Wort Souring

Wort Souring is the process of Mixed Fermentation where lactic acid bacteria, usually Lactobacillus (although there are also Pediococcus cultures available that work well for this technique), is given a "head start" as it's pitched before the yeast so that it will be able to produce significant amounts of lactic acid before the Saccharomyces completes the main fermentation. This is a broadly defined technique and there are many variations on souring wort. These include souring in the primary fermenter, souring in a secondary vessel, or souring in the boil kettle itself (kettle souring). There are also various methods of inoculating the wort with Lactobacillus. Finally, the brewer has the option of pasteurizing the wort by heating it to kill the Lactobacillus before adding the yeast for the main fermentation.

Although this process generally does not include the use of Brettanomyces, some creative brewers have applied wort souring techniques to longer aged Mixed Fermentation beers and barrel aged beers that do contain Brettanomyces.

Many brewers prefer this process over Sour Mashing because it can be easier to control, and when implemented properly, it produces a clean sour beer in a short amount of time. However, the environment created provides an ideal situation for contaminations and great care should be taken to prevent contamination of both yeast and spoilage microbes (see Contamination Concerns below).

The possibility of pasteurizing the soured wort also makes this a good method for making sour beers with a lot of residual malt sweetness (e.g. sour barley wines), and also makes it an attractive process for brewers who are concerned about infection issues in their cold side equipment (equipment that is used post-boil) . When souring wort, some brewers first lower the pH of the wort to 4.0-4.3 before pitching Lactobacillus. This sometimes helps the head retention of the beer and it sometimes helps to protect the wort from contaminating microorganisms. For more information preventing the loss of head retention in sour beers, see the Lactobacillus page section on Foam Degradation.

Generally, Pediococcus is not used with this method (Pediococcus is generally used in long aged Mixed Fermentation sours with Brettanomyces), however Bootleg Biology has released a blend of Pediococcus strains that are reportedly good for souring wort.

Important note regarding aluminum pots: souring in an aluminum vessel may strip the aluminum of the protective oxide layer. The oxide layer is only stable at a pH of 4.5 - 8.5. Therefore, kettle souring in an aluminum pot is generally not recommended . Stainless steel (304 and 316) vessels are safe for holding acidified wort or beer, as well as PET and HDPE plastics .

"Mixed culture fermentation for sour beers produces one thing, and (kettle souring) produces another thing. If you’re going to make a malty red ale that is kettle soured, don’t call it a Flanders Red. Honor the tradition." - Sean Burke, Kettle Souring Presentation, CBC 2015.

Processes

Souring in the Boiler (Kettle Sour)

Also known as kettle souring, souring in the boil kettle is a simple process that is often used if the brewer wants to subsequently heat pasteurize the wort. Pasteurizing the wort has the advantage of allowing the brewer to rack the pasteurized wort into a fermenter and pitch brewer's yeast without fear of an ongoing Lactobacillus infection in their post-boil equipment.

The brewing process is the same for any all-grain batch up until the first wort and sparge runnings are collected in the boil kettle. The temperatures that a typical mash out/sparge are not enough to completely pasteurize the wort . Therefore, the best approach is to heat the wort for a short boil (1-2 minutes) in order to kill a greater degree (2-3 logs more) of thermotolerant microbes . Once all of the wort is collected in the boil kettle (and preferably brought to a boil), the wort is chilled to around 80-115°F (37-46°C), depending on the Lactobacillus culture that is being used (see the Lactobacillus culture charts). Once chilled to the appropriate temperature, the wort in the kettle is inoculated with a culture of Lactobacillus. Hops should not be added at any point before inoculating the wort with a culture of Lactobacillus as most species of Lactobacillus will be inhibited by the presence of even very small amounts hops (1-2 IBU or even just hop material from dry hopping). When using a pure culture of Lactobacillus, it is generally a good idea to create a 500 mL starter for ~5-6 gallons of wort.

There are various ways of inoculating the wort. A reliable method is pitching a pure culture of Lactobacillus or a blend of Lactobacillus cultures. Alternatively, a handful of unmilled malted barley can be added to the kettle for inoculation instead of a pure culture since the husks of grain carry many microorganisms. If unmilled grain is added it is thought that filling the head space of the kettle with CO² will help decrease off-flavors such as "footiness" from Isovaleric Acid which are produced by aerobic microbes that are naturally present on the grain . Keeping the temperature between 109-115°F (42.8-46°C) will encourage the Lactobacillus resident on the grain and will discourage other bacteria. Temperature consistency is critical during this process . Lowering the pH of the wort to under 4.5 (ideally 4.0 - 4.3) will also discourage other bacteria from thriving in the wort during the incubation period. This will also help with head retention . Souring with grains should occur within 1 or 2 days if done correctly . Do not consume wort that has been soured with grains until after it has been fully fermented by yeast because there is a chance that food poisoning pathogens will be present until ethanol is produced. See Blue Owl Brewing's grain inoculation methodology and data for more information on inoculating with grain. Consider Alternative Bacteria Sources for more reliable approaches to using "wild" Lactobacillus, or Lactobacillus from sources other than yeast labs.

