From beersensoryscience.wordpress.com, writer not attributed. Posts as BeerSensor
Cracking cheese, Gromit!
Have you ever smelled cheese in your beer? How about dirty sweatsocks? It’s more common than you may think. If you’re a homebrewer and you don’t use your hop supply as fast as you should, or if you store them improperly, you may be familiar with this aroma. This is isovaleric acid, and it’s a short-chain fatty acid commonly found in cheese, the valerian herb, foot odor, and sometimes beer. Now that’s an interesting selection of sources!
The commonly accepted threshold for isovaleric acid is about 1ppm, but like most other aromatic compounds, this can vary greatly depending on your genetics. This brief article gives some information about the genetic component of isovaleric acid receptors, exploring some of the sources of variability in how subjects perceive this compound. One of the more interesting things mentioned is that its detection threshold can apparently differ between individuals by up to 10,000 times. Personally, I think my nose has what I call an “acquired anosmia” to this compound. To be anosmic to a particular compound means you can not detect it at any concentration. While my case isn’t that dramatic, I think my sensitivity has dropped due to being frequently exposed to the purified compound when I spike it into my samples (despite using a fume hood and taking protective measures, it’s still possible to get it on you). If you get this stuff on your hands, you’ll stink for the rest of the day, if not longer. For this reason, I often have a hard time being able to tell if my spiked samples are at an appropriate level for the panel. Many times, I have to trust my math more than my nose.
So, how does isovaleric acid get into beer?
Most of the time, it’s formed when hops get old, particularly when the alpha acids degrade. I’ve discussed hop acids already in the bitterness article, so if you need a quick overview, head over there and it might clarify some things. This image (from the above-linked article) shows the basic structure of the alpha acids (on the left) and the iso-alpha acids (right) that they isomerize into during boiling in the brewing kettle (at which time they become the source of bitterness in beer). Basically, there are 3 main types of alpha acid (and the 3 corresponding iso-alpha acids) and while they have the same basic structure as each other, there are differences at the “R-group” (top right of the molecule in the images). The differences are minor, but these minor differences can be interesting and influential nonetheless. One of these 3 alpha acids (humulone) has an R-group which is called an isovaleryl group. When this alpha acid oxidizes (due to age and/or improper storage), this R-group can be removed from the molecule and becomes flavor-active, leading to the cheesy/sweatsock flavor I’m on about.
Another way isovaleric acid can get into beer is through a Brettanomyces infection. It’s not the most common source in beer, but infection by this yeast genus can produce cheesy aromas, as well as a host of other undesirable flavor-active compounds like acetic acid (vinegar), 4-ethylphenol (bandages), and 4-ethylguaiacol (smoky). Some breweries intentionally “pitch” Brett into their fermentors as they try to achieve a certain flavor profile or match a particular Belgian style, but more often than not a Brett infection is a bad thing. Brett is also used in winemaking to achieve certain flavors, but it can also be a spoilage organism here depending on the intent of the oenologist.
So limiting undesirable isovaleric acid levels in your beer comes down to using fresh and high-quality raw materials (store hops in a cool, dark environment and, if possible, oxygen-free), and maintaining sanitary brewing conditions and using plentiful and healthy yeast to limit the potential for beer spoilage.