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added section on Kettle Souring
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>.
===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, a sour mashed beer, 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 <ref name="peyer_2017">[http://www.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>.
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