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Sour mashing is a technique for adding acidity to a beer before primary fermentation begins. This is accomplished by the introduction of lactic acid bacteria after the completion of the mashing process. Sour mashing is similar to [[Sour Worting|sour worting]] in that both techniques involve acidification of unfermented wort with lactic acid bacteria before primary fermentation. Many sour brewers prefer [[Sour Worting|sour worting]] due to having more control over off flavors produced versus sour mashing.

Sour mashing procedures fall into the normal all-grain brewday process between completion of saccharification, and any other desired mashing rests, and separation of the wort from the grain. After the normal saccharification rests and a mashout to 170+ F (76.7+ C) to denature enzymes in the mash, the mash is cooled to approximately 120 F (48.9 C). At this point the mash is inoculated with lactic acid bacteria. This may be accomplished by addition of a pure culture of ''[[Lactobacillus]]'' or, more commonly, by the addition of a small amount of unused malt, which has ''Lactobacillus'' in addition to other bacteria, yeasts, and molds on the husks.

Some professional brewers have reported stuck or slow sparges when performing a sour mash. This generally isn't a problem on the homebrew scale. Adding rice hulls and resetting the grain bed will help resolve the issue <ref>[https://www.facebook.com/groups/MilkTheFunk/992202820807932/?comment_id=993485744012973 Conversation with professional brewer Anthony Accardi on MTF.]</ref>.

Due to the mixed culture typically added to a sour mash from the addition of unused grains of malt after the mashing process, unwanted microbes such as ''Clostridium sp''. can take old in a sour mash and produce off flavors such as [[Butyric Acid|Butyric acid]] and possibly [[Isovaleric Acid|Isovaleric acid]].

Should the sour mash display prominent character of these spoilage organisms, such as rancid aroma or moldy patches, it may be advisable to not use the sour mash. If the sour mash is not acidic enough it is possible for human pathogens to be present in the mash, and it may not be advisable to taste the mash, especially if noticeable spoilage aroma is detected. [[Sour Worting|Sour worting]] is generally not as susceptible to these spoilage organisms.

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