If a pure culture of Lactobacillus bacteria is used it has historically been recommended to fill the head space of the fermenter with CO² gas (oxygen does not cause Lactobacillus to produce butyric acid or isovaleric acid, but some brewers have reported that this will help reduce sulfur in the finished beer). However, many homebrewers and some professional brewers have reported having success without purging with CO² (~20:30 mins in), and a Brulosophy blind triangle test was unable to find a significant difference between a kettle sour that was CO² purged and one that was soured in an open vessel. Using some CO² to keep positive pressure in the kettle could help prevent contaminants that create butyric acid and other off-flavors from entering the kettle due to negative pressure, and is often the approach that commercial brewers take . The kettle should be held at the desired temperature for 24-72 hours (in some cases longer, but no longer than 5 days). Depending on the strain of Lactobacillus, and the desired sour level, the time of incubation is ultimately a variable that is up to the brewer (see the Lactobacillus page for suggested temperatures and times for specific strains). The kettle lid should be firmly in place and optionally sealed with plastic wrap so that other microorganisms do not get in. Potential for the formation of Butyric Acid and Isovaleric Acid when using only a pure culture is extremely slight assuming that contamination does not occur. Temperature shifts during kettle souring are not a concern as long as the temperature does not get too high or low for the specific Lactobacillus culture. Some species, such as L. plantarum, create acidity at room temperature so some brewers will pitch this strain at around 90-100°F and let the temperature fall to room temperature during souring. Other species might not perform as well at colder temperatures so maintaining a fairly consistent hot temperature is desirable. If the temperature is allowed to fall, take precautions on not allowing any dust to get sucked into the fermenter since temperature decreases will create a vacuum inside the fermenter (flushing with CO² is a good way to prevent a vacuum).

In order to address the challenges of creating a sealed environment in a commercial boil kettle, which is often left open to the environment through the boil stack, some commercial brewers find it beneficial to rack the wort from the boil kettle to a mashtun, brite tank, fermenter (often dedicated), or another vessel that seals and purges well to sour in. The mashtun or other vessel is first cleaned, sanitized, and often purged with CO². Once the wort is in the vessel, and at the correct temperature for the given culture of Lactobacillus, the Lactobacillus is pitched into the vessel and the vessel is sealed off. If the vessel is sealed and air tight temperature shifts from cooling won't suck air into the vessel and a potentially more sanitary souring fermentation will occur. Using a fermenter or brite tank (if there are multiple) has the advantage of not occupying a bottleneck vessel such as the mashtun or boil kettle, but has more risk if the fermenter is not dedicated to kettle souring. Dedicating removable soft parts like gaskets and hoses will minimize the risk of infection originating from the fermenter or brite tank if it is not dedicated to souring .

Once the level of acidity is reached (this can be tested with a reliable pH meter, or in the case of using a pure culture can safely be taste tested), the wort is brought to a boil. The wort may be boiled normally in the case of any style of beer that requires a longer boiling process, or it may be boiled for no more than a minute or two in the case of making a Berliner Weissbier. Technically speaking, the wort doesn't need to be boiled at all (this is called Raw Ale). Heat pasteurization at 180°F (82°C) for 15 minutes will kill even the most heat tolerant Lactobacillus species . Note that boiling a soured wort that has a pH lower than 5.0 will decrease the amount of protein coagulation, potentially resulting in a hazier beer or more sediment in the fermenter . Haze and sediment can be avoided by choosing to boil long enough to hit a hot break before souring the wort (see example MTF post from Chris Prechel). Raw ales are also fairly hazy in nature due to the lack of hot break from boiling. See Haze below for more information.

Deciding whether or not to boil the wort can also depend on whether or not there was a considerable amount of alcohol produced as commonly happens when the wort is contaminated with yeast. Pure cultures of Lactobacillus do not show typical signs of fermentation that we are used to seeing with yeast fermentations, such as forming a krausen, producing a lot of CO², or fermenting wort more than ~1.005 gravity points (see 100% Lactobacillus Fermentation). See Dealing With Yeast Contamination below.

