Noble Rot Sweet Wines Compared: Sauternes, Loire, Alsace, Germany, Austria and Tokaj

What noble rot does to the berry

Noble rot is the work of Botrytis cinerea, the same grey fungus that causes grey rot. It infects ripe, sound berries. Research on white-skinned grapes shows that during noble rot the waxy cuticle of the berry cracks and the berry dehydrates, so it shrivels and its sugar is concentrated (Blanco-Ulate et al.).

The fungus also changes the berry's chemistry. Sugar alcohols such as glycerol rise, the ratio of glucose to fructose falls and tartaric acid declines, while gluconic and mucic acids accumulate (Blanco-Ulate et al.; the Garganega study). The berry responds differently in noble rot and in grey rot, although the fungus is the same (PMC11333459).

This article gives no figure for how much water a berry loses, because the sources cited here do not state one that Maison has verified.

Why damp mornings and dry afternoons matter

Noble rot needs two kinds of weather in sequence. Moisture lets the fungus infect ripe grapes, and if the damp lasts, the infection turns into damaging grey rot. If drier weather follows, the berries dry out and the rot stays noble (PMC11333459). The Sauternes cahier des charges describes the same alternation: humid nights, then rapid and strong ventilation by day (INAO).

How the Ciron and the Garonne make the autumn mist is explained in the article on Sauternes and the alchemy of noble rot, and is not repeated here. The sections below compare what each country writes into its rules for the grapes that result.

France: Sauternes, Barsac, Quarts de Chaume, Bonnezeaux and Alsace

The Sauternes cahier des charges requires grapes harvested overripe with noble rot, picked by hand in successive passes called tries, and treats 221 grams of sugar per litre of must as the threshold of good ripeness. Sauternes and Barsac wines must each contain at least 45 grams per litre of fermentable sugars (glucose and fructose) when marketed or bottled. The yield for Sauternes is 25 hectolitres per hectare.

On the Loire, Quarts de Chaume and Bonnezeaux are made from Chenin alone. Quarts de Chaume allows concentration by noble rot or by passerillage (drying on the vine) and sets a must sugar of 298 grams per litre. Bonnezeaux requires manual, successive picking, a grape sugar of 238 grams per litre, at least 51 grams per litre of fermentable sugars and a yield of 25 hectolitres per hectare. Maison read these two Loire figures in extracts of the INAO documents and will re-check them against the full texts.

Alsace Sélection de Grains Nobles is made from botrytised berries sorted at harvest, from Gewurztraminer, Muscat, Muscat Ottonel, Pinot Gris and Riesling (CIVA). The cahier des charges of the AOC Alsace sets a minimum sugar for the must at harvest: 276 grams of sugar per litre of must for Riesling and Muscat, and 306 grams per litre for Gewurztraminer and Pinot Gris. For comparison, it sets 235 and 257 grams per litre for the same two groups in Vendanges Tardives. These minimums have applied since the 2001 harvest, when they were raised (INAO).

Germany and Austria: must weight scales

German Prädikat wines are classified by the sugar content of the must, measured in degrees Oechsle (°Oe), and the minimum depends on region and grape variety (Weinverordnung). For Beerenauslese the German Wine Institute gives about 110 to 128 °Oe depending on region and variety, and for Trockenbeerenauslese about 150 to 154 °Oe. For the Rheingau, the Darmstadt regional authority lists 125 °Oe for Beerenauslese and 150 °Oe for Trockenbeerenauslese. The Weinverordnung allows noble-rotted or at least overripe berries for Beerenauslese, and for Trockenbeerenauslese requires largely shrivelled noble-rotted berries, accepting overripe shrivelled berries only when noble rot exceptionally did not occur. Prädikat wines may not be enriched with sugar.

In Austria, § 11 of the Weingesetz 2009 sets minimum must weights in degrees KMW (Klosterneuburger Mostwaage): at least 25 for Beerenauslese, 30 for Trockenbeerenauslese and 27 for Ausbruch. The Oechsle and KMW scales are different, so the two sets of numbers cannot be compared directly.

Tokaj: aszú berries and residual sugar

In Tokaj, the aszú berries are noble-rotted berries picked separately and macerated in must or base wine (Wines of Tokaj). Since the 2013 harvest the product specification has set a minimum of 120 grams per litre of residual sugar in the finished Tokaji Aszú. The former categories of 3 and 4 puttonyos disappeared, and stating a number of puttonyos on the label became optional (Tokaj Today). Aszú is aged for at least 18 months in cask (Wines of Tokaj).

Note what this figure measures: sugar left in the wine, not sugar in the grapes.

Reading the numbers across countries

The figures are not on the same footing. France states sugar in the grapes or must (221, 238 and 298 grams per litre) and, for Sauternes, Barsac and Bonnezeaux, a floor of fermentable sugar in the wine (45, 45 and 51 grams per litre). Germany and Austria state must weight on their own scales (°Oe and °KMW). Tokaj states residual sugar in the finished wine (120 grams per litre).

Converting between them would need the density tables of each law, which are not among the sources here. The table below therefore keeps each unit as its source states it, and ranks nothing.

Practice question: comparing the numbers

[Question]
Quarts de Chaume has a must-sugar figure of 298 g/L and Tokaji Aszú a residual-sugar minimum of 120 g/L. Explain why these two numbers do not show that Quarts de Chaume is the sweeter wine.

[Model answer]
The two numbers measure different things. The 298 g/L is sugar in the must before fermentation, while the 120 g/L is the minimum sugar left in the finished wine. Fermentation turns part of the must sugar into alcohol, so a must figure does not tell you how much sugar remains. To compare sweetness, both wines need the same measure, for example the sugar in the finished wine.

[Self-check (4 points)]
• Says that 298 g/L is must sugar before fermentation (1 point)
• Says that 120 g/L is residual sugar in the finished wine (1 point)
• Explains that fermentation turns part of the sugar into alcohol (1 point)
• Concludes that a like-for-like measure is needed (1 point)