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 starch conversion. Performing a sour mash is similar to sour worting in that both techniques involve acidification of unfermented wort with lactic acid bacteria before primary fermentation. Many sour brewers prefer sour worting (which includes kettle souring) due to having more control over off flavors produced versus sour mashing.
"Mixed culture fermentation for sour beers produces one thing, and (kettle souring/sour mashing) produces another thing. If you’re going to make a malty red ale that is kettle soured, don’t call it a Flanders Red. Honor the tradition." - Sean Burke of the Commons Brewery, Kettle Souring Presentation, CBC 2015.
Methods of 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 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 on the husks (see bacteria found on malted grain for more information).
To help favor the growth of Lactobacillus over the other unwanted microbes, it is important that the mash be kept anaerobic and incubated warm (ideally 113-120°F/45-48.9°C) throughout the duration of the sour mash. This can be accomplished by purging the headspace with CO2 (some breweries go so far as to bubble CO2 through their mash while it is cooling, and periodically throughout the souring time) and covering with saran wrap at the liquid-air interface to eliminate air contact. The favoring of Lactobacillus over spoilage microbes can also be supported by dropping the initial sour mash pH by methods such as the addition of acidulated malt, lactic acid (preferably), or starter wort from a Lactobacillus culture.
Sour mashing may be conducted on the entire mash, or may be conducted on a portion of the mash. On a homebrew scale, sour mashing only a portion of the grist can easily be accomplished by conducting a small stove top mash before the planned brewday. This pre-soured portion can then be blended in with the wort collected from the normal mash of the remaining grist during the brew day.
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.
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 .
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 spp. can take hold in a sour mash and produce off flavors such as Butyric acid and possibly 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 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 .
Additional Articles on MTF Wiki
- Berliner and Beyond: Sour Mashing and Its Applications. Derek Springer's NHC 2015 audio and slides.
- "Year of the Sour Mash" by Derek Springer. This series of articles were written for his NHC 2015 presentation.
- "Sour Mash How-To", by Adam Kielich (Brain Sparging on Brewing blog).
- "Sour Mashing: Techniques", BYO, by Dave Green. October 2008.
- "How to Make a Sour Mash: Techniques", BYO, September 2000.
- "How I Sour Mash & a Recipe"; Sui Generis Blog (includes microbiology information).