Fundamentals of Sour Beer Fermentation

Less Common Sour Beer Microbes

Enteric Bacteria – Bacteria from genera such as Enterobacter, Citrobacter, Klebsiella, and Hafnia are part of a rod-shaped gram negative family of prokaryotes called the Enterobacteriaceae.  While these organisms are not intentionally included in most mixed culture fermentations, their ubiquitous presence in the environment gives them access to beer undergoing traditional spontaneous fermentation.  When present, these organisms tend to deplete a beer of free amino acids, leading to sluggish initial fermentation by Saccharomyces.  These bacteria metabolize sugars into a variety of potential byproducts including lactic acid, acetic acid, ethanol, and carbon dioxide.  Additionally, enteric bacteria influence the flavor profile of a beer through the production of compounds such as DMS (canned corn), indole (fecal), and phenols.  These organisms also produce a class of chemicals called biogenic amines.  Some of these compounds, when ingested in high quantity (example: an all night sour beer bender!), can have negative health effects such as allergic type reactions and increased blood pressure.  Of particular note, and interest to me as a pharmacist, is the fact that patients taking monoamine oxidase inhibitors (MAOIs) are at a particularly high risk of developing hypertensive crisis after the consumption of spontaneously fermented beers, as this enzyme system is used by the body to detoxify biogenic amines.

A "Vinegar Mother" is created by the bacteria Acetobacter aceti and is composed mainly of living cells and cellulose.

A “Vinegar Mother” is created by the bacteria Acetobacter aceti and is composed mainly of living cells and cellulose.

Acetobacter – Bacteria from this genus are responsible for the intentional fermentation of vinegar due to their ability to rapidly metabolize ethanol into acetic acid in the presence of oxygen and warm temperatures (above 77° F).  They are generally considered an unwanted bacteria in sour beer fermentations for this same reason.  While acetic acid plays an important role in the flavor profile of some sour beers, its presence at appropriate levels should be achieved through Brettanomyces activity.  If Acetobacter infects a batch of sour beer, it can rapidly produce too much acetic acid.  Such beers should be dumped rather than blended, because the inoculation of Acetobacter into the blend can result in the same rapid acetic acid production that plagued the initial beer.

Oenococcus oeni – This ovoid gram positive bacteria, one of only two species in its genus, is commonly used within the wine industry as an agent of malolactic fermentation.  It is this ability to convert malic acid to lactic acid which has recently brought this organism to the attention of sour brewers.  As a general rule, lactic acid is softer and more palatable than malic acid (the most common acid in many fruits).  This process releases carbon dioxide, and will generally result in the transformation of the majority of malic acid into lactic acid in around 2 weeks.  Due to the creation of diacetyl from any citric acid present, this process gives wines like chardonnay their buttery flavor, and can impart this same compound to a sour beer, where it is generally considered a flaw.  It’s important to note that this organism is not isolated in its ability to perform malolactic fermentation.  Many species of both Lactobacillus and Pediococcus also share this capacity.

Oxidative Yeasts Candida (anamorphic) and Pichia (teleomorphic) yeasts are another group of organisms uncommon to mixed cultures but commonly present in spontaneously fermented beers.  These yeasts are pellicle forming and utilize oxygen to metabolize ethanol into acetic acid.  Unlike Brettanomyces however, they do not produce alcohol.  Rather, these yeast may contribute to the cidery flavors found within lambic beers.  When present, these yeast also have a high propensity for the creation of ethyl acetate from acetic acid.

Other Non-Conventional Brewing Yeasts – Recently, both scientists and brewers have shown interest in screening a variety of families of yeast such as Torulaspora, Kluyveromyces, Cyberlindnera, and Wickerhamomyces for potentially unique aroma and flavor profiles.  While, so far, none of these genera show the versatility towards beer fermentation that Sacc and Brett have proven, there is still much research to be done.


Bugging Out with Mixed Culture Fermentations

Now that we’ve familiarized ourselves with the most common microbes involved in sour beer fermentations, lets that a look at various ways to manage those fermentations.  The goal of this section will be to review an assortment of the most common mixed fermentation schemes and gain an understanding of the control points available for these practices.

