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Noble rot, known scientifically as Botrytis cinerea, is a fungus allowed to grow on certain grapes in order to produce sweet or dessert wines. In particular, most sauternes, dessert Rieslings and some Muscat wines utilize the introduction of noble rot on grapes designated for sweet wines. The name noble rot is a translation of the French name pourriture noble, but evidence exists that this beneficial fungus was likely discovered by accident in the wine-producing Tokaji region of Hungary.
Generally, mold or fungi on grapes is avoided. After the Turks invaded Hungary in the mid-17th century, Abbott Maté Szepsi attempted to salvage the decaying harvest of grapes for wine. If you’re producing any kind of standard wine, decay of the grapes isn’t desirable. The result was a sweet and complex wine.
Another accidental discovery of noble rot may have occurred in Germany in the late 18th century when a harvest at a Benedictine monastery was unintentionally delayed. Again this fungus formed on the grapes, the grapes were made into wine, and people raved about the results. However, neither the Austrians nor Germans can be credited with intentionally introducing Botrytis to grapes. The French Chateau d’Yquem may have been the first to induce the rot on purpose to make sweet wine.
Noble rot grows best in slightly humid climates. Grapes grown for the purpose of dessert wine may be strategically planted near rivers or lakes, or in any location where cold, foggy mornings give way to sunny afternoons. Once botrytis begins to grow on grapes, heat is needed to dry the grapes so that they are protected from inward decay.
Since the grapes are both rotted on the outside and dried, considerable effort is needed to produce enough juice to bottle. This explains the reason for the price on some Botrytis rotted wines like Sauternes. You can expect to pay over $100 US Dollars (USD) for some of the finest Sauternes in the world because the wine can be incredibly expensive to produce. Grapes may need to be pressed many times, and the wine may need to be stored longer to produce fermentation, because the sugar content of the juice tends to mean that yeast added to wine to create “alcohol” won’t work as well. Less expensive Sauternes and Rieslings may achieve the fermentation process more quickly by heating the wine and destroying some of the Botrytis.
Some wines bottled as late harvest wines also seek to use noble rot to their advantage. The German gewürztraminer grapes have become famous in the US — especially in Northern California at the Navarro Winery in Philo — for a noble rot late harvest wine that has notes of berry, honey, and peach. Other late harvest wines don’t depend on Botrytis, but merely the overly ripened grapes to produce sweet wine.
As much as wineries may seek to induce noble rot conditions on some grapes, they are just as eager to avoid it on others. In misty or foggy conditions grapes may be sprayed with anti-fungal sprays to avoid Botrytis development. Late rainy seasons can mean multiple sprayings to avoid destroying a grape harvest intended for standard wines.