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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 into the mash after the completion of the mashing processstarch conversion. Sour mashing Performing a sour mash is similar to [[Sour Worting|sour wortingWort Souring]] in that both techniques involve acidification of unfermented wort with lactic acid bacteria before primary fermentation. Many sour brewers prefer [[Sour Worting|sour wortingWort Souring]] (which includes ''kettle souring'') due to having more control over off flavors produced versus sour mashing.

Sour mashing procedures fall into the normal all-grain brewday process between after completion of saccharification, and any other desired mashing rests, and before the 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 more or less pasteurized from the hot temperatures of the mash. In order to reintroduce bacteria into the mash it 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, and yeasts, and molds on the husks(see [[Grain#Malted_Barley|bacteria found on malted grain]] for more information).

Sour mashes typically last between roughly 12 hours and 3 days. After the mash has reached the desired acidity the wort is separated from the grain and boiling and fermentation are carried out as normal. It is important that wort from a sour mash is not consumed, especially if it was inoculated with grain, because pathogens have been found to grow in wort soured with grain (see [[Grain#Malt_Inoculated_Wort|Malt Inoculated Wort]]).

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/permalink/992202820807932/?comment_id=993485744012973 993343157360565&comment_tracking=%7B%22tn%22%3A%22R0%22%7D Conversation with professional brewer Anthony Accardi on MTF.]</ref>.

Due to the wide variety of microbial species typically added to a sour mash from the addition of unused grains of malt after the mashing process, unwanted microbes such as ''Clostridium spspp''. can take hold 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/vomit/fecal aromas 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 (over 4.4 pH), and it may not be advisable to taste the mash, especially if noticeable spoilage aroma is detected. [[Sour Worting|Sour wortingWort Souring]] is generally not as susceptible to these spoilage organisms. ==Microbiology and Biochemistry==The buffering capacity of the mash is higher than wort due to the grain material. This has a positive affect on the growth of lactic acid bacteria. Peyer et al. (2017) found a slight increase in cell growth of ''Lactobacillus'' when incubated in a soured mash versus when souring in wort. However, due to a much higher concentration of sugar content and buffering capacity of the sour mash versus plain wort, less lactic acid was actually produced. This study also found that sour mash beers have almost four times as much acetaldehyde as the co-fermented and kettle soured versions, although this was speculated to be caused by the oxidation of ethanol in the sour mash, which converts alcohol back into acetaldehyde <ref name="peyer_2017">[http://www.asbcnet.org/publications/journal/vol/2017/Pages/ASBCJ-2017-3861-01.aspx Sour Brewing: Impact of Lactobacillus amylovorus FST2.11 on Technological and Quality Attributes of Acid Beers. Lorenzo C. Peyer, Martin Zarnkow, Fritz Jacob, David P. Schutter, Elke K. Arendt. 2017.]</ref>.

* [[Sour WortingWort Souring]]

* [http://suigenerisbrewing.blogspot.com/2015/07/how-i-sour-mash-recipe.html "How I Sour Mash & a Recipe"; Sui Generis Blog by Bryan Heit (includes microbiology information).]