BREWING BEER IS usually an exercise in control. Vats and apparatus must be sterilized, water purified, air scrupulously excluded, lest off-flavors and impurities creep into the carefully crafted flavors of your particular brew. The wide, fecund world must be kept at bay, as in an operating room, protecting the product from the teeming soup of life that swirls around us every moment of every day. 

Unless, of course, you’re making sour beer. 

Sour beers, also known as “wild-fermented” beers, are produced by harvesting the wild yeasts and organisms that float invisibly in the open air. Effervescent and tangy and unpredictable, such beers grate against everything that most beer brewers hold as sacrosanct.

While other brewers fight for control, walling up their beer like virgin damsels in sterilized fortresses to keep their purity intact from the “corrupting” influence of the outside air and inoculating the wort with a carefully bred strain of brewer’s yeast, brewers of lambics let their beer ferment free-range in vats called “coolships,” exposed to the outside world like a vast primordial soup of sugars inviting the ministrations of Mother Nature’s most micro of brewers.

This has its downsides, of course — allowing your beer to completely wild-ferment is unpredictable, and sometimes whole batches are lost to off-flavors caused by unwelcome bacteria and impurities. But when it works, the result is — for those of us who like kombucha, drinking vinegar and tart shrubs — the perfect brew. 

Belgium, land of fine chocolates and Hercule Poirot, is the ancestral home of sour beers, known as lambics (or, variously, Gueuzes, or Krieks, or Mars, among other monikers). And, while they have certainly perfected the art, it is perhaps not quite correct to say that Belgian brewers actually “created” the concept of sour beer.

Surely, the earliest beer brewers (Mesopotamians or Egyptians, depending on whom you ask) had to capture wild yeasts for their very first brews. And for beer’s earliest brewers, this magic of fermentation must indeed have seemed like a gift from the gods. But this phenomenon is still oddly relevant; during the early days of the coronavirus pandemic, when a flurry of stress-baking led to a temporary shortage in baking yeast, newbie home bakers across the world got back to their ancestral roots and tried sourdough, snagging the yeasts they already cohabitated with in their homes to rise their loaves. The same principle produces sour beers. 

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But proper lambic and Gueuze beers, sometimes referred to as the “Champagne of Belgium,” are produced only in Belgium, with those names theoretically limited to ales created by the wild yeasts floating untethered in the air of the Senne Valley (although you’ll see the term “lambic” used often enough outside Belgium for beers produced the same way).

The sour beers you will find on your local grocery shelf might not be quite as wild as those Belgian lambics, some of which are so small-batch and unpredictable, they are served only in bottles a few miles from where they are produced.

There are methods for mass production of sour beers that involve adding lactobacilluspediococcus or Brettanomyces yeasts directly to the beer instead of traditional brewer’s yeast, or brewing them in steel kettles instead of wood vats (hence, “kettle-soured” beers), resulting in a somewhat kombuchalike New World hybrid. These are intriguing beers, rebelling against the clean, crisp, hoppy flavors that have dominated beer for so long. 

Flavor-wise, sour beers are often enhanced with fruit, and Belgian lambics often contain the juices of cherries, peaches or currants. Similarly, most modern sour beers will have added fruit juices to complement and boost the sourness. Despite being fruity, these beers are dry, cidery and sometimes puckeringly tart, and you can find local versions like Raspberry Sour from Lucky Envelope Brewing and even several different sours, like the Pear Sour or Guava Sour, from the avowedly experimental brewery Tin Dog Brewing.

Depending on the methods, you might be drinking beers fermented with the very local yeasts you’ve been walking through, breathing in and rising your pandemic loaves with all year long.