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added ref to not purging with CO2
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 reach should be enough to pasteurize the wort <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>, however, we advise heating 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>. 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]]). 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 [[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 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 CO<sup>2</sup> 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)]] will also discourage other bacteria from thriving in the wort during the incubation period. This will also help with 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 CO2 CO<sup>2</sup> gas (oxygen does not cause ''Lactobacillus'' to produce [[Butyric Acid|butyric acid]] or [[Isovaleric Acid|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 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 CO2CO<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. Retreived 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 CO2 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). 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 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 CO2CO<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>.
Once the level of acidity is reached (this can be tested with a reliable [[PH_Meter|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 [http://www.garshol.priv.no/blog/331.html Raw Ale]). Heat pasteurization at 180°F (82°C) for 15 minutes will kill even the most heat tolerant ''Lactobacillus'' species <ref>[[Lactobacillus#Tolerance_of_Extreme_Temperature|Lactobacillus wikipage; Tolerance of Extreme Temperature]]</ref><ref name="pasteurization" />. 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 <ref>[http://beerandwinejournal.com/proper-boil-ph/ Chris Colby. "Easy way to Hit The Proper Boil pH". Beer and Wine Journal blog. 07/01/2013. Retrieved 03/02/2018.]</ref>. Haze and sediment can be avoided by choosing to boil long enough to hit a hot break ''before'' souring the wort (see [https://www.facebook.com/groups/MilkTheFunk/permalink/2008265249201679/?comment_id=2008553569172847&reply_comment_id=2008577055837165&comment_tracking=%7B%22tn%22%3A%22R%22%7D example MTF post from Chris Prechel]). Raw ales are also fairly hazy in nature due to the lack of hot break from boiling. See [[Wort_Souring#Haze|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 CO2CO<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, and WY3726 to name a few (see reference) <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1105185972842949/ Conversation on MTF about using specific yeast strains in acidic wort. 7/6/2015.]</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.