Sour Worting

Sour Worting is a process in which Lactobacillus is given a "head start", pitched before the yeast so that it will be able to produce significant amounts of lactic acid before the Saccharomyces completes the main fermentation. There are several variations on this method, including souring in the primary fermenter, souring in a secondary vessel, or even souring in the boil kettle itself. 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. 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. 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 should also make it attractive to brewers who are concerned about infection issues in their cold side equipment (equipment that is used post-boil) . When sour worting, some brewers first lower the pH of the wort to 4.5-4.8 before pitching Lactobacillus. This has sometimes been found to help the head retention of the beer. For more information, see the Lactobacillus page section on Foam Degradation.

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 .

"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 of the Commons Brewery, Kettle Souring Presentation, CBC 2015.

Processes

Souring in the Boil Kettle

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 into the boil kettle. The temperatures that a typical mash out/sparge reach should be enough to pasteurize the wort , however we advise heating the wort for a short (1-2 minutes) boil 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 80-120°F (37-48°C), depending on the Lactobacillus culture that is being used. Once chilled to the temperature that is appropriate, the wort in the kettle is inoculated with a culture of Lactobacillus.

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 recommended to fill the head space of the kettle with CO2 because oxygen can encourage off flavors such as rancid cheese from Butyric Acid and/or Isovaleric Acid, which are produced by microbes that are naturally present on the grain. Keeping the temperature between 113-120°F (45-48.8°C) will encourage the Lactobacillus resident on the grain and will discourage other bacteria. Lowering the pH of the wort to 4.5 will also discourage many other bacteria from thriving in the wort during the incubation period. 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 is ideal but not necessary to fill the head space of the fermenter with CO2 gas (some brewers have reported that this will help reduce sulfur in the finished beer). 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 formation of Butyric Acid and Isovaleric Acid when using only a pure culture is extremely slight to none assuming no other microbes are allowed inside the kettle.

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 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 170°F (76.6°C) for 15 minutes should kill the Lactobacillus culture being used to sour the wort .

Deciding whether or not to boil the wort can also depend on whether or not there was a considerable amount of alcohol produced, which 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). 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), 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. Another and perhaps more important consideration is that, unlike unfermented wort, 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 beer . In the case of an accidental yeast contamination during the souring process, another option is to simply dump the batch and start again with a pure culture of Lactobacillus.

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 . It is recommended to pitch a healthy starter of yeast, possibly with a higher cell count than normal. In the case of using dry yeast, re-hydrating as per the manufacturer's recommendations and 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) .

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 has the advantage 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 sour worting technique, except the wort is never pasteurized after it is soured. The all grain brewing process is the same for any all grain brewing process, except 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 not advised 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 is little concern of developing Butyric Acid or Isovaleric Acid with this method. 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 90-120°F or 32.2-48.8°C), the wort is racked to the primary fermenting vessel. Note that the wort should contain a low amount of IBU's when using this process since IBU's can inhibit many (but not all) species of Lactobacillus. Using no hops is a good approach to getting more acidity, but if hops are required then using less than 6 IBU's is a good guideline in general (consider mash hopping; mash hopping has been reported to reduce IBU's by ~70% ). It may or may not be advised to create a Lactobacillus starter (see the Lactobacillus page) before hand. 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 2-5 days with the before mentioned target temperature maintained throughout the incubation period (some Lactobacillus species/strains may continue to produce acidity under lower temperatures). 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 fill the head space of the fermenter with CO2. Some brewers have reported 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 the desired acidity level is reached from the incubating Lactobacillus bacteria, the brewer can crash cool the fermenter down to the temperature that is desired 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 non-aerated wort. Re-hydrating the dry yeast as per the manufacturer's instructions and with 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 in Another Vessel Before Racking to the Primary Fermenter

This process is very similar to sour worting in the kettle. This method is ideal for those who wish to use grains to introduce Lactobacillus to the beer. 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 113-120°F (45-48.8°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, a handful of fresh malted unmilled grain is added to the mash and allowed a few minutes to inoculate the mash with the microbes found naturally on the grain husks. The wort is then transferred to a second vessel such as a glass carboy. 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 113-120°F (45-48.8°C) for 1 to 4 days depending on how much acidity the brewer wants (3 days is a good rule of thumb) .

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 170°F (76.6°C) for 15 minutes , or at 140°F (60°C) for 15 minutes if a yeast contamination produced a significant amount of alcohol. 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 .

How to Pre-Acidify

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. This has several benefits, such as discouraging unwanted microbes that may have accidentally been introduced into the wort, and helping to prevent Lactobacillus foam degradation. The idea is to get the wort down to a pH of 4.5-4.8 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 , but 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 Lacto. Begin by adding 5 mL of food grade lactic acid to the wort for a target of 4.5 pH. Stir gently, then take another pH reading. Continue to add 1-2 mL of lactic acid until the wort has the desired pH. Once the desired pH is reached, pitch the Lacto culture. This small amount of lactic acid shouldn't have a much of an overall flavor impact.

See Also

Additional Articles on MTF Wiki

• Sour Mashing
• Alternative Bacteria Sources
• Lactobacillus
• Mixed Fermentation

External Resources

• "How I Sour Mash & A Recipe"; Sui Generis Blog (includes Sour Worting 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.