If you have tried a Belgian lambic beer, then you have tasted the results of spontaneous fermentation. Lambic beer is exposed to naturally occurring yeast and bacteria in the open air, and matured in oak barrels for months or years (as opposed to other brewing methods, which use highly controlled single-yeast fermentations).

"Spontaneous fermentation is something that can happen all the time," says Greg Engert, beer director for the Neighborhood Restaurant Group in Washington, D.C. "If it is controlled a little bit, it can produce some amazing flavors."

After this interview, we asked Engert for a thorough explanation of the lambic brewing process. He did not disappoint.

Spontaneous fermentation

Rebecca Sheir: How does spontaneous fermentation work?

Greg Engert

Greg Engert: It works just like any fermentation. It's just one that happens wildly, and will happen without a brewer intending for it necessarily to happen.

When you're in college -- I'm not saying that any of us ever did this -- if you happened to leave a can of beer out overnight and then tasted it the next day to see if it was OK, it would typically have developed a vinegary note. That's because swimming around us unseen every day all the time are billions and billions of cells of bacteria and yeasts that are all starving for sugar sources. If you leave your can of beer out or your wine and you forget to cork it, it turns to vinegar. There's a particularly hungry yeast in the air called Acetobacter that will jump in in the presence of oxygen. It's an aerobic bacteria. It pretty quickly metabolizes any remaining sugars into acetic acid, which is what we call vinegar.

Spontaneous fermentation is something that can happen all the time. If it is controlled a little bit, it can produce some amazing flavors.

RS: How do you control it?

GE: There are a number of ways we can go about this. The first part is what kind of grains you use. Using grains that are filled with complex starches will make sure that certain bacterias and certain yeasts can't metabolize the sugars. They just can't handle how complex they are.

Another way is to only brew these beers in cooler months. One of the things that happens with these beers is that after you boil them, were you to expect anything to naturally occur immediately, it won't. The sugar water is too hot. If you do that in the cooler months, you get the stuff you want. If you do it in the warmer months, you might get too much acidity, too much funk.

You're going to age this now-cooled-off, starting-to-spontaneously-ferment wort, which is the sugar water, in oak barrels. Over time we're going to make sure that even as the beer is fermenting in oak and all the other wild yeast and bacteria in the barrels are working, there's going to be some evaporation. We call this the "angel's share." The levels of the beer itself are going to drop. We want to keep that oxygen out, or else we're going to get vinegar. We do that by just adding more beer on top.

[Previously from Engert: Beer for people who hate beer and The 7 flavor categories of beer: What they are, how to pair them.]


A lambic beer tasting

1. De Cam Oude Lambiek 2012

  • Geuzestekerij De Cam; unblended lambic; Gooik, Belgium; 5%

  • Unblended, spontaneously fermented still lambic, aged 3 years in oak barrels

beer Photo: adamjackson1984 / Flickr

GE: The first one is from a blender named De Cam. The blender himself is named Karel Goddeau. He's a brewer at a brewery called Slaghmuylder in Belgium, but his weekend job is blending spontaneously fermented lambics.

This is a beer that was bottled in 2012 that is called De Cam Oude Lambiek. It is spontaneously fermented and then aged in oak barrels, where it continues to ferment. All those wild yeasts, ale yeasts, lager yeasts and bacterias that are in the air are also in the oak barrels because oak has sugar in it. These wild yeasts and bacteria will feed on the oak until a new sugar source presents itself. Once we get this lambic in a barrel, it's going to continue over the next 3 years to develop sour flavors, funky flavors, and pick up some nice oaky tannins and vanillins as well.

RS: The color is beautiful.

GE: It's a little bit darker gold; a lot of that comes from the oak itself.

RS: It has quite an aroma compared with other beers I've tried.

GE: You'll also notice that it's not carbonated. This is actually blended from 3-year-olds and then it is not refermented in bottles, so it's still. But with the amount of acidity we have from this -- it's like with wine -- it doesn't need carbonation because it's naturally uplifting on the palate from its natural acidity.

