Yeasts and Bacteria Used in Sour Beer Production

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If you are a beer lover, you have probably heard some of the long and complicated names of the yeasts and bacteria used in sour beers and their production.

They are hard to spell, even harder to pronounce, and it can be confusing to tell them apart.

Many people often refer to these sour beer yeasts and bacteria without fully knowing what they are, so it is worth taking some time to establish a clear record of each one.

To better understand these terms, we asked Michael Tonsmeire (known for his blog The Mad Fermentationist) for an overview of each.

Understanding wild fermentation

To start, it is key to understand the difference between wild fermentation and single-culture fermentation.

Sour beers are beers subjected to wild fermentations or mixed fermentations; this means that various yeasts and bacteria will work together to create a beer with a “funky” character—a term widely used in this style of beer that can be defined as a set of aromas and flavors considered off-flavors in “traditional” beers, yet create a unique profile outside the spectrum produced by conventional brewing yeast.

In the wild ale category, beers can be controlled—meaning brewers have selected exactly which yeast and bacteria to inoculate—or open-fermented, which is when fermenters are left fully or partially open to allow wild yeasts and bacteria from the environment to enter.

Yeasts and bacteria used in sour beers

The yeast responsible for fermenting all “traditional” beers is called Saccharomyces, while Brettanomyces, Lactobacillus, and Pediococcus are something like the “Three Stooges” of sour beer production.

Together they develop funky, unpredictable beers, but there is also a variety of bacteria that add something extra to sour beers.

1. Saccharomyces

Saccharomyces, commonly known as brewer’s yeast, is the only genus of yeast responsible for fermenting all non-sour beer styles, but it is also used in sour beer production.

Brewer’s yeast is responsible for most of the gravity reduction and alcohol production in nearly all sour beers.

No strain of Saccharomyces yeast will ruin a sour beer, but some strains work better than others for certain sour beer types.

Saccharomyces is a fast-working yeast, highly tolerant to bitterness (IBU), acting as a base yeast for sour beer production.

In general, brewer’s yeast protects the wort and sets the stage for a traditional, slow-developing wild fermentation.

2. Brettanomyces

Brettanomyces, often referred to simply as “Brett,” is a genus of yeast, not bacteria, as too many brewers tend to believe.

It is the primary wild yeast used in sour beer production. The specific flavors, aromas, esters, and phenols produced in the beer will depend on the brett strain and species, ranging from pineapple and hay to horse blanket and acrid smoke.

The character of the beer is also influenced by the acids and alcohols available for combination into esters during fermentation.

Tonsmeire notes:

Brett doesn’t turn the beers you make into sours; it turns a sour beer into a delicious beer.

Brett also doesn’t contribute much to the acidity of sour beers, since acid production is the responsibility of the bacteria.

The only exception is when large amounts of oxygen are available, which causes Brett to produce acetic acid, creating a vinegar-like acidity.

Brettanomyces fulfills the same function as Saccharomyces—it ferments beer. However, Brett works more slowly, which means a beer that might have fermented in just a few weeks with brewer’s yeast could take months or years to exhibit its full character when Brett is introduced.

The great beer writer Michael Jackson once compared Saccharomyces to a dog and Brettanomyces to a cat:

Saccharomyces is trainable, generally predictable, and will come back to you when called. Brettanomyces, on the other hand, will run away whenever it feels like it and will probably scratch you when you bring it back.

In other words, Brett can be difficult to predict and manage, but if you can respect the yeast for what it is and what it can do, you will be rewarded in the end.

Brett works well in tandem with the other microbes listed here, or on its own terms when enough is inoculated into the wort, as in 100% Brett beers.

3. Lactobacillus

Lactobacillus, also known simply as “lacto,” is a bacterium, not a yeast. It acts similarly to yeast in that it consumes sugars from the wort, but instead of converting them into alcohol, it converts them into lactic acid.

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Lactic acid lowers the pH of the liquid very quickly, sometimes in as little as 24–48 hours, and gives the beer a sour but clean flavor.

