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| Sautéed mushrooms, quickly cooked in butter and extra virgin olive oil, then finished with a flambé of bourbon. |
Mushrooms are tasty on pizza, battered and deep fried, or stuffed with crabmeat. Maybe you like grilled portabellas with polenta, or shiitakes in a stir fry. Or perhaps you'd enjoy a tasty side-dish like the one pictured above. Bourbon-flavored mushrooms? Sure. Mushroom-flavored bourbon? Maybe not.
But when white oak intended for whiskey barrels is seasoned naturally, mushrooms of a microscopic sort, usually referred to as fungi, play a vital role. Scientists call it fungal colonization. It is an early part of the wood’s natural decomposition process.
During seasoning, a succession of different fungal species send out roots (hyphae) that penetrate the wood structure and release hydrogen peroxide, a natural bleaching and oxidizing agent that helps break the wood down chemically, softening tannins and caramelizing hemicellulose among other salutary effects.
A fresh-cut oak is about 60 percent water by weight and needs to get below 18 percent for the coopers to do their thing. First in the pool is Aureobasidium pullulans, one of the species of common mildew, the same black stuff you clean off your shower tiles. As the wood dries it becomes inhospitable to pullulans which pulls out (okay, dies) and is replaced by another type that thrives in the slightly drier environment. One after another a succession of different fungal species (eumycota) and sub-species each have a go at it, including the one from which the medicine penicillin is made.
By studying fungal colonization in American white oak (Quercus alba), scientists proved the superiority of a traditional cooperage practice–air drying–that was widely abandoned in the United States after World War II in favor of kilns. Kilns remove moisture effectively, but they stop the biological processes, fungal and bacterial, that make many of the wood’s flavor components available for absorption by maturing spirit.
In the first stage of natural seasoning, if humidity and other weather variables are favorable, fresh-cut logs are simply left in the field for days or weeks. From there they go to a stave mill, close to the forest, where they are roughly broken down into staves and head pieces. From there they are shipped to the cooperage, where they are neatly stacked in the yard, fully exposed to the elements. There they will remain for anywhere from three months to two years, and in some cases even longer. Often wood that is given only a short time outside is finished via kiln.
As you can probably guess, it’s a cost issue. You pay a premium for long natural seasoning. A good question to ask when someone tries to sell you an expensive whiskey is, "How long were your barrel staves air seasoned?”
Don't be surprised if they have no idea what you’re talking about.
3 comments:
Thanks so much for your post here. I always heard that it is best to season wood naturally, outside and exposed to all the elements, but I never really knew why until now.
I do some home aging with wood chips in 60% NGS. But while the woodchips are first kiln-dried (according to the packaging) and are not exposed to the elements, they’ve been in my garage for about two years. So heat, cold, humidity, and dryness all get to them. I even took the liberty of punching extra holes in the bags so humidity & temps get to the wood easier. I suppose insects have been crawling all over them too, but they’ve not endured a direct hit of rain or snow or anything like that. So your post here gives me some perspective on what I’m doing, or perhaps what I’m doing wrong.
I listened to a fascinating interview with Nancy Fraley where she waxed on about this very topic, how longer periods of air drying and the gradual decrease in moisture content promotes fungi growth, wood decomposition and its chemical breakdown, and how that influences the aroma and flavor of whiskey.
One of the key fungal enzymes may be laccase, which breaks down lignin so that the fungus can access all of that delicious cellulose and hemicellulose. Oxidized lignin components include vanillin, guiacol, and similar aromatic compounds.
I wonder if wood that has been spalted (a woodworking term referring to the coloration due to fungal growth in the living tree) has yet another flavor profile.
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