Once the soured wort is boiled or heat pasteurized, it can be safely added to the primary fermenting vessel without worries of future infections. The wort is aerated as normal, and brewer's yeast, or Brettanomyces yeast, is then pitched into the wort as normal (usually brewer's yeast is used if infection of cold side equipment is a concern). At a pH of 3.4 or lower, the acidity of the wort can reportedly effect the fermentation of some strains of brewer's yeast . For example, a published study showed that growth of US-05 was 82% at a pH of 3.51, and 53% at a pH of 3.17. Fermentation was delayed by 2-4 days (the lower the pH, the longer the start of fermentation was delayed) . It is recommended to pitch a healthy starter of yeast, possibly with a higher cell count than normal. When using dry yeast, re-hydrating as per the manufacturer's recommendations, using a yeast nutrient like Go-Ferm is recommended. Many yeast strains have been successfully used by MTF members to ferment pre-acidified wort: US05, S04, WY1098/WLP007/OYL-006, Belle Saison, Sacch Trois, Bret brux, B. clausenii, B. custersianus, Bret Drie (BSI), WY3711, and WY3726 to name a few (see reference) . Michael Tonsmeire has shown results that suggest that English yeast strains might attenuate slightly more and give better flavor results than other strains; Richard Preiss from Escarpment Labs expressed similar observations . Brewers who are having difficulty fully fermenting pre-acidified wort can try growing their yeast in the soured wort (pasteurize the soured wort first if needed) with yeast nutrients (Fermaid K + DAP, for example). This assumes that the wort still has a lot of sugar left over after souring (if not, DME can be added). See Acid Shock Starters for more information on how to acclimate yeast to a highly acidic environment to improve fermentation.

Souring in the Primary Fermenter

Wort can be soured in the primary fermenter before adding other yeasts. This is generally a good approach for brewers who aren't concerned with pasteurization and infections of their cold side equipment. This also has the benefit of possibly producing a more complex sour beer overall or at least a sour beer that will evolve over time. It has the advantage over a more traditional Mixed Fermentation in that Lactobacillus is used to guarantee at least a certain level of sourness. This is also a good process to use for making a Berliner Weissbier.

The process is very similar to the kettle souring technique with the exception that the wort is never pasteurized after it is soured. This all-grain brewing process is the same for any all-grain brewing processexcept that, after the boil, the beer is only chilled to the recommended temperature for the Lactobacillus strain that the brewer is going to use. Using grain husks for souring with this method is inadviseable since the grain will stay in the fermenter during primary fermentation and unwanted microbes on the grain husks would potentially have a longer exposure to the wort. Instead, the brewer should use a pure strain of Lactobacillus. As a result of not using grains to sour the wort there remains less concern of developing Butyric Acid or Isovaleric Acid with this method. Even still, lowering the pH of the wort to under 4.5 (ideally 4.0 - 4.3) will also discourage contaminating bacteria from thriving in the wort during the incubation period. This will also help with head retention . There is also the option of using a sour yeast cake from another sour beer as the bacteria inoculation.

Once cooled to the desired temperature, usually around 90-115°F or 32.2-46°C (see the Lactobacillus culture charts for desirable temperature ranges for different Lactobacillus cultures), the wort is racked to the primary fermenting vessel. Note that the wort should contain a low amount of IBUs when using this process since IBUs can inhibit many (but not all) species of Lactobacillus. Eliminating hops can be a good approach to getting more acidity, but if hops are required, then using less than 6 IBUs is a good guideline in general (see the Lactobacillus culture charts for tips on hop tolerance for different Lactobacillus cultures). To achieve a low IBU wort consider mash hopping. Mash hopping has been reported to reduce IBUs by ~70% . It is generally a good idea to create a 500 mL starter beforehand for ~5-6 gallons of wort (see Lactobacillus starters). Once the wort is racked to the primary fermenting vessel the Lactobacillus culture is added directly to the fermenter. No other yeasts are added at this time. The Lactobacillus bacteria is allowed to incubate by itself in the wort for 1-3 days with the target temperature maintained throughout the incubation period (some Lactobacillus species/strains may continue to produce acidity under lower temperatures, for example L. plantarum). Small temperature fluctuations should not adversely affect the souring process as long as the temperature stays within the desirable range for that particular strain of Lactobacillus, although the brewer should take care not to allow any dust to get sucked into the fermentation vessel as a result of temperature decreases. During the incubation time, as long as Lactobacillus is the only microbe growing in the wort, the gravity will not drop more than a few points and the fermentation will be calm (see Lactobacillus fermentation. The exact time frame of incubation depends on the species/strain of Lactobacillus, the manufacturer's recommendation, and the brewer's desired acidity level. Acidity can safely be measured with a reliable pH meter throughout this time. If possible, it is advised that the brewer fills the head space of the fermenter with CO2. Some brewers report that this helps to reduce sulfur production, but if Brettanomyces is added to the beer later on in the process and allowed to age, this shouldn't be a concern.

