Walk into a modern American or European brewery for a tour and you’ll most likely encounter towering steel tanks and a maze of shiny pipes glistening with condensation. For most commercial brewers, the brewing process begins and ends in these metal chambers, which are designed to be sterile vessels that leave the developing beer unperturbed while it undergoes its transformation. But some breweries have a corner, or a room, or an entire building that’s devoted to a very different approach, an organic process that involves a key element typically left off the list of ingredients: wood.

Compared to steel tanks, wood barrels are a whole new (or, really, much older) ballgame. Wood is porous and, as a result, it’s nearly impossible to keep its interior completely sterile. Thus, brewers aging their beer in wood may inadvertently introduce native yeasts, bacteria and other bugs that can change the flavor. The brew may also pick up traits from the wood itself, depending on the type of tree. Done correctly, or when adjusted by blending multiple batches together, you get beer that’s entirely new, fresh and unique, and the drawback of wood becomes a benefit.

Brewers have been barrel-aging beer for centuries — mostly out of necessity, before the advent of modern industrial production. In the early years of the new millennium, as craft beer got more popular and the first mainstream barrel-aged varieties, such as Goose Island’s Bourbon County Stout, hit the market, a new era of experimentation began and attempts to create the most robustly wood-flavored beers reached a fever pitch.

Perhaps as a consequence of palate fatigue, the barrel-aged craze slowed for a while, but lately it’s been making a comeback in more subtle and refined ways. Instead of smothering a stout with boozy bourbon, brewers are creating new mash-ups that result in remarkably creative and complex craft beers unlike anything you’ve tasted before.

As I noted, the characteristics of the wood can be imparted to the beer. Oak barrels, for example, can give beer a vanilla-like taste without the introduction of a single vanilla bean. The previous contents of the barrel can also infuse flavors into beer, as happens when brewers use barrels (or chips of barrels) that were once used to age bourbon, rum, tequila or other spirits.

Some brewers have found creative uses for cast-off wine barrels, making beverages that begin to blur the line between beer and wine. Fruit and other natural additives can also be introduced into the barrels to give the beer more depth and nuance of flavor. The barrel-aging process — which can last for weeks, months, or even years — can completely change the character of the beer that’s initially poured inside.

The creativity of craft brewers, combined with the increasing availability of local ingredients and of barrels from small-scale distillers, has introduced a wide variety of new styles to a sub-market that was previously dominated by heavy, booze-steeped stouts. There’s a golden opportunity to sample the new diversity of wood-kissed brews this month. On Sept. 23, at the Portland Company Complex on Munjoy Hill, the folks from BeerAdvocate, a magazine and online forum based in Boston, are teaming up with Allagash Brewing Company to host a festival called Beer Meets Wood. The event boasts the largest selection of wood-barrel-aged beers on the East Coast. Over 200 varieties are slated to be poured, including brews made in the United States and in Belgium, where the practice of barrel-aging has been steadily developing for hundreds of years.

Maine has a number of breweries that excel at this tricky art. Barrel-aged beers that would be rare in other places are almost commonplace around here, like the infallible Allagash Curieux, a tripel aged in bourbon barrels that was first brewed in 2004. The aptly named Barreled Souls, in Saco, has had great success experimenting with their small batches. The tequila-barrel-aged version of their tart, limey Space Gose tastes just like a margarita. Oxbow recently released a barrel-aged version of their flagship Farmhouse Pale Ale that resists definition. The wood adds a slightly funky character to the refined base beer, teasing out just a hint of vanilla from the French oak barrels in which it was aged.

The sterile predictability of steel has its function, for sure, but what better time and place than autumn in Maine to get lost in the woods?