Before we dive any further into this topic, I’d like to take a brief sidetrack into the mathematical concept of chaos.  Overly simplified, this concept basically states that if any system is sufficiently complex, then very minor alterations in starting conditions can have very large, and difficult to predict, effects on the ending conditions.  Because all fermentations involve living organisms, they are inherently complex systems.  Sour beer fermentations are notoriously unpredictable because elements like multiple microorganisms, longer aging times, and inconsistency between barrels introduces more variability than exists when brewing “clean” beers.  Due to this reality, we can never predict the exact outcome of any single mixed fermentation with any high degree of reliability.

I bring all of this up not to discourage, but rather to emphasize the importance of keeping the big picture in mind when brewing and aging sour beers.  Many, if not all, of these beers should be brewed with an expectation that blending will occur to create your final product.  Additionally, the unpredictable nature of these fermentations should instill the importance of true “fermentation management” to a sour brewer.  Unpredictable does not mean unmanageable.  If attention is paid to a sour beer through both its initial fermentation and aging process, several control points will become available to help a brewer course correct a sour beer that is not developing as planned.  This attention will also help to identify beers that have developed irreparable off-flavors, so-called “blend killers”, that should be dumped in order to free up both time and resources that would otherwise be wasted upon them.

With these concepts in mind, lets dive into the most common types of mixed culture fermentations available to a sour brewer:


All The Bugs Up Front

Nothing beats this method for sheer simplicity.  Basically you add your house mixed culture or whatever combination of cultures you would like to use at the very beginning of fermentation.  There are both pro’s and con’s to this approach.  The addition of yeast fermenters at the very beginning of fermentation reduces the risk for certain types of off-flavors associated with unwanted bacterial contamination such as isovaleric or butyric acid.  On the other hand, these beers tend to develop their acidity more slowly unless a very robust culture of Lactobacillus is included in the mix.  In addition to other flavor characteristics, both the timetable for acidification and total ending acidity become greater variables.  Most of the lab created mixed cultures have pretty wimpy Lactobacillus or Pediococcus strains included.  For this reason, I think this method tends to benefit from the inclusion of some bottle dregs from lambic or other commercial sour beers.  The addition of dregs will generally increase overall flavor complexity and speed up souring when using this method.  Keep in mind that it may still take 6-24 months for these beers to fully develop.

Control Points:

  • Strain Selections – Allows for dramatic changes in fermentation characteristics.
  • Pitching Rates – Allows the brewer to increase or decrease the speed of acidification, alcohol fermentation, and potentially the intensity and variety of fermentation characteristics.
  • Addition of New Microbes at Any Point During The Beer’s Aging Cycle – Allows the brewer to course correct issues such as diacetyl off-flavor, lackluster acid development, or lack of ester or phenol development.
"Bug Farm" by East Coast Yeast is an example of a complete mixed culture that can be used to produce excellent sour beer.

“Bug Farm” by East Coast Yeast is an example of a complete mixed culture that can be used to produce excellent sour beers.

With the right cultures, this method can make some of the world’s best and most complex sour beers.  Because the acidity tends to develop more slowly with this method, Brettanomyces and Saccharomyces tend to produce a wider range and intensity of phenolic compounds, increasing the overall “funkiness” of these beers.  This is the most old-school method around for producing sour beer and when a lackluster blend of microbes is employed this method can easily yield lackluster beers with little to no acidity.  If this happens, it’s best to pitch a fresh culture of Pediococcus or a nice portion of dregs from beers known to contain Pedio.

Keys to Success:

  • Use of a robust and tested house culture or the assembly of a new blend of microbes with at least one yeast fermenter with an adequate cell count to carry out a timely initial fermentation.
  • Build variation into your aging stock by altering the microbes used.
  • Monitor flavor development while avoiding excess oxygen exposure.
  • Have patience. These beers often take 1 to 2 years to fully mature.