After reaching the desired acidity level from the incubating Lactobacillus bacteria the brewer can crash cool the fermenter down to the desired temperature for the primary fermenting yeast. Both Saccharomyces and Brettanomyces, or a blend can be used as primary fermenting yeast. Brettanomyces is often chosen because of it's higher tolerance of a low pH environment (3.4- pH ), although many Saccharomyces strains have been successfully used (see the Souring in the Boil Kettle section above). If the chosen yeast requires aeration and the brewer has the ability, the sour wort should be aerated before pitching yeast. Brewers have had good luck using Fermentis dry yeast products in the non-aerated wort after, of course, rehydrating the dry yeast as per the manufacturer's instructions and with using a yeast nutrient such as Go-Ferm is effective . The wort is then fermented out as normal. The brewer can consider other Brewing Methods such as pitching Brettanomyces, a mixed culture, or commercial sour beer dregs into secondary.

Souring with Malted Grains in Another Vessel Before Racking to the Primary Fermenter

This process is very similar to souring the wort in a kettle. This method is ideal for those who wish to use grains to introduce Lactobacillus to the beer, or for breweries that do not want to tie up their boil kettle. If done properly, the formation of Butyric Acid and Isovaleric Acid should be minimal.

The wort is mashed and sparged as normal (and alternatively brought to a short boil) and is then lowered to somewhere between 109-115°F (42.8-46°C). This temperature favors Lactobacillus while discouraging Enterobacteriaceae. Optionally, the mash pH can be lowered to 4.4 with lactic acid or acidulated malt to further discourage Enterobacteriaceae activity. Once the desired temperature (and optionally pH) is reached, the wort is then transferred to a second vessel such as a glass carboy. A pure culture of Lactobacillus can be pitched, or a handful of fresh malted unmilled grain is added. The vessel should be filled to the very top, minimizing the oxygen levels inside the vessel. The vessel should be stored in a heated environment that maintains a temperature between 109-115°F (42.8-46°C) for 1 to 3 days depending on how much acidity the brewer wants (the faster the souring process the better; Jeff Young from Blue Owl achieves the desired acidity in about 18 hours usually) .

Once the 1 to 4 day time period has been reached, the wort is transferred to the boil kettle and boiled as normal. Boiling will kill all of the microorganisms in the wort, and will provide the option for adding hops and other kettle additions. Just as with kettle souring, the wort doesn't have to be boiled, but can be instead heat pasteurized at 140°F (60°C) for 15 minutes if a yeast contamination produced a significant amount of alcohol (this temperature effectively heat pasteurizes most organisms while not driving off the aromatics and volatiles from the yeast fermentation). Once boiled or pasteurized, the wort can be chilled and handled in the same way as the above methods for wort souring.

James Spencer provides an article that fully explains his process, as well as a step by step video guide and tasting on Beer and Wine Journal .

An alternative to this method would be to culture Lactobacillus from the grain first and then use it in a kettle souring process. See Culturing From Grains and Blue Owl Brewing's grain inoculation methodology and data for more information.

Tips on Maintaining Heat for Homebrewers

Keeping the temperature as steady as possible for a pure culture Lactobacillus fermentation is not that important. Just try to stay in the range of the temperature best suited for a given species (see the Lactobacillus culture charts). Here are some tips from MTF members on maintaining warm temperatures for wort souring :

Editor's note: please be cautious when using heat sources to heat plastic fermenters; PET bottles will melt or warp when applying too much heat to them.

• Electric heating blanket or heating pad.
• A room space heater with towels or a t-shirt to insulate.
• If it fits, place the fermenter in an insulated 10 gallon Rubbermaid cooler (like the kind that many people use as mash tuns).
• For metal vessels such as a kettle, keep it on the stove burner and turn the burner on when heat is lost.
• Put the wort in a plastic bucket and seal with lead, then lower that bucket into a ten gallon water cooler and close the lid. Optionally, insulate with blankets.
• Use a glass carboy FermWrap™ heater or another carboy heater attached to a sensor thermowell through the cap and a temperature controller.
• Use a Brew Belt Fermentation Heating Belt and wrap a "hoody" sweater or some other fabric around it to insulate, and a temperature controller.
• Use an aquarium heater (Enheim brand recommended), fill a large plastic cooler with water and keep the water warm with the aquarium heater. Set the souring vessel in the cooler.
• Use an Anova sous vide device - place the vessel in a larger vessel with water, and keep the heat applied to the outside water with the Anova. Works well for maintaining heat for starters too.
• Similar to the Anova, James Spenser of BasicBrewing.com video on using an InstantPot® to maintain "yogurt" temperatures for 1 gallon batches.
• Build an insulation box out of Styrofoam. See this MTF post and this MTF post.
• Sour in a keg, and heat the keg. If the keg is not fitted with a spunding valve or an airlock, be sure to burp the keg in case of an accidental yeast contamination as that would result in a lot of built up pressure. See this MTF post.
• If not using a plastic vessel, keep the vessel in an oven (if it fits) or an electric smoker.
• Use a ZooMed Reptile Heater Cable inside of a fermentation chamber. Line the inside of the chamber walls with the wire and secure with duct tape.
• If maintaining heat is not possible with any of the techniques mentioned, use a species such as L. plantarum that produces acidity at room temperature. Omega Yeast Labs OYL-605 works well without hot temperatures.

