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'''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 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>.

''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 <ref name="Fenaroli_ethylisovalerate">[https://books.google.com/books?id=15HMBQAAQBAJ&pg=PA638&lpg=PA638&dq=ethyl+valerate+threshold&source=bl&ots=avVr8PQQ_p&sig=zm81_lhLU86VJ4jBNnm4I9nnxDw&hl=en&sa=X&ved=0CDIQ6AEwBGoVChMImYrEl6usxwIVAjmICh1HGwEs#v=onepage&q=ethyl%20isovalerate%20threshold&f=false Fenaroli'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, and 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 rate of metabolic breakdown of isovaleric acid into ethyl isovalerate has not been researched, as far as the authors 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 this wiki knowisovaleric 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. 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 <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>.