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'''Wort Souring''' is the process of "[[Mixed Fermentation]]" (in the case of lactic acid bacteria being left alive to co-exist with yeast) or "Sequential Fermentation" pre-boil acidfication (in the case of kettle sours) where lactic acid bacteria are given a "head start" as it's pitched pre-ferment and acidify wort before the yeast so that it will be able is pitched to produce significant amounts 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-fermentation with lactic acid before the ''Saccharomyces'' completes the main fermentationbacteria 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''.
===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.