How to Pre-Acidify

Photo provided by Mark Horsley. Left is a 5.30 pH wort and the right is a 4.75pH wort. Both were kettle soured with L. delbrueckii. Both reached 3.30 pH post ferment with finishing gravity of 1.7 Plato.

Note: pre-acidifying applies to the wort that will become the batch of sour beer, not Lactobacillus starters. Pre-acidifying a Lactobacillus starter will slow the growth of the Lactobacillus in a starter. The idea of the starter is to grow an effective amount of cells while pre-acidifying the main batch of wort aims to inhibit contaminating microbes and retain foam positive proteins. See Lactobacillus starters for more information.

After the production of the wort, but before pitching the culture of Lactobacillus, some brewers like to slightly lower the pH of the wort with food grade lactic acid (available at homebrew stores) or phosphoric acid before adding the Lactobacillus. Pre-acidifying wort can be applied to any wort souring method in general, and while using any species/source of Lactobacillus including lab cultures, probiotics, yogurt, grains, sauerkraut, etc. Acidifying the wort before pitching Lactobacillus has several benefits, such as discouraging unwanted microbes that may have accidentally been introduced into the wort, and helping to prevent Lactobacillus from degrading foam proteins. There have also been observations by Jeff Mello of Bootleg Biology and others that pre-acidification results in a lower final pH than if pre-acidification is not applied (this pH difference is not explained by the addition of acid because pH is logarithmic; see this MTF thread). The idea is to get the wort down to a pH of 4.0 - 4.4 before adding Lactobacillus.

There currently is no formula for how much lactic acid to add to a volume of wort due to the different buffering capacities of wort . Water chemistry spreadsheets and formulas geared towards mash pH adjustments may not be accurate for wort pH adjustments since wort does not contain grain material, however there has been reports on MTF and recommendations from Martin Brungard (author of Bru'n Water) that Bru'n Water can accurately determine how much lactic acid is needed to lower a wort's pH, or at least provide a starting point . We encourage readers to experiment with water chemistry calculators to see if they can accurately predict wort pH adjustments or get a starting point for how much acid to add .

Another method of finding out how much acid to add would be to pull a measured portion of the wort out, and add acid in measured amounts until the desired pH is reached. The amount of acid added can then be scaled up to the full volume of the wort. A.J. Delange suggests that the buffering capacity of wort might be half that of the mash (based on the kilograms of malt used in the mash) .

Trial and error might be the most practical approach for homebrewers that don't have an abundance of wort to spare for finding out how much acid to add to a sample and scaling that up. Post boil, the wort pH is generally around 5.0 - 5.2. Adjusting the pH of wort before pitching Lactobacillus can then be done fairly easily by taking a trial and error approach. Using 1 mL of 88% lactic acid per .1 shift in pH for 5 gallons of wort is a good starting measurement. As an example, say that 5 gallons of wort has a pH of 5.0 just before pitching the Lactobacillus culture. Begin by adding 5 mL (1 US teaspoon) of food grade lactic acid to the wort for a target of ~4.4 pH. Stir gently, then take another pH reading. Continue to add 1-2 mL of lactic acid until the wort has the desired pH. Derek Springer has observed that it takes about one tablespoon (15 mL) of 88% lactic acid to reach a pH of 4.2 - 4.5 for 5 gallons of wort , however a higher amount may be required if the brewer's water is high in bicarbonate (24 mL for 5 gallons of wort to reach a pH of 4.4 was reported by Sean McVeigh for his water which contains 375ppm of bicarbonates ). Once a pH of 4.0 - 4.4 is reached, pitch the Lactobacillus culture. This small amount of lactic acid shouldn't have much of an impact on flavor. If a more precise method for determining the required amount of lactic acid is required, a sample of the wort can be pulled and lactic acid or phosphoric acid can be added to it until the target pH is reached, and then that amount can be scaled up (a micropipette might be required to measure very small amounts of lactic/phosphoric acid).

