Fatty acids in beer have concentrations that are low in your wort but then increase as fermentation and maturation progresses. Medium-chain acids are a normal constituent of and contribute to the characteristic flavors you associate with beer. When the concentrations of these fatty acids in beer becomes high, off-flavors develop.
The main culprit is Butyric Acid, also known under the systemic name “butanoic acid.” Butyric Acid is a carboxylic acid with the structural formal CH3CH2CH2COOH. It is an important flavor compound in a number of foods as well as beer.
At concentrations above its flavor threshold (2 mg/L), butyric acid will cause cheesy, rancid, baby vomit, or putrid off-flavors in your beer. I don't know about you, but baby vomit is not something I want in my beer.
Excessive levels of butyric acid are usually caused by infections from anerobic bacteria of the genus Clostridium . Other off-flavors attributed to these acids are described as "goaty" and come from hexanoic, octanoic, and decanoic acids being produced in excess during fermentation.
Excessive concentrations of these acids can also cause a decrease in a beer's foam stability and should be avoided for obvious reasons.
A similar "cheesy" off flavor comes from Isovaleric Acid. Some describe this off flavor as being like stale cheese, hard cheese, sweaty or putrid. This off flavor comes from the use of old or degraded hops. The two off flavors are often confused. Remember that Butyric Acid has "rancid cheese" notes and Isovaleric Acid has more of a "stale cheese" flavor and aroma. You would have to purchase an off flavor sensory kit to learn the difference between the two. FlavorActiv Sensory Kits have standards for both of these off flavors.
Information for this page was adapted from Volatile compounds in foods and beverages By H. Maarse and FlavorActiv.com-GMP Flavour Standards-The Sensory Professional's Choice.
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