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==Introduction=='''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 rancid Parmesan, "pungent cheese" or "foot odor". it 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 <ref name="aroxa">[http://www.aroxa.com/beer/beer-flavour-standard/isovaleric-acid/ Aroxa. Isovaleric acid. Retreived 12/30/2015.]</ref> and 1.5 mg/L <ref name="peyer_2017" />. It has a boiling temperature of 347-351°F (175-177°C) <ref name="wikipedia" />.

''[[Brettanomyces]] '' can create isovaleric acid <ref name="wikipedia">[http://en.wikipedia.org/wiki/3-Methylbutanoic_acid Wikipedia article]</ref>. 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) <ref>[https://scholar.sun.ac.za/bitstream/handle/10019.1/4141/Botha,%20J.J.pdf?sequence=1 Botha, Janita J. Sensory, chemical and consumer analysis of ''Brettanomyces '' spoilage in South African wines. March 2010. Pg 2, 13, 17, 18]</ref><ref>[https://scholar.sun.ac.za/bitstream/handle/10019.1/1239/oelofse_investigating_2008.pdf?sequence=1 Oelofse, Adriaan. Investigating the role of ''Brettanomyces '' and Dekkera during winemaking. December 2008.]</ref>.

Other bacteriasIt 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 [https://en.wikipedia.org/wiki/Keto_acid 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). <ref>[http://aem.asm.org/content/70/7/3855.full Helinck, Le Bars, Moreau, and Yvon. Ability of thermophilic Lactic Acid Bacteria To Produce Aroma Compounds from Amino Acids.]</ref>. 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]])<ref>[http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.1970.tb03256.x/pdf Green, C. P. The Volatile Water-Soluble Fraction Of Hop Oil. Aug 24, 1909.]</ref><ref>[https://books.google.com/books?id=oWQdjnVo2B0C&pg=PA498&lpg=PA498&dq=oxidation+of+hop+resins+3-Methylbutanoic+acid&source=bl&ots=wmM8jX-qJY&sig=_wFzzuUA40eg0vMNc-7vfU6tneA&hl=en&sa=X&ei=XX2PVJHDI4bcoATjoYK4BA&ved=0CEcQ6AEwBQ#v=onepage&q=oxidation%20of%20hop%20resins%203-Methylbutanoic%20acid&f=false Oliver, Garret. The Oxford Companion to Beer. 2001. Pg 498.]</ref>.

