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This process is very similar to sour worting in the kettle. This method is ideal for those who wish to use grains to introduce Lactobacillus to the beer. If done properly, the formation of [[Butyric Acid]] and [[Isovaleric Acid]] should be minimal.
The wort is mashed and sparged as normal, and is then lowered to somewhere between 113°F113-120°F (45°C45-48.8°C). This temperature favors Lactobacillus, while discouraging Enterobacteriaceae. Optionally, the mash pH can be lowered to 4.5 with lactic acid or acidulated malt to further discourage Enterobacteriaceae activity. Once the desired temperature (and optionally pH) is reached, a handful of fresh malted unmilled grain is added to the mash and allowed a few minutes to inoculate the mash with the microbes found naturally on the grain husks. The wort is then transferred to a second vessel such as a glass carboy. The vessel should be filled to the very top, minimizing the oxygen levels inside the vessel. The vessel should be stored in a heated environment that maintains a temperature between 113°F113-120°F (45°C45-48.8°C) for 1 to 4 days depending on how much acidity the brewer wants (3 days is a good rule of thumb) <ref name="james_spencer">[http://beerandwinejournal.com/sour-wort-berliner/ Spencer, James. December 15, 2014. Beer and Wine Journal.] </ref>.
Once the 1 to 4 day time period has been reached, the wort is transferred to the boil kettle and boiled as normal. Boiling will kill all of the microorganisms in the wort, and will provide the option for adding hops and other kettle additions. Just as with kettle souring, the wort doesn't have to be boiled, but can be instead heat pasteurized at 170°F (76.6°C) for 15 minutes <ref name="pasteurization" />. ''Caution: the wort gravity may also be tested to check for alcohol presence. Alcohol can be more easily boiled off since it boils at 173.1°F (78.37°C) <ref name="boiling_of_alcohol" />.'' Once boiled, the wort can be chilled and handled in the same way as the above methods for wort souring.