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Wort can be soured in the primary fermenter before adding other yeasts. This is generally a good approach for brewers who aren't concerned with pasteurization and infections of their cold side equipment. This also has the benefit of possibly producing a more complex sour beer overall or at least a sour beer that will evolve over time. It has the advantage over a more traditional [[Mixed Fermentation]] in that ''Lactobacillus'' is used to guarantee at least a certain level of sourness. This is also a good process to use for making a [[Berliner Weissbier]].
The process is very similar to the kettle souring technique with the exception that the wort is never pasteurized after it is soured. This all-grain brewing process is the same for any all-grain brewing process except that, after the boil, the beer is only chilled to the recommended temperature for the ''Lactobacillus'' strain that the brewer is going to use. Using grain husks for souring with this method is inadvisable since the grain will stay in the fermenter during primary fermentation and unwanted microbes on the grain husks would potentially have a longer exposure to the wort. Instead, the brewer should use a pure strain of ''[[Lactobacillus]]''. As a result of not using grains to sour the wort there remains less concern of developing [[Butyric Acid]] or [[Isovaleric Acid]] with this method. Even still, lowering the pH of the [[Wort_Souring#How_to_Pre-Acidify|wort to under 4.5 (ideally 4.0 - 4.3when souring with grain)]] will also discourage contaminating bacteria from thriving in the wort during the incubation period. This will also help with head retention <ref>[[Lactobacillus#Foam_Degradation]]</ref>. There is also the option of [[Mixed_Fermentation#Reusing_a_Sour_Yeast_Cake|using a sour yeast cake]] from another sour beer as the bacteria inoculation.
Once cooled to the desired temperature, usually around 90-115°F or 32.2-46°C (see the [[Lactobacillus#Culture_Charts|''Lactobacillus'' culture charts]] for desirable temperature ranges for different ''Lactobacillus'' cultures), the wort is racked to the primary fermenting vessel. Note that the wort should contain a low amount of IBUs when using this process since IBUs can inhibit many (but not all) species of ''[[Lactobacillus]]''. Eliminating hops can be a good approach to getting more acidity, but if hops are required, then using less than 6 IBUs is a good guideline in general (see the [[Lactobacillus#Culture_Charts|''Lactobacillus'' culture charts]] for tips on hop tolerance for different ''Lactobacillus'' cultures). To achieve a low IBU wort consider mash hopping. Mash hopping has been reported to reduce IBUs by ~70% <ref>[http://www.homebrewersassociation.org/how-to-brew/resources/conference-seminars/ ''Putting Some Numbers on First Wort and Mash Hop Additions''. David Curtis NHC 2014 Presentation.]</ref>. It is generally a good idea to create a 500 mL starter beforehand for ~5-6 gallons of wort (see [[Lactobacillus#Starters_and_Pitching_Rate|''Lactobacillus'' starters]]). Once the wort is racked to the primary fermenting vessel the ''Lactobacillus'' culture is added directly to the fermenter. No other yeasts are added at this time. The ''Lactobacillus'' bacteria is allowed to incubate by itself in the wort for 1-3 days with the target temperature maintained throughout the incubation period (some ''Lactobacillus'' species/strains may continue to produce acidity under lower temperatures, for example ''L. plantarum''). Small temperature fluctuations should not adversely affect the souring process as long as the temperature stays within the desirable range for that particular strain of ''Lactobacillus'', although the brewer should take care not to allow any dust to get sucked into the fermentation vessel as a result of temperature decreases. During the incubation time, as long as ''Lactobacillus'' is the only microbe growing in the wort, the gravity will not drop more than a few points and the fermentation will be calm (see [[Lactobacillus#100.25_Lactobacillus_Fermentation|''Lactobacillus'' fermentation]]. The exact time frame of incubation depends on the species/strain of ''Lactobacillus'', the manufacturer's recommendation, and the brewer's desired acidity level. Acidity can safely be measured with a reliable [[PH Meter|pH meter]] throughout this time. If possible, it is advised that the brewer fills the headspace of the fermenter with CO2. Some brewers report that this helps to reduce sulfur production, but if ''Brettanomyces'' is added to the beer later on in the process and allowed to age, this shouldn't be a concern.