Whisky Fungus: Understanding the Dark Growth Around Warehouses

Black growth around some spirit warehouses has an unexpected connection with the contents inside. Fungi in the genus Baudoinia can use airborne ethanol, helping explain their association with places where alcohol is stored.

The association is interesting, but it is not an authentication test. A dark surface cannot tell you the age, quality or origin of a particular whisky cask.

Whisky fungus: the dark growth near warehouses

Some alcohol and water evaporate during cask maturation, a loss known as the angel's share. The rate varies with conditions and the cask; a single annual percentage should not be used to reconstruct the life of an individual barrel.

Where conditions suit it, Baudoinia can form dark growth on surrounding surfaces. Its extent is not a simple measure of how many casks are present or how long they have been stored.

This is not unique to Scotch whisky. The connection is with suitable conditions and airborne ethanol, rather than one particular spirit or country.

Black Trees and Angry Neighbours

A black coating alone is not enough to identify a fungus or assess the health of a tree. Other deposits and organisms can darken surfaces too.

A Scottish Government review published in 2023 noted limited research on health effects associated with Baudoinia. That does not support an absolute claim that every exposure is harmless. Concerns about a local site should be addressed through the relevant environmental-health service.

For neighbours, surface soiling can be a practical concern even when visitors see it as part of a distillery's appearance. It should be discussed carefully rather than romanticised.

For a whisky enthusiast, the useful lesson is that maturation also involves losses from the cask. The resulting environment can be worth learning about without calling a black coating proof of quality.

What dark wood can and cannot tell us

Reclaimed cask wood can have dark areas for several reasons, including heat treatment, metal contact, storage and previous use. Appearance alone is not enough to identify a particular deposit.

Heavy darkening does not prove decades in a dunnage warehouse. It cannot establish the cask's filling history or show how long one whisky matured.

In my workshop, the source of the material and its condition matter more than the darkness of its surface. I prepare genuine reclaimed cask oak for its next use and describe only the provenance I can support.

An interesting clue, not a certificate

Whisky fungus is a subject worth exploring because it connects maturation with the surroundings of a warehouse. It is not a certificate of age, authenticity or careful production.

When you visit a distillery, ask the guide about warehouse conditions and the angel's share. For a piece of reclaimed wood, ask about its source cask instead of relying on the colour of its surface.

Sources and further reading

https://www.gov.scot/publications/review-human-health-environmental-impacts-non-methane-volatile-organic-compound-emissions-malt-whisky-maturation-scotland-final-report-prepared-scottish-government-aether/pages/5/