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updated bacteria section in Production
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 <ref>[http://www.ncbi.nlm.nih.gov/pubmed/16699586 Ara K, Hama M, Akiba S, Koike K, Okisaka K, Hagura T, Kamiya T, Tomita F. Can J Microbiol. 2006 Apr.]</ref>. Leucine is also present in beer <ref>[http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.1981.tb03992.x/pdf Hall, Nutfield, Redhill, Surrey. Amino Acid Esters in Beer. Brewing Research Foundation. July 14, 1980.]</ref>.
Other bacteriasbacteria, 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 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]]).
Isovaleric acid can also be produced by the oxidation of hops <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>.