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'''Wort Souring''' is the process of [[Mixed Fermentation]] where acidifying wort by inoculating it with 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 ferment and produce significant amounts of lactic acid before inoculating with yeast to perform the ''Saccharomyces'' completes the main primary alcoholic fermentation. 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 note that [[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|mixed fermentation]] methods such as souring in the primary fermenter, souring in for a secondary vesselshort time period before adding yeast, or pre-boil acidification 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 ('''[[Wort_Souring#Souring_in_the_Boiler_.28Kettle_Sour.29|kettle souring]]'''). There are also various methods of inoculating the wort with ''Lactobacillus''<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 FinallyDysvik, A., Liland, K. H., Myhrer, K. S., Westereng, B., Rukke, E.-O., de Rouck, the brewer has the option of pasteurizing the wort by heating it to kill the ''Lactobacillus'' before adding the yeast for the main 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>.

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) <ref>[http://sourbeerblog.com/fast-souring-lactobacillus/ Miller, Matt. Dec 20, 2014. "Fast Souring with ''Lactobacillus'' – Best Practices, Sensory, & Science". Sour Beer Blog.]</ref>. When souring wort, some brewers first [[Wort_Souring#How_to_Pre-Acidify|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#Foam_Degradation|''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, however [[Pediococcus#Commercial_Pediococcus_Cultures|Bootleg Biology]] has released a blend of ''Pediococcus'' strains that are reportedly good effective to use for all wort souring wortmethods.

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'', and specifically the 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.

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 . <blockquote style="background-color: lightgrey; border: solid thin grey; padding:10px;">The optimal fermentation temperature is different for different species of ''Lactobacillus'' as well as different strains within the same species, and fermenting outside of the recommended range can result in the lack of souring. Consult your yeast lab's website and documentation to determine the optimal fermentation temperature range for your given ''Lactobacillus'' product. See also the [[Lactobacillus#Culture_Charts|''Lactobacillus'' culture charts]]. If your ''Lactobacillus'' fermentation temperature range is unknown, the [[Lactobacillus#Culture_Charts|''Lactobacillus'' culture charts]]may provide some guidance based on species if the species is known. Otherwise, experimentation will be needed to find the best temperature (we recommend making different starters and holding them at different temperatures and measuring pH drop over time to find the optimal fermentation temperature). Note that some ''Pediococcus pentosaceus'' cultures such as [[Pediococcus#Bootleg_Biology_on_Sour_Weapon_P|Bootleg Biology's "Sour Weapon P"]] can also be used to kettle sour.</blockquote>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.

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 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 CO² <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 CO². 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 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|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 CO² is a good way to prevent a vacuum).

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.

* [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.]