Maintaining a Lactic Acid Bacteria Culture

It is possible to maintain a pure culture Lactobacillus culture from batch to batch of beer. One method that is common with commercial brewers is to collect an appropriate volume of the soured wort (after souring, but before pitching yeast). Commercial brewers will often use a designated corny keg or sanke keg (depending on the volume that is required). After souring the wort a small portion will be transferred using a closed system into the keg. The keg is then refrigerated until it is needed for the next batch. Storing the soured wort cold is recommended for lengthening the viability of the Lactobacillus culture while also inhibiting the growth of any potential contaminants, however, the Lactobacillus might perform differently based on a number of variables such as how long it is stored (longer storage means less viability). Re-pitching might lead to an increase in acid production as well. We recommend experimenting with this process and finding the right pitching volume/storage time until repeatability can be achieved. For commercial brewers, Breakside Brewery recommends using 5-7% by volume of stored Lactobacillus buchneri (WY5335) soured wort for a new batch, although this might vary based on many factors such as the strain of Lactobacillus, the storage pH, and the storage time .

Homebrewers can use a similar process. For a 5 gallon batch of soured wort, collect around 1 liter of wort in a jar before pitching yeast. Keep the culture cold until the next use. Don't seal the jar completely tight in case refermentation occurs in the jar. If the culture is kept for more than a couple of months, then create a 500 mL starter to ensure that the bacteria culture is still viable.

Yeast contamination is a concern when storing soured wort, especially in a sealed vessel such as a keg. Pure cultures of Lactobacillus cannot effectively ferment most sugars in wort (see 100% Lactobacillus fermentation), therefore there are a lot of sugars available for contaminants to take advantage of when storing wort that has been soured but not fully fermented with yeast. Good sanitation practices are of the utmost importance. Equipment used should be thoroughly clean and ideally sterilized, or at least boiled if possible (or exposed to 82°C/180°F water for 15 minutes). If a yeast contamination goes unnoticed, then the keg can become heavily pressurized and potentially dangerous even when stored cold. We recommend regularly checking a keg of soured wort to ensure that over-pressurization does not occur, or use a spunding valve to ensure that over-pressurization does not occur.

These methods only work with pure cultures of Lactobacillus (or at least cultures that do not contain an unwanted microbe that can take advantage of the residual sugars in the soured wort, such as yeast). Cultures from grains, mixed cultures, or cultures from other fermented foods such as those listed on the Alternative Bacteria Sources wiki page, should be treated as a mixed culture that potentially contains yeast. See Storing a yeast cake or sample of a mixed culture.

Contamination Concerns

When working with lactic acid-producing bacteria, the brewer’s goal is usually to attain clean-tasting sourness, while obtaining desirable flavor contributions from these bacteria, and simultaneously minimizing off-flavors. It should be noted that off flavors span the range from undesired by most, to desirable to some. For example, isovaleric acid is a compound known for its footy aroma that would be considered an off flavor in many beers, yet it gives a highly desired flavor to certain French cheeses and is acceptable by some in small amounts in mixed fermentation beer.

With that said, contamination issues are among the biggest challenges when pre-souring wort with Lactobacillus. This is because Lactobacillus does not fully ferment wort by itself (see 100% Lactobacillus fermentation). When yeast fully ferments wort into beer, alcohol, hops, and a low pH all work together to prevent most spoilage microorganisms from contaminating the beer (although contamination can certainly happen with beer spoilage microbes such as Brettanomyces, Pediococcus, etc.) . When fermenting with Lactobacillus by itself, either no alcohol is produced or not enough alcohol is produced to have an antimicrobial effect. Usually, hops are not used when souring wort with Lactobacillus because even small amounts of hops completely inhibit most commercial strains, but they also inhibit some spoilage microorganisms. The high available sugars, warm temperatures typically used in souring wort, and a lack of alcohol and hops therefore increase the chances for contamination during souring wort with Lactobacillus.

Contaminates can include a variety of molds, yeasts, and bacteria. Contaminations can have a variety of potentially unfavorable flavor effects on soured wort depending on the type of microbe(s) that caused the contamination. One common off-flavor in kettle soured beers has been associated with butyric acid, which is in and smells like human vomit. Although the exact source of butyric acid in kettle soured beers has not been identified that we know of, butyric acid is produced by anaerobic contaminates and not when Lactobacillus is exposed to oxygen (see Effects of Oxygen on Lactobacillus and butyric acid). Isovaleric acid is another off-flavor that can be produced by both anaerobic and aerobic contaminates.

