Isovaleric Acid, also known as 3-Methylbutanoic acid, is an organic compound with the formula (CH3)2CHCH2CO2H. The flavor and aroma are often described as "pungent cheese" or "foot odor". It is not to be confused with Butyric Acid, which specifically has a more bile or vomit aroma and flavor. The flavor threshold of isovaleric acid has been reported to be 1 mg/L  and 1.5 mg/L . It has a boiling temperature of 347-351°F (175-177°C) .
Production in Beer and Wine
Brettanomyces can create isovaleric acid . The compound generally takes a few months to produce in beer by Brettanomyces. Brettanomyces breaks down leucine present in beer into isovaleric acid (controversial, but generally accepted) .
Isovaleric acid can also be produced by a bacteria that lives naturally on human skin and is responsible for foot odor called Staphylococcus epidermidis. It does so by degrading leucine, an amino acid present in sweat . Leucine is also present in beer .
It has been shown in Swiss-type cheese that other bacteria, including species and strains of Streptococcus (more so) Lactobacillus (less so) can produce various amounts of isovaleric acid from leucine, as well as other compounds from other carboxylic acids. Lactobacillus is not capable of producing isovaleric acid without the presence of an alpha-keto acid, which is produced by Streptococcus thermophilus, so the presence of S. thermophilus or another alpha-keto acid producing microorganism is required for Lactobacillus to produce isovaleric acid (as well as a range of other acids). . This has not been shown to occur in beer, but this may be the reason that sour mashes often have a rancid cheese off flavor (although this may also be at least partially due to Butyric Acid production during Sour Mashing) .
Professional brewer Khristopher Johnson has observed the taste of isovaleric acid in kettle soured beers. Purging with CO2 has been quoted as something that has resolved this issue, the hypothesis being that oxygen could promote the formation of isovaleric acid if aerobic contaminates are present .
One study compared the levels of isovaleric acid in a kettle soured beer (flushed with CO2), a sour mashed beer (flushed with CO2), and a beer co-fermented with Lactobacillus amylovorus and US-05. They found that there was a significant increase in isovaleric acid when the Lactobacillus and US-05 were co-fermented (see the table below). The authors suggested that this might be due to the release of the amino acid precursors by the Lactobacillus towards the end of its life cycle when it was allowed to survive in a co-fermentation, versus being killed in the kettle souring/mash souring batches .
|Method||Amount of IVA (mg/L) |
|US-05 Only (control)||0.83|
|Mash Souring with L. amylovorus and primary fermentation with US-05||0.72|
|Kettle Souring L. amylovorus and primary fermentation with US-05||0.50|
|Co-fermentation with L. amylovorus and US-05||1.15|
Other bacteria have been shown to create isovaleric acid, and may contribute to the production of it in beer in unsanitary conditions. Bacillus amyloliquefaciens and B. atrophaeus are Gram-positive, aerobic (require oxygen for growth) bacteria and have been shown to produce isovaleric acid . Xanthobacter agilis is a Gram-negative aerobic bacteria that can produce isovaleric acid . Paenibacillus macerans is a facultative anaerobe (can utilize oxygen if present, but oxygen is not required) that can produce isovaleric acid. P. macerans is a Gram-variable bacteria, meaning that it can produce Gram-positive and Gram-negative rods . The strict anaerobe, Megasphaera cerevisiae, which has been identified as a beer spoiler, can also produce isovaleric acid (as well as butyric acid, valeric acid, caproic acid, and hyrodgen sulfide) .
Brettanomyces can break down isovaleric acid into an ester called ethyl isovalerate. This ester is described as fruity, sweet, berry-like with a ripe, pulpy fruit nuance . The rate of metabolic breakdown of isovaleric acid into ethyl isovalerate has not been researched, but anecdotes from brewers indicate that only very small amounts of isovaleric acid seem to be broken down over time, and Brettanomyces will not significantly "clean up" a beer with high amounts of isovaleric acid . See also Brettanomyces ester production.
Status as an Off-Flavor
Isovaleric acid is widely thought to be an off-flavor in beer, particularly non-sour beer. It is also generally thought of as an off-flavor in kettle sours and in some mixed fermentation beers. However, many brewers find small amounts to be acceptable in mixed fermentation sour beers and lambics. Other brewers consider any amount of isovaleric acid to be an off-flavor in mixed fermentation sour beers and lambic, and they consider it a sign that the beer has not been allowed to age long enough .
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