The Chemistry of Noble Rot Wines

This is another post on noble rot wines, focussing on the chemistry of the wine. In order to get a final product that can be considered to be a noble rot wine, it is important that the colonization of B. cinerea on the grapes is of high purity. The higher the purity of the noble rot, the higher the quality of the final product.
Botrytized wines are typically aged for a long time either in barrels (like Tokaji Aszú) or in bottles (Sauternes, TBA wines). The length, the oxygen ingress, the level of SO₂, and the material of the cooperage have a huge impact on the aroma composition of the new botrytized wines.¹
Lactones

• γ-nonalactone, γ-decalactone and δ-decalactone has distinct aromas of apricot and peach
• γ-3-methyloctalactone, known as whisky lactone, is a tertiary aroma compound with coconut flavor extracted from wood during barrel aging
• 2-furanone or γ-crotonolactone enhances the aroma profile with caramel and stone fruit notes
• 2-nonen-4-olide, is a 2-furanone type lactone, which has the second lowest perception threshold (10.8 μg/L) after γ-3-methyloctalactone (whisky lactone), and is mainly associated with fruity and minty aromas only found in this type of wines
• 2-nonen-4-olide along with γ-3-methyloctalactone (whisky lactone), eugenol and γ-nonalactone seem to be responsible for the creation of the synthetic aroma of candied orange

Sotolon

• One of the first compounds identified as a key odorant was sotolon (4,5-dimethyl-3-hydroxy-2(5H)-furanone), having a sweetish, caramel, or nut taste
• Sotolon, is a compound landmark for Port aged wines with sensory character of curry, fenugreek, nuts and dried fruits, with a threshold of 19 μg/L
• The impacts of botrytization on grapes, such as oxidation and sugar concentration, seem to create the perfect substrate for sotolon production after aging

Thiols

• 3-Mercaptohexan-1-ol (3MH) is the most important thiol in botrytized wines enhancing the aroma profile with notes of citrus peel, grapefruit and passion fruit
• 4-mercapto-4-methylpentan-2-one, 4-mercapto-4- methylpentan-2-ol, 3-mercaptopentan-1-ol and 3-mercaptoheptan-1-ol are responsible for aromas of citrus fruits

Phenylacetaldehyde

• Phenylacetaldehyde has an aroma of honey

The ratio between glycerol and gluconic acid is considered to be a noble rot quality index. The higher the ratio the higher the quality of the potential wine.
Some distinctive descriptive terms associated with botrytized wines are peach, apricot, quince, tropical fruit, honey, and caramel. The aging process may add more nuances, eg, chocolate, tobacco, dried fruits, raisins, etc.
References:
1. Kallitsounakis, Georgios, Catarino, Sofia, An Overview on Botrytized Wines REVISÃO: VINHOS BOTRITIZADOS, Ciência e Técnica Vitivinícola, 2020, 35, 76, 10.1051/ctv/20203502076.
2. Magyar I., Soós J., Botrytized wines – current perspectives, International Journal of Wine Research, 2016;8:29-39 https://doi.org/10.2147/IJWR.S100653.
3. Sipiczki, Matthias, Csoma, Hajnalka, Antunovics, Zsuzsa, Pfliegler, Walter, "Biodiversity in yeast populations associated with botrytised wine making", Mitteilungen Klosterneuburg, 2010, 60, pg 387-394.