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'''Wort Souring''' is the process of "[[Mixed Fermentation]] where " (in the case of lactic acid bacteria, usually ''[[Lactobacillus]]'' being left alive to co-exist with yeast) or pre-boil acidfication (although there are also ''[[Pediococcus]]'' cultures available that work well for this technique), is given a "head start" as it's pitched before in the yeast so that it will be able to produce significant amounts case of kettle sours) where lactic acid bacteria pre-ferment and acidify wort before the ''Saccharomyces'' completes yeast is pitched to complete the main alcoholic fermentation <ref>[https://www.researchgate.net/publication/341353855_The_power_of_sour_-_A_review_Old_traditions_new_opportunities Bossaert, Sofie & Crauwels, Sam & De Rouck, Gert & Lievens, Bart. (2019). The power of sour - A review: Old traditions, new opportunities. BrewingScience. 72. 78-88. 10.23763/BrSc19-10bossaert.]</ref><ref>[https://onlinelibrary.wiley.com/doi/full/10.1002/jib.569 Dysvik, A., Liland, K. H., Myhrer, K. S., Westereng, B., Rukke, E.-O., de Rouck, G., and Wicklund, T. (2019) Pre-fermentationwith lactic acid bacteria in sour beer production. J. Inst. Brew., 125: 342– 356. https://doi.org/10.1002/jib.569.]</ref>. While non-sour beer falls in the range of 3.8-4.6 pH, a final pH between 3.0 and 3.7 is the general target range for the soured wort and also the finished sour beer (although [[Titratable Acidity]] is more accurate for measuring perceived sourness). This is a broadly defined technique and there are many variations on souring wort. These include mixed fermentation methods such as souring in the primary fermenter, souring in for a secondary vesselshort time period before adding yeast, or sequential fermentation such as souring in the boil kettle itself with a pure culture of lactic acid bacteria and then boiling to pasteurize the sour wort before yeast is added ('''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''.
<blockquote>"''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.</blockquote>
==Strain Selection==
The strain of ''Lactobacillus'' is very important. As is explained below, the souring process should not take longer than 2 days, and some strains do not perform well within this timeframe. Not all strains or species are efficient to use for wort souring. ''Lactobacillus plantarum'' has proven to be the most successful species for kettle souring because it produces acidity very quickly, and it is not as vulnerable to temperature shifts, although using any hops will usually completely prevent this species from producing lactic acid. ''L. brevis'' has also been used with some degree of success, depending on the strain. While ''Pediococcus'' is generally not used for wort souring, Bootleg Biology's ''Sour Weapon P'', which is ''P. pentosaceus'', performs well at wort souring (see [[Pediococcus#Commercial_Pediococcus_Cultures|Commercial ''Pediococcus'' cultures]]). The ''L. delbruekii'' culture offered by White Labs is meant for long term souring and is not a good choice for wort souring (other strains of ''L. delbruekii'' might perform better) <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1212455192116026/?comment_id=1212475888780623&reply_comment_id=1212476575447221&comment_tracking=%7B%22tn%22%3A%22R3%22%7D Conversation with Andrew Addkison on MTF. 01/12/2016.]</ref>. Review the [[Lactobacillus#Culture_Charts|''Lactobacillus'' commercial cultures]] before selecting one for wort souring, and review the [[Alternative Bacteria Sources]] page for tips on using alternative sources of ''Lactobacillus'' such as probiotics, grain, etc.
===''Lactobacillus'' Nutrient Requirements===
Different strains of ''Lactobacillus'' are known to have different nutrient requirements, including some metal ions. ''Lactobacillus'' requires manganese, magnesium, potassium, and sodium, with manganese and magnesium being crucial for their growth. Brewer's wort has many of these nutrients naturally, although zinc is generally deficient. One study looked at WLP672 (''L. brevis'') and found that an increase of magnesium resulted in a faster souring phase, while zinc additions slightly slowed the souring phase. This suggests that in at least the case for this strain of ''L. brevis'', zinc additions should be added after the souring phase for yeast health. Excess magnesium could lead to negative flavor contributions to beer, so magnesium additions should not exceed 30 mg/l <ref>[https://www.brunwater.com/water-knowledge "Water Knowledge." Martin Brungard. Bru'n Water website. Retrieved 12/02/2020.]</ref>. The addition of zinc also had several effects on the flavor and aroma compounds produced. Therefore, experimenting with different nutrient additions for the souring phase may result in widely different tasting beer <ref>[https://www.mdpi.com/2218-273X/10/12/1599 Chemical Composition of Sour Beer Resulting from Supplementation the Fermentation Medium with Magnesium and Zinc Ions. Aneta Ciosek, Katarzyna Fulara, Olga Hrabia, Paweł Satora, and Aleksander Poreda. 2020. DOI: https://doi.org/10.3390/biom10121599.]</ref>. See [https://www.facebook.com/groups/MilkTheFunk/permalink/4107425732618943 this MTF thread] for more commentary on this study.
