Butyric Acid

Butyric Acid (chemical name butyrate ) 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 a Gram-positive 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 at least 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. Fermentation also inhibits growth .

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.

Megasphaera spp

Megasphaera species produce large amounts of butyric acid, as well as some acetic, isovaleric, valeric, and caproic acids. They are considered a true beer spoilage organism in beer M. cerevisiae is a common species found in spoiled beer, and is a Gram-negative bacteria. They are obligate anaerobes, and die in the presence of oxygen. Growth occurs at a temperature range between 59°-98.6°F (15°-37°C), with an optimal growth rate at 82.4°F (28°C) . Growth of M. cerevisiae cannot occur above 5.3% abv, and slow significantly at 2.66% abv. Growth also stops at a pH of 4.0 .

Fusobacterium nucleatum

F. nucleatum is a Gram-netagive bacteria that is commonly found in dental plaque. It is capable of fermenting glucose into butyric acid, and is tolerant of up to 6% oxygen . It has a optimal growth temperature at 98.6°F . F. nucleatum is sensitive to a pH of 5.0 and lower, and temperatures above 140°F (60°C) . Avoid contaminating wort with saliva, and this organism shouldn't be a problem.