Commercial Barrel Aging

February 10th, 2024

The barrel business isn’t always clean. While we pride ourselves on maintaining a well-run and tidy operation, the truth is that used oak casks are naturally a bit messy. And that’s okay—they’re full of character and history, after all.

But when it comes to one particular topic, we’ve noticed some concern popping up lately: the black stuff you sometimes find on casks. It’s easy to assume it’s mold, but here’s the good news: IT’S NOT MOLD—it’s whiskey fungus. And while "fungus" might not sound much better, rest assured it’s completely harmless. Sure, it’s not the prettiest (to be fair, the list of beautiful fungi isn’t long), but whiskey fungus won’t hurt you or the beer, whisky, bourbon, or other spirit aging inside the barrel. In fact, as we’re about to explain, it might even help your barrel-aging process.

Still unconvinced? Hey, we respect someone who does their due diligence. If you’re curious (or concerned) about that dark, fuzzy substance, let’s dive into what it really is, why it’s there, and why you don’t need to worry.

“Whiskey fungus” is the common nickname for the fungus scientifically known as Baudoinia compniacensis, a species that thrives in environments where ethanol is present. You’ll often see it near whiskey distillery rickhouses, where it feeds on the ethanol vapors released during the aging process.

The phenomenon was first documented in 1872 by French pharmacist Antonin Baudoin, who observed a black substance growing around Cognac distilleries. While he initially misidentified it as algae, mycologists later determined it was a fungus. In 2007, mycologist James Scott officially named it Baudoinia compniacensis (Scott et al., 2007), a nod to the man who discovered it. 

Scott’s research extensively documented whiskey fungus around distilleries. In communities with prominent distillers—such as Kentucky’s Bourbon Country—it can be found on buildings, street signs, fences, cars, and yes, barrels. At first glance, it may look like soot, but a closer look reveals its fuzzy texture. Some communities attempt to keep it at bay with regular cleaning, but since it feeds on evaporated alcohol, it always returns. The ethanol vapors that escape from barrels—a process distillers call the “angel’s share”—combine with moisture in the air to create the perfect environment for this fungus to grow.

Understandably, it’s not uncommon to find the substance on used bourbon and whiskey barrels, including near distilleries in our neighboring state of Tennessee (Levenson, 2023). In fact, whiskey fungus has been documented near Canadian whiskey distilleries and Caribbean rum distilleries (Rogers, 2011). There’s a reason it’s also known simply as “distillery fungus.”

But whiskey fungus isn’t exclusive to distilleries. It’s also been found near ethanol plants (Carboline, 2024), bakeries, and other places where alcohol evaporates. In short, if ethanol is in the air, whiskey fungus may appear (Scott et al., 2016).

One common misconception is that whiskey fungus is the same as black mold. It’s not.

Mold and fungus are often used interchangeably, but they are not the same. Mold is a type of fungus, but not all fungi are molds. Molds require warm, damp conditions and pose health risks due to their airborne spores. They thrive in water-damaged environments.

Black mold typically refers to Stachybotrys chartarum, which grows on materials high in cellulose, like wood or drywall, and causes respiratory irritation and allergic reactions in some people (Dyląg et al., 2022).

Whiskey fungus, on the other hand, is an entirely different species of fungus. Unlike black mold, it doesn’t grow on surfaces because of water damage, nor does it release harmful spores. It thrives specifically on ethanol vapors and poses no risk to human health (Indiana Department of Health, 2023). It’s also important to note that whiskey fungus stays on the outside of barrels and doesn’t affect the liquid aging inside. 

Understanding this key difference can help distillers and brewers feel confident that the black substance on their barrels is harmless—and might even be beneficial.

While whiskey fungus might not be the prettiest sight, it can actually benefit the barrel-aging process. In larger quantities, it acts as a natural moisture barrier, slowing oxidation and helping develop deeper, more complex flavors in barrel-aged beer.

One brewer shared their experience with us: they purchased two Wild Turkey barrels of the same age—one with whiskey fungus and one without. The cleaner barrel imparted the bold, classic Wild Turkey flavors, while the cask with whiskey fungus contributed deeper fudge and umami notes to their beer. The difference was clear—whiskey fungus played a role in enhancing those unique flavors.

Brewers sometimes ask us about the black substance on their barrels. We’re quick to ease their concerns—and even let them know how it might help. As our VP of Sales, Ethan Schulenberg, explains in the video below, whiskey fungus on a barrel just means there’s plenty of bourbon or whiskey vapor—and serious flavor—inside the oak.

Let’s address the big question: Is whiskey fungus harmful? The answer is a definitive no.

Baudoinia compniacensis poses no risk to human health or to the products aging inside barrels. It lives on the outside of barrels, feeding on the ethanol vapors that escape during the aging process. There’s no evidence to suggest it harms people, spirits, or beer in any way.

In fact, its presence is a testament to the authenticity and natural aging process of barrels. Whiskey fungus is simply part of the environment where great whiskey, bourbon, and other spirits are made.

We hope this clears up any concerns you might have about the dark, fuzzy substance found on some barrels. Remember: it’s not mold, it’s harmless, and it might even help your product taste better.

If you have more questions—or if you’re ready to talk barrels—reach out! Midwest Barrel Co. is here to make the barrel-buying process easy and educational, with a little bit of fun along the way. Let’s keep your barrel-aging journey moving forward!

Dyląg, M., Spychała, K., Zielinski, J., Łagowski, D., & Gnat, S. (2022). Update on Stachybotrys chartarum—Black mold perceived as toxigenic and potentially pathogenic to humans. Biology, 11(3), 352. https://doi.org/10.3390/biology11030352

Indiana Department of Health. (2023, November). Baudoinia compniacensis "Whiskey Fungus". [PDF document]. Retrieved from https://www.in.gov/health/eph/files/Whiskey-Fungus-UPDT.pdf

Levenson, M. (2023, March 1). Whiskey fungus fed by Jack Daniel's encrusts a Tennessee town. The New York Times. https://www.nytimes.com/2023/03/01/us/whiskey-fungus-jack-daniels-tennessee.html

Rogers, A. (2011, May 17). Unraveling the mystery of the Canadian whiskey fungus. Wired. https://www.wired.com/2011/05/ff-angelsshare/

Scott, J. A., Ewaze, J. O., Summerbell, R. C., Arocha-Rosete, Y., Maharaj, A., Guardiola, Y., Saleh, M., Wong, B., Bogale, M., O’Hara, M. J., & Untereiner, W. A. (2016). Multilocus DNA sequencing of the whiskey fungus reveals a continental-scale speciation pattern. Persoonia, 37, 13–20. https://doi.org/10.3767/003158516X689576

Scott, J. A., Untereiner, W. A., & Ewaze, J. O. (2007). Baudoinia, a new genus to accommodate Torula compniacensis. Mycological Research, 111(8), 919–922. https://doi.org/10.1016/j.mycres.2007.05.014