Changes

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 [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]]). Isovaleric acid can also be produced by the oxidation of hops, and is often found in hops aged for traditional [[lambic]] brewing <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>.