Isovaleric ===Kettle Souring===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 <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1778865588808314/?comment_id=1778870915474448&comment_tracking=%7B%22tn%22%3A%22R9%22%7D Khristopher Johnson. Milk The Funk Facebook group. 08/03/2017.]</ref>.  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 can also when the ''Lactobacillus'' and US-05 were co-fermented (see the table below). The authors suggested that this might be produced due to the release of the amino acid precursors by the oxidation ''Lactobacillus'' towards the end of hops its life cycle when it was allowed to survive in a co-fermentation, versus being killed in the kettle souring/mash souring batches <refname="peyer_2017">[http://onlinelibrarywww.asbcnet.org/publications/journal/vol/2017/Pages/ASBCJ-2017-3861-01.aspx Sour Brewing: Impact of Lactobacillus amylovorus FST2.11 on Technological and Quality Attributes of Acid Beers. Lorenzo C. Peyer, Martin Zarnkow, Fritz Jacob, David P. Schutter, Elke K. Arendt. 2017.]</ref>. {| class="wikitable sortable"|-! Method !! Amount of IVA (mg/L) <ref name="peyer_2017" />|-| 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.wileyamylovorus'' and US-05 || 1.15|-|} ==Other Producers==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 <ref name="Vasquez">[http://bashanfoundation.com/doigmaweb/pdfs/vazquez1.pdf Phosphate-solubilizing microorganisms associated with the rhizosphere of mangroves in a semiarid coastal lagoon. P. Vazquez, G. Holguin, M.E. Puente, A. Lopez-Cortes, Y. Bashan. 2000.]</ref><ref>[http://web.mst.edu/~microbio/BIO221_1998/B_amylo.html Bacillus amyloliquifaciens. Carl Sather's website. Retrieved 10/06/2015.1002]</ref><ref>[http:/j/www.2050researchgate.net/publication/263432119_Bacillus_atrophaeus_main_characteristics_and_biotechnological_applications__a_review Bacillus atrophaeus: main characteristics and biotechnological applications – a review. Sandra R B R Sella, Luciana Vandenberghe, Carlos Ricardo Soccol. 2014.]</ref>. ''Xanthobacter agilis'' is a Gram-0416negative aerobic bacteria that can produce isovaleric acid <ref name="Vasquez"></ref><ref>[http://www.1970thelabrat.tb03256com/restriction/sources/Xanthobacteragilis.xshtml Xanthobacter agilis. The Lab Rat. Retrieved 10/06/2015.]</pdf Greenref>. ''Paenibacillus macerans'' is a facultative anaerobe (can utilize oxygen if present, Cbut 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 <ref>[https://en.wikipedia.org/wiki/Paenibacillus_macerans Paenibacillus macerans. Wikipedia. Retrieved 10/06/2015.]</ref>. The Volatile Waterstrict anaerobe, ''Megasphaera cerevisiae'', which has been identified as a beer spoiler, can also produce isovaleric acid (as well as [[Butyric_Acid|butyric acid]], valeric acid, caproic acid, and hyrodgen sulfide) <ref>[http://www.asbcnet.org/publications/journal/vol/2016/Pages/ASBCJ-2016-4895-01.aspx Monoclonal Antibodies Binding to Lipopolysaccharide from the Beer-Spoilage Bacterium Megasphaera cerevisiae Exhibit Panreactivity with the Strictly Anaerobic Gram-Negative Brewing-Soluble Fraction Of Hop OilRelated Bacteria. Barry Ziola. Aug 24, 19092016.]</ref>. ===Hops===Isovaleric acid can also be produced by the oxidation of hops, and is often found in hops aged for traditional [[lambic]] brewing. See [[Hops#Aged_Hops|Aged Hops]] for more information. ==Metabolic Breakdown==''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 <refname="Fenaroli_ethylisovalerate">[https://books.google.com/books?id=oWQdjnVo2B0C15HMBQAAQBAJ&pg=PA498PA638&lpg=PA498PA638&dq=oxidation+ofethyl+hop+resins+3-Methylbutanoicvalerate+acidthreshold&source=bl&ots=wmM8jX-qJYavVr8PQQ_p&sig=_wFzzuUA40eg0vMNc-7vfU6tneAzm81_lhLU86VJ4jBNnm4I9nnxDw&hl=en&sa=X&ei=XX2PVJHDI4bcoATjoYK4BA&ved=0CEcQ6AEwBQ0CDIQ6AEwBGoVChMImYrEl6usxwIVAjmICh1HGwEs#v=onepage&q=oxidationethyl%20of20isovalerate%20hop%20resins%203-Methylbutanoic%20acid20threshold&f=false OliverFenaroli's Handbook of Flavor Ingredients, Fifth Edition. George A. Burdock. CRC Press, Dec 3, 2004. Pg 587.]</ref><ref name="Joseph">[http://www.ajevonline.org/content/suppl/2015/07/28/66.3.379.DC1/Supplemental_Data.pdf Supplemental Data for: Joseph, C.M.L., E.A. Albino, S.E. Ebeler, Garretand L.F. Bisson. Brettanomyces bruxellensis aroma-active compounds determined by SPME GC-MS olfactory analysis. 2015.]</ref><ref name="lucy_joseph">[http://slideplayer.com/slide/4473144/ Impact of Brettanomyces on Wine. Presentation by Lucy Joseph of UC Davis. Retrieved 08/15/2015.]</ref>. The Oxford Companion 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 Beerbe broken down over time, and ''Brettanomyces'' will not significantly "clean up" a beer with high amounts of isovaleric acid <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/4384393781588802/?comment_id=4386457104715803 Richard Preiss and other brewers. MTF post; anecdotes on isovaleric acid break down over time. 03/09/2021.] </ref>. See also [[Brettanomyces#Ester_Production|''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. 2001However, many brewers find small amounts to be acceptable in mixed fermentation sour beers and lambics. Pg 498Other 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 <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/3752511238110396 Milk The Funk poll on whether or not isovaleric acid is considered an off-flavor and at what amounts. 08/08/2020.]</ref>.