Sour Mashing

Introduction

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 in that both techniques involve acidification of unfermented wort with lactic acid bacteria before primary fermentation. Many sour brewers prefer sour worting due to having more control over off flavors produced versus sour mashing.

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, yeasts, and molds on the husks.

To help favor the growth of Lactobacillus over the other unwanted microbes, it is important that the mash be kept anaerobic and incubated warm (generally 100-120 F/ 37.8-48.9 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 .

Unwanted Microbes

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 sp. 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, 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.

External Articles

• Year of the Sour Mash by Derek Spring. 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.