==Processes==
===Souring in the Boiler (Kettle Sour)===
Also known as '''kettle souring''', souring in the boil kettle (or another vessel) 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. This process was originally invented by a German scientist named Otto Francke, and called the "Francke acidification process". It was designed by Francke as a way to shortcut the mixed culture fermentation of Berliner Weisse; however, this process was never really used in Berliner Weisse production because it did not produce a product that resembled Berliner Weisse that was fermented with a mixed culture of yeast and bacteria (including ''Brettanomyces''). See the [[Berliner_Weissbier|Berliner Weisse]] page for more information on the history of Berliner Weisse production methods <ref name="marshall">[https://drive.google.com/file/d/0B8CshC9nxYHdckhlbXFQN1hPbGc/view Kurt Marshall. CBC 2012 Presentation.]</ref><ref>[https://eurekabrewing.wordpress.com/2012/03/10/44-traditional-berliner-weisse/ Samuel Aeschlimann. Eureka Brewing Blog. "#44 Traditional Berliner Weisse". 03/10/2012. Retrieved 09/02/2017.]</ref><ref>[http://barclayperkins.blogspot.com/2020/09/kettle-souring.html Shut up about Barclay Perkins. "Kettle Souring". Ron Pattinson. 09/17/2020.]</ref>.
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 <ref>[https://onlinelibrary.wiley.com/doi/epdf/10.1002/j.2050-0416.2005.tb00221.x Enhancing the Microbiological Stability of Malt and Beer – A Review. Anne Vaughan, Tadhg O’Sullivan and Douwe van Sinderen. 2005.]</ref><ref>[https://elifesciences.org/articles/04634 Mapping microbial ecosystems and spoilage-gene flow in breweries highlights patterns of contamination and resistance. Nicholas A Bokulich, Jordyn Bergsveinson, Barry Ziola, David A Mills. 2015.]</ref><ref>[http://mmbr.asm.org/content/77/2/157.full The Microbiology of Malting and Brewing. Nicholas A. Bokulich and Charles W. Bamforth. 2013. DOI: 10.1128/MMBR.00060-12.]</ref>. 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 <ref name="Heit_boiling">[https://www.facebook.com/groups/MilkTheFunk/permalink/1180630378631841/?comment_id=1180634488631430&reply_comment_id=1180677581960454&comment_tracking=%7B%22tn%22%3A%22R2%22%7D Conversation with Bryan of Sui Generis Blog regarding boiling versus lower temperature pasteurization. 11/18/2015.]</ref><ref>[http://sourbeerblog.com/lactobacillus-2-0-advanced-techniques-for-fast-souring-beer/ ''Lactobacillus'' 2.0 – Advanced Techniques for Fast Souring Beer. 11/18/2015. Retrieved 11/19/2015.]</ref><ref name="pasteurization">[http://www.mbaa.com/meetings/districtpresentations/DistrictPresentations/2011_03_10PasteurizationTechnologies.pdf "District Michigan MBAA Technical Meeting Grand Ledge, MI". MBAA Presentation. 2011.]</ref>. 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|''Lactobacillus'' culture charts]]; [[Pediococcus#Bootleg_Biology_on_Sour_Weapon_P|Bootleg Biology's "Sour Weapon P"]] can also be used to kettle sour). Once chilled to the appropriate temperature, the wort in the kettle is inoculated with a culture of ''Lactobacillus''. The chilled wort can alternatively be transferred to another vessel where the ''Lactobacillus'' is added for the acid producing fermentation and then afterwards sent back to the boiler to pasteurize the acidified wort and kill the lactic acid bacteria. 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 [[Lactobacillus#Starters_and_Pitching_Rate|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, similar to how [http://www.lowoxygenbrewing.com/ingredients/a-sauergut-reactor/ sauergut] is made, instead of a pure culture since the husks of [[Grain|grain]] carry many microorganisms. If unmilled grain is added it is thought that filling the head space of the kettle with CO<sup>2</sup> (or another gas like argon or nitrogen; always provide proper ventilation when working with any gasses <ref>[https://www.brewersassociation.org/press-releases/brewers-publications-presents-gose-brewing-a-classic-german-beer-for-the-modern-era/ Allen, Fal. "Gose: Brewing a Classic German Beer For the Modern Era". Brewers Publications. 2018. Pg 126.]