What is the impact of botrytis cinerea infection (noble rot) on the chemical composition of grapes and the resulting wine style?

Botrytis cinerea, under specific environmental conditions, causes 'noble rot', a beneficial infection that dramatically alters the chemical composition of grapes, resulting in distinctive sweet wines. The infection requires humid conditions followed by dry, sunny periods to prevent grey rot, a less desirable form of Botrytis. Noble rot concentrates sugars within the grape. Botrytis perforates the grape skin, causing water to evaporate, thus concentrating the sugars (primarily glucose and fructose) along with acids and other compounds. The infection also increases glycerol production, due to the osmotic stress on the grapes. Glycerol contributes to the wine's texture and perceived sweetness. Botrytis introduces unique aromatic compounds. It produces sotolon, a key aroma compound responsible for honey, saffron, and marmalade notes characteristic of botrytized wines. It also degrades certain grape aroma precursors and produces others that impart specific flavors. Organic acids are also affected. While the overall concentration of acids may increase due to dehydration, Botrytis metabolizes tartaric acid, reducing the wine's overall acidity. This lower acidity balances the high sugar levels, preventing the wine from being cloying. Polysaccharides are created during infection of noble rot. Botrytis produces glucans, polysaccharides that increase the wine's viscosity and contribute to its luscious mouthfeel. The resulting wine style is typically a lusciously sweet, complex dessert wine with distinctive honeyed, apricot, and marmalade aromas, balanced acidity, and a rich, viscous texture. Examples of wines made from botrytized grapes include Sauternes from Bordeaux, Tokaji Aszú from Hungary, and Trockenbeerenauslese from Germany. The concentration of noble rot is key. If not carefully managed, less desirable compounds can emerge resulting in off flavors. The degree of Botrytis infection is critical for achieving the desired balance of sweetness, acidity, and aroma complexity. Winemaking techniques are used such as selecting grapes with the right degree of infection, carefully managing fermentation, and aging the wine appropriately.