How Botrytis Affects Taste Profile in Sweet Bordeaux Wines

  • By Jacky Blisson MW
  • 08 May 2021
  • 5 MIN
  • Level 201
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Botrytized grapes - Credit : CIVB

Botrytis cinerea is a fungal mould that attacks certain plants in humid environments. In grape vines, Botrytis generally occurs late in the growing season. Interestingly, the effects of Botrytis on wine grapes can be either negative or positive, depending on factors like grape ripeness and weather cycles.

In underripe grapes, where wet, cool conditions linger throughout the day, grey rot ensues, decimating grape bunches. However, in ripe grapes, when damp early mornings are followed by warm, dry afternoons, the beneficial form of Botrytis, often referred to as “noble rot”, occurs.

Bordeaux is famous for its complex, long-lived noble rot wines. In parts of the Left Bank and Premières Côtes de Bordeaux territories, early morning mists blow off the region’s rivers, giving way to sunny, dry days. This pattern repeats on a near daily basis at summer’s end.

What’s more, one of the region’s two major sweet wine grapes, Sémillon, is particularly susceptible to Botrytis due to its thin skin.

But how exactly does noble rot alter Bordeaux’s sweet wines and set them apart?

How Noble Rot Affects Wine Grapes 

Spores of the Botrytis cinerea fungus penetrate grape skins causing water evaporation. The warm, dry afternoon weather stops grey rot from taking hold and accelerates the rate of evaporation. This results in shrivelled grapes with high concentrations of sugar, acids, and minerals.

The greater the frequency of beneficial Botrytis attacks, the more widespread and concentrated the effects of noble rot throughout the vineyards.

Botrytis and Aromatic Development  

Botrytis oxidizes glucose in wine grapes into gluconic acid. This transformation results in intense, baked stone fruit and honeyed aromas. Studies have shown that concentrations of certain lactones, like soloton, are higher in botrytized wines. Soloton imparts sweet, nutty aromas to wines. At very high levels, it can give off exotic spice nuances like curry leaf or fenugreek.

In general, the nobly rotten sweet wines of Bordeaux are intense and complex on the nose. Common aromatic descriptors include marmalade, baked stone and tropical fruits, honey, roasted nuts, candied ginger, and saffron.

Botrytis and Palate Profile  

Botrytis spores excrete glycerol into affected grapes. These higher glycerol concentrations give excellent ageing potential to botrytized wines and allow them to last for extended periods once opened. The high glycerol levels also impart a rich, rounded mouthfeel to the wines.

The fermentation process for Botrytis-affected grapes stops, either naturally or by winemaker intervention, well before all sugars have been converted to alcohol. This leaves high residual sugar levels resulting in lusciously sweet wines.

Due to this heady sweetness, vibrant acidity is necessary to bring balance to noble rot wines. Research has shown that Botrytis causes tartaric acid to break-down in infected grapes. It is therefore important that varieties with naturally high acidity be used in noble rot wines. In Bordeaux, Sémillon’s main blending partner, Sauvignon Blanc, brings this lively freshness.

Botrytized grapes - Credit : CIVB