</ref><ref>[https://beerandbrewing.com/dictionary/DfG8FdFjfm/nitrogen/ Nick R Jones. "The Oxford Companion to Beer definition of nitrogen". Craft beer & Brewing Magazine website. Retrieved 10/21/2018.]</ref>) will help decrease off-flavors such as "footiness" from [[Isovaleric Acid]] which are produced by aerobic microbes that are naturally present on the grain <ref name="khris_johnson">Personal correspondence with Khristopher Johnson of Green Bench Brewing Co. and Dan Pixley. 05/24/2016.</ref>. 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 <ref name="young_grains">[https://www.facebook.com/groups/MilkTheFunk/permalink/1356058381089039/?comment_id=1356464531048424&comment_tracking=%7B%22tn%22%3A%22R%22%7D Conversation with Jeff Young from Blue Owl Brewing Co on souring from grains. 07/21/2016.]</ref>. Lowering the pH of the [[Wort_Souring#How_to_Pre-Acidify|wort to under 4.5 (ideally 4.0 - 4.3 when souring with grain)]] with lactic acid or phosphoric acid will discourage other unwanted bacteria and wild yeast from thriving in the wort during the incubation period. This will also help with head retention (see [[Lactobacillus#Foam_Degradation|''Lactobacillus'' and head retention]]) <ref>[[Lactobacillus#Foam_Degradation]]</ref>. Souring with grains should occur within 1 or 2 days if done correctly <ref name="young_grains"></ref>. 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 [[Grain#Malt_Inoculated_Wort|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 CO<sup>2</sup> gas or another type of gas like argon or nitrogen to purge the oxygen (oxygen does not cause ''Lactobacillus'' to produce [[Butyric Acid|butyric acid]] or [[Isovaleric Acid|isovaleric acid]], but some brewers have reported that purging oxygen will help reduce sulfur in the finished beer). Purging oxygen could also discourage [[Mold|mold]] growth. Mold growth during the souring process has been reported from time to time by homebrewers, and if mold grows in contact with the wort then it probably should be discarded (see the [[Mold]] page for more information) <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/2144219142272955/ Pavel Dunn. Milk The Funk Facebook group thread on mold growth during kettle souring. 06/23/2018.]</ref>. However, many homebrewers and some professional brewers have reported having success without purging with CO<sup>2</sup> <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1778865588808314/ Various members of Milk The Funk. Milk The Funk Facebook thread on purging kettle sours with CO<sup>2</sup>. 08/03/2018.]</ref><ref>[https://www.milkthefunk.live/podcast/2018/4/2/episode-004-kettle-souring-with-adi-hastings-from-omega-yeast-labs Adi Hastings. Milk the Funk "The Podcast"; Episode #004. 04/02/2018. Retrieved 04/16/2018.]</ref>(~20:30 mins in), and a [http://brulosophy.com/2018/04/16/the-impact-of-open-fermentation-on-kettle-sour-beer-exbeeriment-results/ Brulosophy blind triangle test] was unable to find a significant difference between a kettle sour that was CO<sup>2</sup> purged and one that was soured in an open vessel. Using some CO<sup>2</sup> to keep positive pressure in the kettle could help prevent contaminants that create [[Butyric Acid|butyric acid]] and other off-flavors from entering the kettle due to negative pressure, and is often the approach that commercial brewers take <ref>Personal correspondence with Steph Cope of CraftHaus Brewing Co. 02/06/2016.</ref>. The kettle should be held at the desired temperature for 24-72 hours (in some cases longer, but no longer than 5 days- slower acidification increases the chances of the growth of contaminating microbes and off-flavor production from contaminating microbes and/or unwanted alcohol production from contaminating yeast). 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 CO<sup>2</sup> 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 CO<sup>2</sup>. 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 <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1778027798892093/ Various professional brewers. Milk The Funk Facebook post about souring in a mashtun, brite tank, or other vessel for commercial brewers. 08/02/2017.]</ref>.
