Sour Bugs

In a continuation from yesterdays post talking about what makes a beer a sour and the traditional sour styles, today I'll be covering what actually turns a beer into a sour. There are several bacteria and wild yeast that are generally used to cultivate sours which are colloquially referred to as "bugs" by many brewers. So what are these magical microorganisms that can turn a regular old base into a delectably tart treat? Lets start with the big four fermenters:

Saccharomyces fermenting a beer.

Saccharomyces: The classic brewers yeast, Saccharomyces Cerevisiae, literally known as "sugar fungus" is the same yeast as bakers yeast. Though they share species, this yeast varies wildly in strains, containing both top-fermenting, bottom-fermenting yeasts, and yeasts with varying alcohol tolerance, attenuation, and flavor profiles. Though this yeast isn't a sour yeast, it often plays an important role in sours either doing to the bulk of alcohol production or providing competition for the other bugs to increase stress in the environment which maximizes the production of desired "off-flavors."

A Lactobacillus infection on beer.

Lactobacillus: Cheese and yogurt makers should recognize this bacteria responsible for the production of lactic acid which aids in the production of yogurt and cheese curds. In beer, lactic acid contributes are tart tangy flavor. L. Delbrueckii is the most available strain to brewers and produces only lactic acid and carbon dioxide from the consumption of glucose. Lactobacillus growth is generally retarded by the presence of hop acids in a wort and will die in the presence of too much alcohol or lactic acid. They will generally stop reproducing when the wort reaches a pH of 3.8. Lactobacillus is generally active in the production of flanders sours but fairly limited in lambics.

Pediococcus's ropey and hazy
coating on a beer.

Pediococcus: This bacteria produces the bulk of lactic acid in lambics, though it shares the task with lactobacillus in Flanders style beers. Unlike Lactobacillus, Pediococcus doesn't produce carbon dioxide in the production of lactic acid. This bacteria tends to lose it's viability around 3.4 but is rarely present in high quantities since it's a slow grower compared to other souring bugs. Pedio produces a lot diacetyl (a compound responsible for a slick mouth-feel and buttery taste) which tends to be broken down by yeasts. The ropy hazy slime that this bacteria leaves on the top of beer is usually broken down by Brettanomyces.

A forming pellicle of Acetobacter

Acetobacter: Acetobacter is responsible for the production of vinegar. It works by turning ethanol into acetic acid and can absolutely destroy a beer or wine. This bacteria produces a sharp sour taste and is only palatable in low to moderate quantities in a sour beer. In classic styles, Acetobacter is really only present in significant numbers in the Flanders Red family. Acetobacter requires oxygen to convert alcohol to acetic acid  so as long as its introduction is late in fermentation, it's pretty limited in how much of an effect it will have.

Enterobacter: Enterobacter is a genus of bacteria that produce flavor compounds present in young lambics. Most enterobacter will convert glucose into acetic and lactic acids and produce lots of traditional flaw flavors like dimethyl sulfide (which gives beer a cooked vegetable/corn flavor), smoky moldy, and "baby diaper" flavors. In a successful sour with these present, ther flavors of these byproducts are generally not evident, and most sour producers try to prevent their presence. Enterobacter generally cease their activity around a pH of 4.3. Despite their undesirable qualities, they were historically acceptable in sour beers. Enterobacter presence in beer is important to be aware of since the genus includes E. Coli and Salmonela, fortunately, none of the pathogenic bacteria from this genus can survive all the way to the finished beer. For safety, sours should generally be tasted only after they reach the end of fermentation so the alcohol kills off anything pathogenic. These pathogens aren't the only ones that can't survive finished beer, in fact, no known pathogens can survive in beer, a major reason why beer was consumed instead of water until sanitary practices became common. Nothing in beer can kill you or make you sick, except alcohol of course.

And last, but certainly not least are the strains of Brettanomyces. Though this yeast might have been more appropriately introduced to you along side Saccharomyces, it has several strains used in brewing which each deserve their own entry. Brettanomyces apparently means "British brewing industry fungus," showing it's traditional view as an unwanted presence in classic brewing. There are five recognized species, though generally only three are readily available to brewers.

Brett C. Pellicle

Brettanomyces Anomalus (B. Claussenii): Colloquially referred to as Brett. C. , this strain was originally isolate from British old ales and contribute winelike character. It has a fairly rapid fermentation compared to the other strains and has a fairly large ester profile. Brett C. is often attributed with a pineapple aroma.

Brettanomyces Bruxellensis: Brett B. was isolated from Brussles and is usually used in conjunction with Brett L. and Saccharomyces. Brett B. is responsible for the classic wild characteristics of sour beers, especially lambics, and is generally desired for its "horsy" and cherry characteristics.

A bubbly pellicle on a
100% Brett L. Beer

Brettanomyces Lambicus: Isolated from Belgian Lambics, Brett L. has a cherry-like sour flavor. It also has that distinct "Brett" taste when used in conjunction with Brett B., Saccharomyces, and lactobacillus.


So these are all the major players in sour beer. They may look gross, but hopefully understand the role they play will help people look past their appearances and judge the beers on their delightful aromas and tart refreshing tastes. Though I've only brewed with Brett. B so far, I'm looking forward to doing some experimental batches with each of the bretts and starting my own blended cultures, though this will probably have to wait until after I do some beer with my Jolly Pumpkin starter that I talked about a few posts back.