Staggered Microbe Additions

In comparison to the method we just discussed, the staggered addition of microbes to a sour beer fermentation is a newer practice that offers a greater degree of control over the final product.  The variations within this method all share the concept that if certain microbes are added to a beer at different time points, then the conditions under which they are actively fermenting the beer can be optimized to achieve a variety of results.  Variables such as temperature, oxygenation, amount of fermentable sugars, and microbe pitch rate can be individually controlled for each step of the process.  In its most extreme form, this method can be used to fast-sour a beer using a robust pitch of Lactobacillus, which is then followed up with Sacc, Brett, or both in order to turn around certain sour beer styles in as little as a few weeks.  For more information on fast souring including expanded details on avoiding off-flavors during Lactobacillus fermentations, click here.  Despite the popularity of fast souring, the concept of allowing Lactobacillus to ferment in a beer before the addition of yeast fermenters is not a “one trick pony” useful only for brewing low-complexity Berliner Weisses and Goses.  This practice can be used to establish a healthy colony of Lactobacillus early in the beer’s fermentation process, even if a high degree of acidity isn’t desired or achieved until later during a beer’s aging.  Let’s take a look at a few variations on these practices and review their potential benefits:

Goodbelly (L. plantarum) and a large pitch of Saccharomyces which was used to brew 40 gallons of sour red blending stock.

Goodbelly (L. plantarum) and a large pitch of Saccharomyces which was used to brew 36 gallons of sour red blending stock.

Lactobacillus –> Saccharomyces –> Brettanomyces

This method, sometimes referred to as my “Three Step Fermentation” is the most common process that I personally use to create sour beers for my blending program.  In my opinion, this method can be used to create sour beers with a moderate to high degree of complexity without having to deal with some of the drawbacks of using Pediococcus such as diacetyl production or the possibility of ropiness due to EPS formation.  These beers also tend to be ready for blending or consumption on a somewhat accelerated timetable, generally in the 3 to 12 month range.

Control Points:

  • Strain Selection – Individual strain choices can be made for each of the microbes to be used. This allows for greater variability in batches (beneficial in blending) and also allows a brewer to identify the impact that changing a single strain in the blend can have.
  • Pitch Rate – The results of a mixed culture fermentation can be altered significantly by pitching rate.
    • Lactobacillus pitch rates will affect the speed and, potentially, the degree of acidification.
    • Saccharomyces pitch rates can be optimized to reduce off flavors such as diacetyl, DMS, or acetaldehyde.
    • Brettanomyces pitch rates, in my experience, affect the overall presentation of Brett derived flavors.  Low pitch rates into attenuated wort seem to produce more of the classic “lambic” funky barnyard flavors, while higher pitch rates into less attenuated worts  seem to result in flavors more commonly found in farmhouse ales and Brett saisons.
  • Oxygenation & Oxygen Avoidance – These control points allow for fermentation health and avoidance of unwanted acetic acid production by Brettanomyces. Oxygen levels can be reduced for Lactobacillus fermentation, increased for healthy Saccharomyces growth and fermentation, and kept low for Brettanomyces aging.
  • Temperature – The ability to warm a wort will aid the growth of many strains of Lactobacillus, while cooling and maintaining a stable temperature through Saccharomyces fermentation will help to drive attenuation and reduce off-flavors such as acetone and fusel alcohols. Brettanomyces tends to produce a balance of phenols and esters when held between 70 to 80° F.  At temperatures cooler than this, Brett characteristics seem to develop more slowly but still maintain a normal balance between esters and phenols.  At temperatures above 80° F, many Brettanomyces strains start to behave more like saison yeasts, producing a greater degree of spicy phenols.
  • pH – The measurement of pH allows a brewer to control several aspects of the fermentation. The most obvious of these is the sourness of the beer.  Less obvious are characteristics such as attenuation, ester, and phenol development.  At pH levels below 3.5 the final beer is likely to be assertively sour, but it is also less likely to enjoy a healthy fermentation by Saccharomyces.  This can be combated by increasing Sacc pitch rate, or by relying on Brettanomyces to achieve full attenuation and clean up any off-flavors from Saccharomyces.  Additionally, beers that are quickly soured to pH levels below 3.5 tend to develop less Brettanomyces funk (phenolic compounds) than beers which develop their acidity later during the aging process.
A young sour beer receiving a pitch of Brettanomyces "Dirty Dozen" Blend (Step 3).

