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→Souring in the Boiler (Kettle Sour)
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). 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>.