In yesterday's post about Devil's Kriek, Samurai Artist sparked a conversation about the souring properties of 
brettanomyces.  Since sour beers are becoming more common, it's a timely discussion.  

Sourness  may be tart and clean as in a Berliner Weisse, dry and austere as in  some lambics, our face-puckeringly intense, as in some Flemish reds.   These different qualities come from different microorganisms, and it's  worth spending a post mentioning a few of the biggies.
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.)
Brettanomyces 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.
LactobacillusLactobacillus 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.
PediococcusAs 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.

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PHOTO: Cantillon casks, Thom's Beer Blog / link