Another common contaminate from improper wort souring is brewer's yeast (S. cerevisiae). Brewer's yeast is not greatly inhibited by Lactobacillus. Conversely, Lactobacillus is greatly inhibited by the presence of active S. cerevisiae . The warmer temperatures encourage S. cerevisiae to ferment the wort quickly. This often results in a beer that isn't sour because the Lactobacillus are out-competed by the yeast. Signs that yeast has contaminated the wort include the typical signs of yeast fermentation: the presence of a krausen, a gravity shift of more than 1.005 gravity points (or 0.5-1.0° Plato), and looking at a sample of the wort under a microscope. Sources of yeast contamination can come from poor sanitation, but they can also come from the yeast manufacturer themselves. See see 100% Lactobacillus fermentation for more information.

Another source of contamination, which is arguably desired, happens when using a "wild" source of Lactobacillus. For example, when culturing Lactobacillus using non-plating techniques from grains, fruit, or from some other fermented food such as kefir, sauerkraut, etc., yeast and other microbes can carry over from the culturing process (see Alternative Bacteria Sources). The only way to guarantee that only Lactobacillus from a "wild" source such as these ferment the wort, is to isolate the microbe using plating techniques (see Wild Yeast Isolation). Otherwise, there is a chance that wild yeast will also survive the culturing process.

Preventing Contamination

Several techniques can be applied to prevent contamination in kettle soured beers. Many of these are covered in the sections below when they apply to specific processes, however, we will cover them all in this section as well.

Pre-acidifying wort to 4.5 pH or lower helps to kill many microbes that are not tolerant of low pH. Flushing the wort with CO2 during the souring process is thought to help prevent aerobic contaminates . Maintaining a temperature between 113-120°F (45-49°C) helps encourage some species of Lactobacillus and inhibits heat intolerant contaminates, however, some species of Lactobacillus do not do well at these warmer temperatures (see the Lactobacillus culture charts). Pre-boiling the wort has been shown to help greatly with preventing contamination. This is likely due to the killing power of boiling temperatures versus lower temperature pasteurization, as well as the heat and steam killing microbes on the sides of the boil kettle, the lid of the boil kettle, and the head space above the wort. Boil kettles are inherently unsanitary vessels compared to fermentation vessels, so great care should be taken to sanitize the vessel and prevent any air from getting into the vessel during the souring process. Air can get in when temperatures cool and a vacuum is created inside the kettle. Wrapping the boil kettle and lid with plastic wrap has been a typical approach for homebrewers, as well as maintaining a constant temperature so as to avoid creating a vacuum inside the kettle from cooling temperatures. Commercial brewers must also prevent air from getting sucked into the boil kettle. Some have used sanitized and inflated sports/beach balls or something similar to clog the boiler stack during kettle souring, for example. Continuous flushing with CO2 can also help prevent a vacuum from sucking in air. Finally, achieving the desired acidity as quickly as possible helps to cut down on the chances for contamination. Achieving the desired acid development within 48 hours is an ideal goal, but achieving it within 12-24 hours is even better.

Yeast contaminations can be difficult to avoid if they are coming from the manufacturer of the Lactobacillus culture. Look at the culture under a microscope and check for yeast cells, which will be much larger and circular in shape compared to the much smaller, rod-shaped bacteria. Reputable yeast companies will usually offer a replacement for any contaminated Lactobacillus cultures.

Contamination of the Lactobacillus strain itself in a production brewery is always a concern, especially if the Lactobacillus is not boiled. Thorough cleaning and sanitation regimes are required for preventing cross contamination. Brewers will often advise performing two cycles of near boiling water with caustic, followed by a sanitizing rinse. If metal parts can be removed from a system, they should be boiled. Soft parts such as hoses and gaskets should be reserved for sour brewing. Using a Lactobacillus strain that has been tested to be intolerant of 1-2 IBU's can also reduce the risk of cross contamination . Check with your QC lab or yeast lab for procedures for avoiding cross contamination.

Dealing With Yeast Contamination

If a yeast contamination produces a significant amount of alcohol during the souring process, then this presents a problem when it comes to boiling. Although 100% pure ethanol boils at 173.1°F/78.4°C , the lower the concentration of ethanol in the wort (technically beer at this point if it has been fully attenuated by yeast), the higher the temperature required for boiling off the ethanol. For example, at 5% ABV it takes approximately 197°F/92°C for the ethanol to boil . Time is also required to boil off the ethanol, so this may not be as big of a concern as it first appears (warning: vaporized ethanol is highly flammable). Another and perhaps more important consideration is that boiling and high heat pasteurization temperatures can have a negative effect on the flavor of fermented beer. Beer already has anti-bacterial properties, such as low pH, presence of alcohol, and hops (although hops may not be present in wort being soured), so higher pasteurization temperatures aren't necessarily required for beer. For these reasons, the beer industry commonly heat pasteurizes beer at 140°F/60°C for 15 minutes, and this is also adequate for pasteurizing soured wort that accidentally fermented do to yeast contamination . Although the yeast will be killed during the pasteurization, so will the Lactobacillus, and the brewer can trust that equipment downstream shouldn't get contaminated. Other options are to simply dump the batch and start again with a pure culture of Lactobacillus, or do not pasteurize and let the Lactobacillus live. If the Lactobacillus is not pasteurized and allowed to live, the soured wort/beer should be treated with care so that it does not contaminate the rest of the brewery (see 'Do I need Separate Equipment?' FAQ).