Lacto is also used in many other food fermentations, such as kimchi and yogurt, creating a relatively clean flavor, since it produces little more than lactic acid.

It is most often the primary acidifying microbe in the German styles Berliner Weisse and Gose.

4. Pediococcus

Pediococcus, alias “pedio,” is also a bacterium, not a yeast. Pedio is the other common lactic acid bacterium used in sour beers, as well as in other culinary roles such as acidifying sauerkraut and traditional dried sausages.

Pedio, unlike lacto, requires a long aging period before it initiates a dramatic drop in the beer’s pH, which works as an advantage because it gives time for the primary yeast strain to complete its fermentation before a substantial pH drop occurs.

The difficulty with pedio acidification is that most strains produce diacetyl concentrations above the flavor threshold.

Unlike brewer’s yeast, pedio does not reduce diacetyl by converting it into less flavorful byproducts. Instead, it leaves behind a buttery and/or popcorn flavor.

A good solution is to include Brett in beers that are pitched with pedio in order to eliminate the diacetyl. This will take time, so be patient if your beer tastes like movie popcorn when it is still young.

Another key difference between lacto and pedio is the type of acidity they produce.

While lacto produces a clean acidity, pedio can produce other funky aromas and flavors that result in a rougher acidity. However, pedio gives Brett more fuel to work with, which is why the two are often used together.

This bacterium is responsible for sour beers such as Lambic and Flanders Red.

5. Lactic acid

Lactic acid is the primary acid in sour beers; along with carbonic acid from dissolved carbon dioxide, it is produced by lactic acid bacteria (Pediococcus and Lactobacillus).

It is subtle and sharp at low levels, but can cause a pronounced lip-puckering sensation at higher concentrations.

It is also the same acid found in yogurt, buttermilk, and other sour dairy products.

Lactic acid can also be transformed into fruity esters by the action of Brett, described as a character “mostly expressed on the tongue, rather than the back of the throat like acetic acid.”

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6. Acetobacter

Acetobacter is a less common bacterium, generally kept to a minimum in the fermentation of most sour beers, as it consumes ethanol to produce harshly flavored acetic acid.

These bacteria require a constant supply of oxygen to perform the oxidative fermentation that converts ethanol into acetic acid.

Acetobacter is also airborne, so even if you do not inoculate it, it can easily attach to your equipment if it is left empty for several days or weeks.

Adding or allowing the production of acetic acid before the boil is not recommended due to its high volatility and urine-like odor.

Acetobacter will not do much during early fermentation because carbon dioxide production prevents oxygen from coming into contact with the beer.

However, once primary fermentation is complete, the lack of pressure will allow oxygen to begin seeping in.

If you want a low level of acetic acid—which is an important flavor component in Flanders Red and Lambic—add a small amount of unpasteurized vinegar (which contains live acetobacter) when inoculating the other microbes.

Frequently asked questions

What is the difference between kettle souring and barrel fermentation?

Kettle souring acidifies in a controlled way inside the brew kettle using Lactobacillus for a clean, quick sourness; it is then boiled to kill the bacteria and stop acidification. Barrel souring (as in Lambic or Flanders Red) ferments and matures for months or years in barrels—sometimes with open-air inoculation—using a mix of Saccharomyces, Brettanomyces, Lactobacillus, and Pediococcus, resulting in a more complex acidity and a pronounced funky profile.

Why does Pediococcus take longer to lower pH than Lactobacillus?

Lactobacillus is a fast acidifier that acts within the first 24–48 hours. Pediococcus, by contrast, requires a longer aging period before triggering a dramatic pH drop and often needs Brettanomyces present to metabolize the diacetyl it produces, preventing unwanted buttery or popcorn off-flavors.

Which styles are specifically characterized by acetic acid?

Acetic acid (vinegar flavor) is an important component—though usually at low levels—in classic sour styles such as Flanders Red Ale and Lambic (or Gueuze). These beers are often aged with some oxygen exposure, allowing Acetobacter (and sometimes Brett in the presence of oxygen) to convert ethanol into acetic acid.