving tiny yields after all that shrivelling (10-15 hectolitres per hectare would not be unusual compared to anything from 40 hectolitres upwards for dry wines). As an example a good picker in Tokaj for instance may bring in just 10 kg a day.

Extracting juice from shrivelled grapes: each region has its own technique

Santo Grapes

The next problem is getting the thick juice out of shrivelled berries, requiring long and quite firm pressing, typically using basket presses in regions like Sauternes, but in Tokaj, the climate is such that the region has its own special winemaking method.

Tokaj's aszú berries

Humid foggy mornings followed by bright, windy autumn afternoons are typical in a good vintage in Tokaj, giving both noble rot and extreme shrivelling to form the so-called Aszú berries, which are too dry to give anything more than a trickle of syrup when pressed (this part is the legendary Eszencia, claimed to have miraculous medicinal properties). These Aszú berries instead get soaked in fermenting juice or young wine to dissolve their contents - and of course the quantity of Aszú grapes helps determine quite how sweet the wine is.

Whatever the details of the winemaking method, if winemakers are able to harness the magical power of noble rot, and avoid the scourge of its grey alter ego, the result can be utterly sublime and give luscious wines that can age beautifully for decades too.

> Explore noble rot wines