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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 recommended to fill the head space of the kettle with CO2 because oxygen can encourage off flavors such as rancid cheese from [[Butyric Acid]] and/or [[Isovaleric Acid]], which are produced by microbes that are naturally present on the grain. Keeping the temperature between 113-120°F (45-48.8°C) will encourage the ''Lactobacillus'' resident on the grain and will discourage other bacteria. Lowering the pH of the [[Sour_Worting#How_to_Pre-Acidify|wort to 4.5]] will also discourage many other bacteria from thriving in the wort during the incubation period. 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 is ideal but not necessary to fill the head space of the fermenter with CO2 gas (some brewers have reported that this will help reduce sulfur in the finished beer). Keeping positive pressure in the kettle with CO2 will help prevent contaminates 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 formation of [[Butyric Acid]] and [[Isovaleric Acid]] when using only a pure culture is extremely slight to none assuming no other microbes are allowed inside the kettle.
Once the level of acidity is reached (this can be tested with a reliable 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 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 170°F (76.6°C) for 15 minutes should kill the ''Lactobacillus'' culture being used to sour the wort <ref name="pasteurization" />.