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 CO<sup>2</sup>, or fermenting wort more than ~1.005 gravity points (see [[100% Lactobacillus Fermentation]]). See [[Wort_Souring#Dealing_With_Yeast_Contamination|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 <ref name="low ph">[http://www.homebrewtalk.com/f127/no-hop-berliner-weisse-415067/index3.html#post5280971 Michael Tonsmeire on HBT]</ref><ref>[http://www.themadfermentationist.com/2016/12/quick-sour-then-what-acid-tolerance-of.html "Quick Sour, then what? Acid Tolerance of Brewer’s Yeast." The Mad Fermentationist blog. Michael Tonsmeire. 12/13/2016. Retrieved 12/14/2016.]</ref>. 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) <ref name="peyer_2017">[http://www.asbcnet.org/publications/journal/vol/2017/Pages/ASBCJ-2017-3861-01.aspx Sour Brewing: Impact of Lactobacillus amylovorus FST2.11 on Technological and Quality Attributes of Acid Beers. Lorenzo C. Peyer, Martin Zarnkow, Fritz Jacob, David P. Schutter, Elke K. Arendt. 2017.]</ref>. 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, WY3726, and [[Kveik|kveik]] to name a few (see references) <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1105185972842949/ Conversation on MTF about using specific yeast strains in acidic wort. 7/6/2015.]</ref><ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/2666743113353886/ Multiple MTF members. Milk The Funk Facebook group thread about fermenting kettle sours with kveik. 05/15/2019.]</ref>. [http://www.themadfermentationist.com/2016/12/quick-sour-then-what-acid-tolerance-of.html 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 <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1509509862410556/?comment_id=1509520015742874&comment_tracking=%7B%22tn%22%3A%22R4%22%7D MTF conversation with Richard Preiss about English ale strains and pH tolerance. 12/13/2016.]</ref>. 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 [[Packaging#Acid_Shock_Starters|Acid Shock Starters]] for more information on how to acclimate yeast to a highly acidic environment to improve fermentation. Escarpment Labs reported seeing stalled fermentations in kettle soured wort when the selected ''Lactobacillus'' produced acetic acid on the higher end, and they suggest using a species like ''L. plantarum'' that does not produce high amounts of acetic acid relative to other strains from species like ''L. brevis'', and that perhaps limiting oxygen can help reduce acetic acid production <ref>[https://www.youtube.com/watch?v=LuVoydgzK8w Nate Ferguson. "Quick Souring with Lactobacillus 101". Escarpment Labs video presentation. 06/09/2020.]</ref> (~19:50 mins). Some brewers have reported a pH shift, either up or down, by 0.1 to 0.3, or no shift at all from the point of after the second boil and after the final yeast fermentation <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/3177062738988585/ Survey of brewers on pH shift between post-souring and yeast fermentation. 01/03/2019.]</ref>. This change in pH caused by the yeast fermentation might be due to many variables such as compounds released by the yeast, consumed by the yeast, strains of yeast/bacteria, water profile, malt recipe, dry hopping, etc.
The usual kettle souring process involves fermenting with a normal ale strain of some sort, and lacks ''Brettanomyces''. Without ''Brettanomyces'', there is a lack of ''Brettanomyces'' character which is the result of the unique esters, phenols, and fatty acids that this genus of yeast produces (see [[Brettanomyces#Secondary_Metabolites|''Brettanomyces'' secondary metabolites]]). For example, traditional [[Berliner_Weissbier|Berliner Weisse]] was fermented with a mixed culture containing ''Brettanomyces'', and this was considered the most important aspect of achieving the fruity ester character of that beer style historically (see [https://docs.google.com/spreadsheets/d/1CNrO46TPSFpjhO3HX1-CbKK5rhFd7uGWdpONf7AJAlU/edit#gid=0 Benedikt Koch's table comparing esters of traditional Berliner Weisse versus kettle soured Kindl Weisse and Belgian gueuze]). Kettle sours can also be racked into oak barrels and have ''Brettanomyces'' pitched at that time. This approach is taken by some commercial brewers who don't want to expose their fermentation vessels to living ''Lactobacillus'' or ''Brettanomyces''. It is also one method for controlling the level of sourness in the beer since the ''Lactobacillus'' is killed off.