RS: That funkiness you're talking about -- it's definitely there.

GE: This is tart. It's obviously more sour than your IPA or something that is quite hoppy and bitter. It has this really well-integrated acidity. You get the tannins, that woodsy drying quality from the oak. Everything is in balance. I think it's wonderful. I don't think it needs carbonation. I think it's wonderful as a still beer.

2. Oude Quetsche Tilquin à l’Ancienne

  • Gueuzerie Tilquin; fruit lambic (plums); Rebecq-Rognon, Belgium; 6.4%

  • Blend of spontaneously fermented lambic, matured 1 and 2 years in oak barrels then aged on plums, refermented in bottle and sparkling

beer Photo: Allagash Brewing / Flickr

GE: This is a fruited lambic from Pierre Tilquin. He's also a blender, so he buys this spontaneously fermented beer from other brewers, then ages it in his barrels and blends it together.

This is called Tilquin Quetsche, which is a plum lambic. You're going to taste this funky, wild, mildly tart, drying oaky flavor that we tasted in the De Cam Oude Lambiek. But now you're going to add this really interesting influence of plum. He steeps this spontaneously fermented beer on plums, which is very interesting.

RS: I definitely taste the fruit.

GE: It's a great reminder that fruit is not sweet. In steeping this, the wild yeasts and the bacterias continue to ferment a lot of the sugars of the plum, but they leave a lot of that color, so you get this beautiful violet gold color coming through. The aromatics of the plum work nicely with that perfumed, earthy note. But then on the palate, it has this plum-like juiciness that is so incredible.

"Any beer that is well made and filled with beautiful complexity belongs in a wine glass."
-Greg Engert

I really wanted to pour this for you because I think a lot of times when people think of lambic, they think of sweet fruit lambics. Brewers like Lindemans, Belle-Vue and De Troch, these brewers from the same region, they make spontaneously fermented beer.

Throughout most of the 20th century, people weren't interested in sour beer. They weren't interested in funky, wild beers that were reminiscent of barnyards and horses. They were looking for something that was a little more simple and direct, frankly more like soda pop. But in order to survive, these brewers would take their funky beer, they would filter it, they would pasteurize it, and they would add fruit juices and syrups, sweeten it out. Then they would sell it as kriek, which is cherry lambic, or framboise, which is raspberry lambic. Luckily, today the traditional lambics are coming back, so we have unfiltered, unpasteurized, all-natural, dry, tart and aromatically complex lambics from a number of producers.

RS: These beers are so different, so wild, compared with a lot of the more mainstream beers we have. If I go to a bar and I want to order this kind of beer, I just ask for a sour beer?

GE: Nowadays, that's probably your best bet. The thing about these real lambics from Brussels and the Pajottenland is that they're getting increasingly rare and hard to find.

Luckily though, our burgeoning American craft beer scene is picking up the slack and producing a host of sour, funky beers, some spontaneously fermented, others just deliberately fermented with wild yeast and bacteria. If you're out at a bar, just ask to see their sour list. You're going to find something that you like.

RS: Will they be impressed that I'm asking for a sour beer?

GE: Yes. They will be impressed.

RS: These beers that you're pouring today -- you're serving them in wine glasses. Is that typical with this kind of beer?

GE: It is, actually. To be honest, any beer that is well made and filled with beautiful complexity belongs in a wine glass as well as any wine. You get to swirl, you get to focus aromas with that glass, really dig in with your nose and pick up the bouquet. Certainly these beers that are so complex and alluring, they really benefit from being in a wine glass.

Rebecca Sheir
Rebecca Sheir is the host of Metro Connection on WAMU 88.5 in Washington, D.C. She previously served as host of AK on Alaska Public Radio Network and reported for NPR member station KTOO in Juneau. Her stories have won numerous awards, airing on public radio programs such as All Things Considered, Weekend Edition, Marketplace, Latino USA, Only a Game, Here & Now, Interfaith Voices and Voice of America.