A young sour beer receiving a pitch of Brettanomyces “Dirty Dozen” Blend (Step 3).

As a beginning sour brewer, I began experimenting with this three step fermentation process in 2010, and I’ve continued to successfully use it to this day.  My scientific side has always been drawn to this method because of the control it gives you over strain choice and pitch rates per strain.  I think that this level of control helps to remove some of the larger variables from the sour brewing process and offers more predictability than methods which rely on unknown or un-quantified blends of microorganisms.  On the other hand, my artistic side still appreciates the complex beers that this process can feed into my blending program.  On a typical sour brew day, I may produce up to 40 gallons of wort to be split into numerous fermentors.  Each fermentor will then receive a different assortment of strains for each step in the fermentation.  Doing so creates the flavor and aroma variety that I think really helps to make my blending program a success.

Keys to success:

  • Monitor aspects of your fermentation such as gravity, pH, and flavor/aroma at each step in the process.
  • Keep detailed notes on strain selections, pitch rates, temperatures, lengths of fermentation, and wort/beer measurements. Doing so will help you to identify trends or recreate beers that turned out particularly well.
  • For intensely sour “acid beers”, use large pitches of Lactobacillus, select aggressive strains such as Omega Yeast’s L. plantarum or Gigayeast’s L. delbrueckii, and allow these strains to survive throughout the Saccharomyces & Brettanomyces portions of fermentation (no fast-sour second boil). Keep in mind that both of these strains can create worts so acidic, that even Brettanomyces will have difficulty reaching full attenuation.
  • For beers with a balance between funky phenols and fruity esters, I like to select slower acidifying strains of Lactobacillus such as White Lab’s L. delbrueckii. These are followed up by fruity Sacc strains such as those used to produce English Ales.  I then age these beers on classic strains of B. bruxellensis or anomalus.
  • For beers that really maximize the farmyard funk, I again select slower acidifying Lacto strains, but I then follow them up with spicy Saccharomyces strains such as those found in Saisons or Farmhouse Ale Blends. I age these beers with relatively funky Brett strains such as White Lab’s B. bruxellensis “lambicus” or East Coast Yeast’s “Dirty Dozen” Brett Blend.

(The Milk The Funk Wiki maintains an excellent list for selecting your Brettanomyces strains or blends)

Other Staggered Microbe Fermentations:

The staggered addition of microbes can naturally be modified to utilize a greater or fewer variety of microorganisms than the Lacto-Sacc-Brett approach that I’ve just described.  Here are a few variations that I feel are worth mentioning:

Lactobacillus –> Brettanomyces (or) Brettanomyces –> Pediococcus

The combination of a single genus of lactic acid bacteria with Brettanomyces is a method which has been used effectively to produce sour beers.  If choosing a Lactobacillus first approach, the degree of initial acidification will have effects upon the final presentation of “funk” as described earlier.  The Pediococcus last approach has the benefit of allowing Brettanomyces to be more expressive, but also carries the potential for longer maturation times.

Lactobacillus –> Saccharomyces –> Brettanomyces +- Pediococcus +- Bottle Dregs

One method that can be used to establish a minimum level of acidity in a sour beer while also allowing it to enjoy the flavor possibilities afforded by a robust mixed culture is to use a three step fermentation process in which Pediococcus and/or bottle dregs are added along with Brettanomyces in the last step.


Spontaneous Fermentation

Spontaneous fermentation, a term used by brewers to describe the inoculation of wort with an unknown assortment of microbes from the air, is the classic method used to produce lambic beers.  As an overview: In spontaneous fermentation a wort is pumped hot into a large, shallow, open air cooling vessel called a koelschip (pronounced and sometimes spelled “coolship”) and allowed to cool overnight to near room temperature.  During this time the wort becomes exposed to a variety of microbes floating in the open air.  The following day, the wort is pumped into a homogenization tank and then loaded into wooden vessels (oak or chestnut barrels, or larger vessels such as foudres) to undergo both primary fermentation and aging.