See also:
* [https://www.milkthefunk.live/podcast/2018/4/2/episode-004-kettle-souring-with-adi-hastings-from-omega-yeast-labs MTF "The Podcast" interview with Adi Hastings of Omega Yeast Labs on kettle souring] and [https://www.milkthefunk.live/podcast/2018/11/26/episode-007-lactobacillus-microbiology-with-dr-bryan-heit-of-sui-generis-brewing-blog episode #007 ''Lactobacillus'' microbiology with Dr. Bryan Heit].* [https://fb.watch/5I4mUUvQKC/ Brewing Network, Brew Strong interview with Fal Allen from Anderson Valley Brewing (author of "Gose: Brewing a Classic German Beer for the Modern Era") on kettle souring in commercial breweries.]
===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 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 inadvisable 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_Souring#How_to_Pre-Acidify|wort to under 4.5 (ideally 4.0 - 4.3when souring with grain)]] will also discourage contaminating bacteria from thriving in the wort during the incubation period. This will also help with head retention <ref>[[Lactobacillus#Foam_Degradation]]</ref>. There is also the option of [[Mixed_Fermentation#Reusing_a_Sour_Yeast_Cake|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|''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|''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% <ref>[http://www.homebrewersassociation.org/how-to-brew/resources/conference-seminars/ ''Putting Some Numbers on First Wort and Mash Hop Additions''. David Curtis NHC 2014 Presentation.]</ref>. It is generally a good idea to create a 500 mL starter beforehand for ~5-6 gallons of wort (see [[Lactobacillus#Starters_and_Pitching_Rate|''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#100.25_Lactobacillus_Fermentation|''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|pH meter]] throughout this time. If possible, it is advised that the brewer fills the headspace 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.
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) <ref name="delange_brungard">[http://www.homebrewtalk.com/showthread.php?t=587124 A.J. Delange. Homebrewtalk Thread. 07/19/2016.]</ref>.
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 around 4.4 pH (or somewhere between 4.2 and 4.8; target 4.0 - 4.3 if souring with grain or some other non-purified source of bacteriato help inhibit the growth of unwanted bacteria that could produce off-flavors). 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 - 4.8 for 5 gallons of wort <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1290987234262821/?comment_id=1291031577591720&comment_tracking=%7B%22tn%22%3A%22R3%22%7D Conversation with Derek Springer on MTF regarding acidifying wort. 04/24/2016.]</ref>; 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 <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1290987234262821/?comment_id=1291031577591720&reply_comment_id=1291118190916392&comment_tracking=%7B%22tn%22%3A%22R%22%7D Conversation with Sean McVeigh on MTF on acidifying wort. 04/23/2016.]</ref>). Once a pH of 4.2 - 4.5 8 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===
==Various Other Concerns==
===Failed Souring===
There are instances where wort souring fails for seemingly unknown reasons. Use a species or strain of ''Lactobacillus'' that is adapted well to souring wort within 24 hours, such as ''L. plantarum''. The lactic acid bacteria sometimes do not survive well during storage over long periods of time, so viability should be checked for long stored or mishandled cultures. Contamination with yeast can also prevent souring in some instances but not all. The use of hops should be avoided even for hop tolerant species (unless the brewer is experienced enough to predict the souring based on their lactic acid bacteria), and especially avoided if using ''L. plantarum''. Residual iso-alpha acids on the walls of kettle tanks could be enough to inhibit some strains of ''Lactobacillus'', specifically ''L. plantarum'', so kettles that have previously held highly hopped wort might need to be scrubbed clean before being used as a souring tank <ref>[https://www.stitcher.com/show/craft-beer-brewing-magazine-podcast/episode/episode-199-alex-flores-of-urban-south-kettle-sours-the-hard-way-85931878 Alex Flores. Craft Beer & Brewing Podcast. Episode 199: Alex Flores of Urban South Kettle-Sours the Hard Way. Retrieved 09/16/2021.]</ref>(~21 mins in).
===Not Boiling and Dimethyl Sulphide (DMS)===
This is generally not a concern. See [[Dimethyl Sulfide]] for an explanation why.
===External Resources===
* [https://www.youtube.com/watch?v=LuVoydgzK8w Kettle souring; video presentation by Escarpment Labs.]
* [http://suigenerisbrewing.blogspot.com/2015/07/how-i-sour-mash-recipe.html "How I Sour Mash & A Recipe"; Sui Generis Blog (includes Wort Souring and microbiology information).]
* [http://www.fivebladesbrewing.com/year-sour-mash/ "Year of the Sour Mash" by Derek Springer. This series of articles were written for his NHC 2015 presentation.]