Noble Rot Fermentation

A prudent search of the Internet usually can result in the perfect reference that one is looking for. In this case, my husband asked me how a Noble Rot Fermentation is done. In 2020, I made, by chance, a very, very small amount of Noble Rot wine that I wrote about in a blogpost called The Most Expensive Wine.
This year, we don't really want to leave our fermentation to chance, so I found what I was looking for in a reference titled, An Overview on Botrytized Wines REVISÃO: VINHOS BOTRITIZADOS published in 2020!
Here is a summary of the main points of Noble Rot Fermentation from the reference:¹
Grape Varieties

• The main grape varieties used for noble rot fermentation include Semillon, Sauvignon Blanc, Chenin Blanc, Furmint, Riesling, Picolit, Pinot Blanc and Gewürztraminer.
• These varieties have characteristics such as type of skin as well as oenological parameters, such as potential alcohol, sugar content and acidity that make them ideal for botrytization.

Harvest

• The harvest for noble rot wines cannot be done mechanically. Each grape that goes into the fermentation is harvested by hand and is based on botrytization process that promotes an intense enzymatic maceration of the grape skin.

Pressing

• After harvest, a sorting table is recommended for separating parts of the clusters that are less or not infected or co-infected by other fungus and bacteria.
• Whole cluster pressing in which stems facilitate pressing and their extracted tannins is also recommended to facilitate protein stabilization, with subsequent maceration for sugar and aromas extraction.
• Whole cluster pressing avoids the release of particles such as glucans in the must as these colloids will produce later clarification problems and will hamper the stabilization processes.
• The use of gravity versus pumps during berry juice transfer decreases the harshness of every operation.

Settling before fermentation

• Settling at temperatures lower than 10°C for 28 to 48 hours is beneficial but excessive clarification increases the possibility of stuck or sluggish fermentation as lees is a source of fatty acids, sterols, among others, which are important for yeast development.
• The optimum turbidity, without facing problems of bitter, reductive and herbaceous notes is between 500 and 600 NTU.
• Addition of lees, saved from settling of previously produced dry white wine, in tank or barrel filled with botrytized must, is a boosting factor for yeasts development during fermentation.

Alcoholic fermentation kinetics

• Following a spontaneous alcoholic fermentation of noble rot must with a sugar content of 320 g/L showed that after the yeasts’ growth phase, fermentation rate stayed stable and maximum during stationary phase, until the 11th day of fermentation when the death phase started.
• Fermentation stopped after 40 days, with a level of residual sugars at 100 g/L.
• The extremely high sugar content of these wines affected the yeasts’ growth phase due to the hyperosmotic environment that eventually led to ‘‘stuck’’ fermentation.

Vat types for fermentation

• Two types of containers can be used during fermentation, stainless-steel tank or wooden barrel.

The reference also provided a work flow for the fermentation of noble rot grapes:¹

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.