A view of the koelschip room at Allagash Brewing

A view of the koelschip room at Allagash Brewing

Because this method takes strain selection and pitch rates out of our control it is easily the least predictable of the three major options presented here.  In classic lambic beers, an early bloom of bacteria that would be considered food spoilers such as Enterobacter and Klebsiella are soon replaced with a vigorous fermentation by Saccharomyces.  The modest pH lowering of Saccharomyces and its production of alcohol prevent further activity by bacteria that could potentially cause illness.  After Saccharomyces reaches the limits of its ability to attenuate the beer, both Brettanomyces and Pediococcus take over and begin a long and slow period of conditioning during which dextrins are further consumed, lactic acid is produced, and a wide variety of flavors and aromas emerge.

The idea that the flavors of traditional lambic are impossible to achieve through spontaneous fermentation in areas outside of the Senne Valley in Belgium is largely a myth.  Additionally, I think that it is a myth that such flavors require spontaneous fermentation to develop at all.  So where does spontaneous fermentation (outside of lambic production) fit within a modern sour brewing culture?  I think the answer to this is two-fold.  The first benefit of spontaneous fermentation is a cultural connection to a more ancient brewing process, one which gave us our first sour beers.  The second benefit to this method is the ability to capture and utilize strains of microbes that are not available from a lab, they are truly local and will produce unique beers.

Due to the very nature of spontaneous fermentation, there isn’t much to be done to manage these fermentations aside from the control of environmental factors such as temperature and oxygen exposure.  While it is not the purview of this article, the majority of steps used to control spontaneous fermentation are employed on the brew day and through recipe design.  We will cover all of these in detail in the upcoming third article of this series.

The use of small test batches to collect wild cultures is one method that I think can help to give us the benefits of wild microbes without all of the major drawbacks (when things go wrong, spontaneous beers can be riddled with off-flavors, unwanted microbes like mold, or can be downright undrinkable).  Using this method, small batches of wort are allowed to spontaneously ferment.  These batches are then measured for attenuation and pH drop and then tasted.  The ones that turn out well can be grown up as mixed cultures to be used for larger batches or employed in staggered additions in the same way bottle dregs can be used.


Methods for Measuring and Managing Sour Beer Fermentations

Now that we have an understanding of the microorganisms involved in sour beer production and a variety of methods used to introduce them into our worts, this final portion of the article will discuss measurements and adjustments that can be made to a fermenting or aging sour beer:


Measuring Gravity

One of the most universal assessments of a fermentation’s progress is the measurement of sugars remaining in the beer.  This is generally achieved in one of two ways: by measuring the optical rotation of light passing through a sample or by measuring the density of a sample.  The first method is achieved through the use of a refractometer.  Refractometers have the benefit of being quick to use and requiring very low volumes of sample wort.  Unfortunately, their readings become inaccurate once lactic acid or alcohol are present in the solution, so these tools are only recommended for use during the brew day.


A table saison with Brettanomyces that has reached near 100% attenuation.

The measurement of wort or beer density is typically achieved via a classic (glass) or digital hydrometer.  Glass hydrometers are suspended in a testing cylinder filled with wort or beer, whereas the much more expensive digital hydrometers draw up a small sample for analysis via built-in pipette.  The measurement these devices give is accurate regardless of the beer’s stage of fermentation.  It is important to remember that all forms of gravity measurement are based on a calibration temperature for your device.  If the sample is colder or warmer than your calibration temperature, a conversion factor must be applied to get an accurate measurement.

It is worthwhile to mention that the transformation of sugar into lactic acid does not have a significant effect on the density of a wort.  A statistically significant change in the density of a beer only occurs when carbon dioxide is evolved from solution.  Therefore, a change in gravity has the same correlation to alcohol production for sour beers as it does for non-sour beers.  The only exception to this would be the conversion of ethanol to acetic acid, which does not evolve CO2.  Beers with a higher proportion of acetic acid may, in turn, have slightly lower true ABV measurements than the calculations using change in gravity would indicate.  In these cases, ABV can be measured in a lab using fractional distillation.

