BRUSSELS -- The most esteemed brewer of Belgian lambic beers, Brasserie Cantillon, sits in a mixed industrial and residential district far from the city's tourist haunts. The pale gray light of a winter day was fading fast when we hopped off a trolley at Square d'Aviation and studied our map, determined to reach the brewery before it closed at 5 p.m. We knew we had arrived when we rounded a corner and nearly bowled over a teamster offloading 110-pound bags of wheat.
Seeing our rush, Claude Van Roy assured us (in French) that we had plenty of time to tour the brewery and sample the wares. When she heard our accented thanks, she switched effortlessly to English.
Lambic beer is made with an ancient style of brewing, depending on spontaneous fermentation to produce a bone-dry, profoundly tart, and naturally effervescent drink that improves with years in the bottle. By European Union regulation, it can be produced only in a small area of Belgium.
An aura of mystery clings to the sour, powerful lambics, which are often spoken of with the reverence otherwise reserved for grand cru wines or ancient brandies. Yet the Van Roy family blithely lets visitors see every step of the process at Cantillon because their particular lambics cannot be replicated.
The brewery dates from 1900, and looks every day of its age. In the normally antiseptic world of beer making, this would not be a good thing, but lambic beer bears about as much relation to the amber fizz of commercial lagers as a great Roquefort does to Velveeta.
Most brewers sterilize everything in sight, then introduce a carefully guarded strain of yeast to their malted grain. Traditional lambic brewers let wild yeasts do the work. The proprietors of Cantillon estimate that at least 100 strains of yeast and 50 kinds of bacteria produce their unique brews. Fermentation depends entirely on the colonies of microorganisms in the building, and the blending and secondary fermentation proceed to suit the palate of the master brewer and no one else.
By tradition, lambic beers are made from 35 percent raw wheat and 65 percent malted barley with three-year-old dried hops added at a specific ratio.
"It is a very, very old process," Van Roy explained. "When the fermentation finishes after eight or 10 days, the beer is done. We put it away in barrels like wine. When it is one, two, three years old, we taste, we blend, then we bottle." She didn't mention it, but the blending process is at least as complex as making a sherry or a port.
Then she shooed us toward a rickety flight of stairs.
"Go!" she said. "Look it over and come back for a drink."
The "tour" was self-guided, but with signs and handout sheets in French, Flemish, and English, we hardly needed a guide to pick our way through the Rube Goldberg apparatus. We climbed the stairs past the copper vats ("mash tuns") where the grain and malt are cooked together at a temperature just below boiling. Darkness suddenly turned to a glimmer of dim light as an automatic switch illuminated the grain storage bins above the mash tuns.
Perhaps the most important piece of equipment in the brewery stood nearby: a gigantic copper tray where the hot wort (as the extract of grain and malt is called) cools overnight as wild yeasts and bacteria settle onto the surface. In the morning, the seeded wort drains into a stainless tank and is pumped into sterilized barrels to ferment.
When the foam stops pouring out after a week or so, the Van Roys simply seal the barrels and let them age until the brew is ready for blending into gueze (pronounced "gooz"), the noblest and most straightforward of lambic beers. Alternately, some barrels are set aside to receive doses of macerated fresh cherries, raspberries, or Italian white grapes to make Kriek, Ros de Gambrinus, or Vigneronne, respectively.
Jean Van Roy, who took over the master brewing and blending tasks from his father, Jean-Pierre, three years ago, is something of a purist. Most of the year, he prefers the classic gueze, with its thick texture and complex sour flavors and dry finish. In summer, he'll switch to fruit beers for a lighter touch and an appealing tartness.
Unlike most producers, Cantillon does not sweeten its lambic beers. In a traditional tavern, lambics are served with sugar cubes and a "stamper" to crush the sugar in the bottom of the glass. Sugar does cut the assertive acidity of a lambic, which can have overtones variously described as "citric" or sometimes (and this is a compliment from beer aficionados) "barnyard" or "horse blanket."
Cantillon's traditional lambics have more in common with wine than with other beers. They acquire some of their flavor from the oak and chestnut casks where they're aged (most of Cantillon's barrels come from Bordeaux wineries), and they oxidize like fine sherries. Moreover, gueze and the fruit-infused lambics cellar well for up to 20 years.
When we settled in by an upended barrel to taste some beers, Jean-Pierre and Claude Van Roy joined us. With great amusement, she told of an American who came in on a hot day and downed a large glass of the tart Kriek in a single gulp.
"And Americans," she said in a disapproving tone, "want their beer very cold."
Jean-Pierre snorted as he poured glasses of gueze, observing that it was exactly 8 degrees Celsius (about 46 degrees Fahrenheit). Cold, he explained, kills the flavor. He sipped slowly, drawing some beer through his lips with lots of air, nodding his approval.
"It has only been in the bottle for six months," he said. "It will be better in five years. Let's try the raspberry."
Patricia Harris and David Lyon are freelance writers in Cambridge.
