My source material here is Jeff Sparrow's Wild Brews, which I recommend highly for anyone interested in a deep understanding of sour beers. Let's start with a pithy opening from the start of his fourth chapter, "Beer-Souring Microorganisms." Here, he describes the actors that create the funk:
"Four dominant types of microorganisms commonly ferment and acidify wild beers: brettanomyces, lactobacillus, pediococcus, and saccharomyces. Sever other important players also merit a mention, including acetobacter, enterobacter, and various oxidative yeasts."Now, in this next pithy passage, he describes the particular nature of the funk those actors produce:
"The acids most important to wild beers include lactic and acetic acid. Acetic acid, present in copious amounts in vinegar, is sharp, pungent, and greatly increases the perception of sourness. Lactic acid, found in spoiled milk, is less objectionable and contributes a 'tangy' character, sometime perceived as 'sweet' by brewers in contrast to other acids."(There are actually a host of other acids that contribute flavor like caproic acid, which Sparrow says gives a "goaty," "sweaty," or "zoolike" character. But you can read his book if you want the full monty.)
This wild yeast inspires the most awe and fear among brewers. It will eat anything, including dextrins and sugars that other yeasts find unpalatable, achieving nearly 100% attenuation. (Brewers joke that it will start eating the glass in a bottle if you leave it long enough.) Attenuation is the percentage of available sugars a yeast will eat. Wyeast's Northwest ale yeast, a non-brettanomyces yeast, attenuates at about 70%, for example. Brett will produce both acetic and lactic acids, but the former only under certain circumstances. There are at least five species of brettanomyces and many strains within each. The most common is brettanomyces bruxellensis, named for a strain from Brussels.
Lactobacillus is a type of bacteria that gives Flanders beers (red and brown) their character, as it does to some German ales like Gose and Berliner Weisse. It is not a major player in lambics, however--the lactic there comes from pediococcus (see below). As the name suggests, lactobacillus produces lactic acid. Lactobacillus is far more finicky than brettanomyces, preferring warm temperatures, a low-oxygen environment, and low levels of hop acids.
As alluded to above, pediococcus is the beastie that gives lambics their lactic, not lactobacillus. This is mainly a function of the life cycle of a lambic. Pediococcus ferments in beer with little or no oxygen; likewise, it gives off no carbon dioxide. In a lambic, the pediococcus kicks in after 3-4 months when, fascinatingly, the wort is exceptionally sour as a result of early enterobacter production. The pediococcus begins when the lambic warms up, creating "long strands of slime" on top of the wort. You can drink the beer at this stage, but it's oily and known as the "sick" stage. But from the sickness comes the lactic, and eventually, the slime is reabsorbed as the brettanomyces begin gobbling up everything that's left.
The upshot? "Sour" isn't a fixed flavor. Different beers have different compounds and acids that contribute characteristics that define style. Brewers have very different attitudes to the kinds of sour beers produce. When I was at Allagash last year, Jason Perkins and Rob Tod described their efforts to cultivate native brettanomyces. On the other hand, Ron Gansberg doesn't want brett in his brewery; he's a lactobacillus man. Matt Swihart is a brett man, but is he only a brett man, or will future batches exhibit the character of other funky bacteria? I guess we'll have to wait and see.
By the way, Sparrow reproduces a lot of very cool graphs he got from Raj Apte, and you can find those at Raj's site. One I particularly enjoyed is a graph showing the waves of activity in lambic fermentation, particularly in the first year. Click on it to see an enlarged version. You'll find more cool stuff if you follow the link to his site, too.
PHOTO: Cantillon casks, Thom's Beer Blog / link