Boiling soured wort that hasn't had an accidental yeast contamination (and thus still has a high specific gravity) probably has less of an effect on the flavor than it does on fully fermented beer. Other than lactic acid, the flavor components that different strains of Lactobacillus produce are not well defined, so it will be difficult to determine if boiling soured wort will have a negative impact. However, brewers who boil kettle soured wort don't often report that the boiling causes flavor issues .

Various Other Concerns

Not Boiling and Dimethyl Sulphide (DMS)

This is generally not a concern. See Dimethyl Sulfide for an explanation why.

Haze

The pH of boiling wort influences how well proteins are precipitated out, with a pH of about 5.2 being ideal for this mechanism. At a pH lower than 5.0, these proteins gain negative charges, and at a higher pH they gain positive charges, making them resistant to settling out during boiling. Boiling a low pH wort that has been acidified by the fermentation of Lactobacillus therefore does not effectively precipitate out haze-forming proteins. However, haze forming polyphenols, which are also required for haze formation, are greatly reduced by an unknown mechanism during kettle souring, which can potentially lead to good clarity in the final beer regardless of the protein level (although other factors might still lead to a hazy beer) . Additionally, some strains lactic acid bacteria are known to cause haze as a side effect of their lactic acid fermentation.

Color

Maillard and various browning reactions lead to darkening of beer in general. These reactions are both temperature and pH related. A low pH will actually prevent these reactions and results in a quantitatively lighter color beer compared to a control beer that is not acidified .

Oxygen

There is no scientific basis for the idea that a small amount of oxygen itself that is dissolved during wort souring has a great effect on the flavors that Lactobacillus produces. See Effects of Oxygen on Lactobacillus and this MTF thread that discusses the anecdotal experience of brewers who do not purge with CO2. The effects of oxygen on the beer could lead to other stability issues with the beer because the exposure to oxygen during wort production is thought to play a role in beer staling (see Aging and Storage), although hot side aeration is also thought to be a minor problem if at all , and the impact of hot side aeration on sour beer has not been studied. Some brewers believe that purging all oxygen out of the system will help to prevent contaminating microbes that are aerobic (microbes that require oxygen) from creating isobutyric acid. Oxygen does not play a role in the production of Butyric Acid in wort soured beers because this compound is produced by anaerobic bacteria (bacteria that requires the absence of oxygen). Air/dust can carry contaminating microbes, which is a potential problem (see Contamination Concerns above), however, if the air is filtered somehow before entering the fermenting vessel then the oxygen itself shouldn't pose a problem to souring wort. Follow the links in this paragraph for more information.

Yeast Harvesting

It is generally recommended to not re-use yeast that has fermented a soured wort. This has a stressful impact on the yeast. However, some commercial brewers have claimed that repitching WY3711 as many as 100 (in one case) to 2000 (in a second case) generations in soured wort conditions has not posed a problem for them. If this is attempted, repitching the yeast from the middle of the yeast cake will select for the more acid tolerant cells. Some yeast strains may not be able to adapt to these conditions as well as others (more data is needed), however, it has been shown that S. cerevisiae can adapt to acidic conditions and become more tolerant of low pH conditions from generation to generation. See terminal acid shock for more information.

See Also

Additional Articles on MTF Wiki

• Sour Mashing
• Alternative Bacteria Sources
• Lactobacillus
• Butyric Acid
• Isovaleric Acid
• Mixed Fermentation

External Resources

• "How I Sour Mash & A Recipe"; Sui Generis Blog (includes Wort Souring and microbiology information).
• "Year of the Sour Mash" by Derek Springer. This series of articles were written for his NHC 2015 presentation.
• "Lactobacillus 2.0 - Advanced Techniques for Fast Souring Beer", by Matt Miller of Sour Beer Blog.
• "Overnight Acidification" by Michael Tonesmeire, article for Brew Your Own Magazine on Jan/Feb 2016.
• "Boil vs. No-Boil Berliner Weisse | exBEERiment Results!" on Brulosophy blog.
• Catharina Sour – A Brazilian kettle-soured fruit beer.