Gravity measurements give us a window into how actively the microbes in a sour beer are metabolizing.  The calculation of attenuation allows us to decide whether a fermentation is proceeding and allows us to estimate about how much further it may go.  When it comes time to blend or package a beer, consistency in gravity measurements over a period of 6 to 8 weeks tends to be an indication that the beer has stabilized, and the risk of over-carbonation in the bottle is minimized.


Adjusting Gravity

While the gravity of a wort is typically established on brew day, there are a few situations where you may make adjustments to a fermenting or aging beer’s gravity:

  • A beer is near 100% attenuation, but lacks fermentation character – While Brett can continue to create flavor changes in a fully attenuated beer, it often does so faster if it has a little sugar to feed its growth. In these cases, you may choose to add a fresh culture of Brett while also adding more sugar to the beer in the form of fresh wort, a sugar source like honey or table sugar, or via an ingredient addition like fruit.
  • A beer has a stable final gravity that seems too high – In some cases, the strains of microbes we have chosen may not attenuate the beer enough for the style / our goals. This can occur because certain strains of Lacto, Brett, or Pedio may lack the necessary enzymes to further attenuate the beer.  In these cases, it is best to add a fresh culture to the beer, choosing a strain that is known to be able to fully attenuate a dextrinous beer.
  • A beer’s ABV is higher or lower than desired – While most craft brewers focus on flavor and not alcohol measurements, in some cases legal requirements or the goals of the brewer may necessitate an adjustment to the final ABV of a beer. These adjustments can be made by adding sugar sources, adding water, or blending with a different ABV beer.


Measuring pH and Titratable Acidity

pH and titratable acidity (TA) are two measurements that describe different aspects of the acidity within a sour beer.  The sour flavors that we taste in these beers are the result of a variety of organic acids.  The most common of these are lactic and acetic acid, but, depending upon the fermentation and any specialty ingredients, malic, tartaric, citric, and other less common acids may also be present.  All acids are designated as such due to their ability to release one or more free protons (H+ Ions) into an aqueous solution.  All of the acids in sour beers are considered to be weak acids, meaning that some portion of the total molecules present will have released their proton(s) into solution, while the remainder will have not.   This equilibrium concept lies at the heart of the difference between pH and titratable acidity.

I personally use the Milwaukee MW102 pH meter with automatic temperature compensation. I find this to be an accurate, reliable, and easy to use meter.

I personally use the Milwaukee MW102 pH meter with automatic temperature compensation. I find this to be an accurate, reliable, and easy to use meter.

pH is a measurement of the number of free protons in solution.  This number, while constant for any given beer, is decided both by the quantity of acid molecules present AND by the quantity of a wide variety of buffering compounds.  Buffering compounds in beer can include mineral ions from brewing water as well as compounds released from malt, hops, the microbes fermenting the beer, and any special ingredients.  Therefore, pH is an indirect measurement of how many acidic molecules are in a beer, and as such, this measurement does not correlate exactly to how sour a beer will taste.  However, pH is an important measurement because it describes the biologic activity of acids in the beer.  Microorganisms don’t care about acid molecules that haven’t released a proton, they only care about the protons themselves.  This is due to the fact that the free protons have an effect on a wide variety of chemical reactions, ranging from the activity of enzymes to the ability for a cell to metabolize, reproduce, or survive.

On the other hand, Titratable Acidity (TA), is a measurement that determines the actual quantity of acid molecules in a beer (in any solution).  This measurement uses a chemical called a strong base to forcibly remove all of the protons from all of the acid molecules in a sample of beer.  By knowing how much strong base we had to add to the beer to achieve this, we can know how much acid was in the beer to begin with.  Unlike pH, TA correlates strongly to how much sourness we taste in a beer.

When it comes to sour brewing, an accurate pH reading can only be achieved with an electronic pH meter.  Those pH test strips used to measure mash pH (even the really good ones) aren’t accurate enough to tell us anything useful about the pH of our sour beers during fermentation.  Like gravity measurements, the pH of our beer is also affected by the temperature of that beer, so choosing a pH meter with a secondary temperature probe and automatic temperature adjustment is beneficial.  Otherwise, click here for a calculator to do this correction.

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