Butyric Acid

Butyric Acid is a carboxylic acid that is produced by aenarobic bacteria during glucose fermentation, and is generally considered an off flavor in sour beer. If not done right, Sour Mashing can be a big producer of butyric acid. Butyric acid is produced by aenarobic bacteria such as Clostridium butyricum, C. kluyveri, and Fusobacterium nucleatum . The aroma and flavor is often described as a vomit, bile, or rancid cheese. It is also present in the human gut, and is the cause of the aroma of vomit . This is not to be confused with Isovaleric Acid which has a more "feety" aroma and flavor, and is produced by Brettanomyces. Brettanomyces can convert butyric acid into Ethyl Butyrate at low levels, which has a pineapple, tropical fruit aroma and flavor .

Microbes and Metabolism

Clostridium spp

Clostridium is an anaerobic genus of bacteria that is found in plants, wounds, soil, and feces. Several species of the genus Clostridium produce butyric acid, and is produced in less than 3 days. Clostridium are generally anaerobic, but some can be active in aerobic conditions (aerotolerant), such as T. tertium. Species of Clostridium that can produce butyric acid from glucose include C. butyricum, C. fallax, C. paraputrificum, C. sartagoformum, C. tertium, C. thermosaccharolyticum, and to a lesser degree C. pasteurianum.

Clostridium contamination in brewing methods such Sour Mashing can be controlled by lower the pH of the wort/mash. However, some species of Clostridium are acid tolerant. For example, C. butyricum and C. tertium were found to survive a pH as low as 4.0 . Therefore, pre-acidifying during the Sour Mashing process to a pH of 3.9 or lower may produce the best results.

Temperature resistance varies from species to species of Clostridium. Most species have an optimal growth at around 104°F (37°C) . However, C. thermosaccharolyticum can survive temperatures as hot as 149°F (65°C). Additionally, spores of some species of Clostridium (such as C. tertium) can survive boiling temperatures for longer than 1.5 hours .

Although C. botulinum and other identifiably pathogenic Clostridium species do not tend to grow in Sour Mashing conditions, it is advisable to not consume wort produced from a sour mash until after it is boiled. Seek specific food and safety advice from a physician.