Modern beer is produced through fermentation by a pure culture of yeast. Brewing yeast is a strain of either Saccharomyces pastorianus (for lagers) or Saccharomyces cerevisiae (for ales), and its purity is constantly maintained. However this practice is only a few hundred years old, evolving alongside (and in a large part motivating) the study of microbiology.
Before the invention of the microscope the various fermentation microbes were indistinguishable. The yeasts and bacteria would form a mixed culture referred to simply as “Godisgood”. This cake would be repitched from batch to batch indefinitely, a method much like making homemade sourdough bread.
Brewing yeast (especially lager yeast) is prized because it ferments alcohol yet generates minimal byproducts. There are many other microorganisms (bacteria and wild yeasts) that do much the same work less cleanly. The vast majority of these are considered beer spoilers.
With the right species under the right conditions, a mixed fermentation of bacteria and yeast will produce something with more depth than any wine. The addition of strains of bacteria to beer adds a unique flavor: sour. This comes from organic acids, predominantly lactic acid (yes, like sour milk) and acetic acid (yes, like vinegar). If not handled properly sour beer will be worse than vinegar or sour milk, a lesson understood by many beginning homebrewers (from pouring their beer down the drain).
However, when made with style and grace a sour beer is simply divine. If you already appreciate the complexity found in the binary balance between malt and hops, imagine the possibilities afforded by the addition of this new dimension of flavor.
Certainly sour beer takes dedication to appreciate. Doesn’t all beer? If you were to try Pliny the Elder or some other ridiculously hoppy, ultra-bitter beer, without previous experience with IPAs, that too would seem undrinkable. Given the proper introduction, however, it is not just drinkable but delicious.
The most striking character of sour beer is its complexity. Certainly there is the strange balance between malt and acid and hops. Fruity character is accented by the acidity in the same way it is in a high quality wine (or an orange or grapefruit for that matter). But my favorite part of the complexity is the way the taste of a sour beer will change as it sits on your tongue. The organic acids will slowly neutralize after coming into contact with your mouth and saliva and different aspects of the beer will shine through at different times.
There are basically three types of sour beer: the weissbiers of Berlin, the brown and red ales of east and west Flanders, and the lambics and gueuzes of the Zenne (Senne) valley. Blending is common if not the rule for all of these styles, both to temper and accentuate the sourness.
Berliner weisse is a pale wheat style with a clean and crisp acidity that comes from lactic acid. It is usually brewed to 2.5 to 4 percent alcohol, making it a shankbier or “small beer”. Traditionally it is served with a sugar syrup that comes in two flavors: green is woodruff and red is raspberry.
Flanders red-brown ales have a strong fruity malt character and a balancing acidity. Unlike Berliner weisse the red-browns have a prominent acetic component in addition to the lactic sourness. This is stronger in the red ales of west Flanders, which are aged in oak. The east Flanders brown ales are aged in steel and have more dark malt complexity that makes up for the relatively restrained acidity.
Lambic is a style of beer that properly can be brewed only in spring and fall in the valley of the Zenne, a river that flows under downtown Brussels. This valley is home to a particular microbiocoenosis: the balance of various bacterias and wild yeasts is just right to produce the true champagne of beers. Lambic is fermented in wood casks, which produces two important effects. First, the microbiota can “hibernate” in the wood, something more and more important as the area around Brussels gets paved over. Second, all the carbon dioxide escapes through the wood, so unblended lambic is flat.
Lambic is almost never consumed straight anyway. There are three common preparations. Faro is lambic that has had Belgian candi sugar added. Gueuze is a bottled blend of old (two or three year) and new (six month) lambics. The sugars remaining in the younger beer ferment into carbon dioxide producing a sparkling beverage. The most common lambic preparation is the addition of fruit. Raspberries and cherries are the traditional candidates, with Shaarbeekse cherries being the real classic (Shaarbeek, former home of endless fields of cherry trees, is only a kilometer from downtown Brussels).
Sour beer must be savored with patience, but your dedication will be rewarded many times over with an intensity and depth beyond that of any other beverage. And that is why sour is my favorite kind of beer.